tag:blogger.com,1999:blog-79536162387991558572024-02-19T11:38:46.469+05:30Chemistry for CompetitionsShilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.comBlogger10125tag:blogger.com,1999:blog-7953616238799155857.post-68246056061411772802011-08-02T12:54:00.000+05:302011-08-02T12:54:20.498+05:30Regulation of carotenoid synthesis and accumulation in plants<div dir="ltr" style="text-align: left;" trbidi="on">INTRODUCTION<br />
The amounts and identities of the various carotenoids in the photosynthetic membranes of green plants<br />
are relatively well conserved. A handful, typically including lutein, β-carotene, violaxanthin, neoxanthin,<br />
and zeaxanthin, account for most of the carotenoid pigment in the chloroplasts of many plant and<br />
algal species [1]. The biosynthesis and accumulation of these carotenoids in developing chloroplasts<br />
proceed in concert with the assembly of the light-harvesting antennae and reaction centers with which<br />
these pigments are in large part associated [2]. Genetic modifications that reduce or prevent synthesis<br />
of one or more of these carotenoids may be compensated by increases in others so that the total<br />
carotenoid content in the photosynthetic membranes is not much affected [3–7]. Such observations<br />
make clear that robust feedback mechanisms exert control over carotenoid synthesis and accumulation<br />
in plant chloroplasts.<br />
Carotenoid pigmentation in non-green plant plastids, in contrast, ranges broadly both in quantity<br />
and composition. The total amount of the carotenoids may vary from little or none (as in white flower<br />
petals) to quite substantial quantities (as in the dark orange petals of certain marigold varieties). The<br />
pigments may include those common in the photosynthetic membranes (e.g., the lutein of marigold<br />
flower petals), consist of earlier pathway intermediates (e.g., lycopene in red tomato fruits), or be<br />
derived from carotenoids normally found in the chloroplasts (e.g., capsanthin and capsorubin, formed<br />
from violaxanthin in red pepper fruits).<br />
What mechanisms are employed by plants to specify and adjust the amounts and identities of the<br />
various carotenoids that are accumulated in green and non-green plastids? The answer to this question<br />
has many parts, and much remains to be learned. There is abundant evidence to indicate that the reaction<br />
catalyzed by phytoene synthase (PSY) is an important control point for regulation of flux into and<br />
through the carotenoid pathway [8]. This reaction will not be discussed here. Instead, following a brief<br />
update on genes and enzymes of the pathway, I will review what has been learned recently regarding<br />
two other likely control points of the carotenoid pathway in plants: the availability of substrate and<br />
branching of the pathway.<br />
<br />
Table 1 Carotenoid pathway genes in Arabidopsis thaliana.<br />
Gene Enzyme Family Members*<br />
Ipi isopentenyl diphosphate isomerase 2<br />
Ggps geranylgeranyl diphosphate synthase 11<br />
Psy phytoene synthase 1<br />
Pds phytoene desaturase 1<br />
Zds ζ-carotene desaturase 1<br />
CrtISO carotene isomerase 1<br />
Ptox plastid terminal oxidase 1<br />
Lcy-b lycopene β-cyclase 1<br />
Lcy-e lycopene ε-cyclase 1<br />
Chy-b β-ring hydroxylase 2<br />
Chy-e ε-ring hydroxylase not identified<br />
Zep zeaxanthin epoxidase 1<br />
Vde violaxanthin de-epoxidase 1<br />
Nsy neoxanthin synthase no ortholog<br />
<br />
SUPPLY OF SUBSTRATES (IPP AND DMAPP) FOR CAROTENOID BIOSYNTHESIS<br />
Carotenoids are isoprenoids. The five carbon building blocks that serve as precursors for the synthesis<br />
of carotenoids and other isoprenoid compounds, isopentenyl diphosphate (IPP) and dimethylallyl<br />
diphosphate (DMAPP), are produced in two different compartments and by two different pathways in<br />
plant cells (Fig. 1). The well-known mevalonate (MVA) pathway in the cytosol/endoplasmic reticulum<br />
begins with acetyl-CoA and proceeds in linear fashion to IPP, which is then reversibly converted to<br />
DMAPP in a reaction catalyzed by IPP isomerase (IPI) [15].<br />
The recently recognized methylerythritol (MEP) pathway occurs in plant plastids, in cyanobacteria,<br />
and in certain other bacteria [16; see 17 for a recent update on this incompletely elucidated pathway],<br />
and utilizes pyruvate and glyceraldehyde-3-phosphate (GAP) as the initial substrates (Fig. 1). In<br />
contrast to the MVA pathway, DMAPP and IPP are produced separately via a branching of the MEP<br />
pathway [18]. Even so, IPP isomerase, the enzyme that serves as the terminal enzyme of the cytosolic<br />
MVA pathway (Fig. 1), is also present in plastids [19,20].<br />
Carotenoids in plants are synthesized in the plastids. Are the IPP and DMAPP utilized for<br />
carotenoid synthesis produced solely via the plastid MEP pathway or does the cytosolic MVA pathway<br />
also contribute? Does the source of IPP/DMAPP for plastid isoprenoid synthesis depend on the stage<br />
of development, the type of tissue, or the type of plastid (e.g., etioplast, chloroplast, chromoplast, or<br />
amyloplast)?<br />
Although there are some indications of compartmental “crosstalk”, isoprenoid synthesis in both<br />
green and non-green plastids of many plants has been found to rely primarily on IPP and DMAPP produced<br />
via the MEP pathway. Much of the evidence in support of an MEP pathway origin for plastid isoprenoids<br />
comes from analyses of the distribution of label in certain isoprenoid pathway end-products<br />
after incubation of plants or algae with 13C-labeled glucose or 13C-labeled 1-deoxy-D-xylulose (DOX)<br />
[reviewed in 21]. The effects of specific MVA and MEP pathway inhibitors and the phenotypic consequences<br />
of a mutation in an Arabidopsis gene encoding the MEP pathway enzyme deoxyxylulose-5-<br />
phosphate synthase (DXS) lend further support. The application of the MEP pathway inhibitor fosmidomycin<br />
(an inhibitor of deoxylulose-5-phosphate reductoisomerase, DXR, the first enzyme specific<br />
to the MEP pathway; see Fig. 1) reduces lycopene accumulation in tomato fruits [22], whereas the MVA<br />
© 2002 IUPAC, Pure and Applied Chemistry 74, 1409–1417<br />
Regulation of carotenoid synthesis and accumulation in plants 1411<br />
Fig. 1 Distinct isoprenoid pathways exist in plastids and cytosol of plant cells. Enzymes that may limit flux through<br />
the MEP pathway (the evidence is largely from studies of the bacterium E. coli) are in white text in black boxes.<br />
Abbreviations: diPG, diphosphoglyceraldehyde; GAP, glyceraldehyde-3-phosphate; GAPD, glyceraldehyde-3-<br />
phosphate dehydrogenase; HMG, hydroxymethylglutaryl; MEP, methylerythritol-5-phosphate; PEP, phosphoenolpyruvate.<br />
</div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com0tag:blogger.com,1999:blog-7953616238799155857.post-54678758187276205662011-08-02T12:42:00.000+05:302011-08-02T12:42:41.745+05:30Troponin I and troponin T<div dir="ltr" style="text-align: left;" trbidi="on"><ul><li><b>Troponin I and troponin T</b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/003503.htm">CPK</a> and <a href="http://www.nlm.nih.gov/medlineplus/ency/article/003504.htm">CPK-MB</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/003663.htm">Serum myoglobin</a></b></li>
</ul><b>If you had a heart attack, you will need to stay in the hospital, possibly in the intensive care unit (ICU). You will be hooked up to an ECG machine, so the health care team can look at how your heart is beating.</b><br />
<b>Life-threatening irregular heartbeats<span style="background-color: black;"> (</span><a href="http://www.nlm.nih.gov/medlineplus/ency/article/001101.htm" style="background-color: black;">arrhythmias</a><span style="background-color: black;">)</span> are the leading cause of death in the first few hours of a heart attack. These arrythmias may be treated with medications or electrical cardioverson/defibrillation.</b><br />
<b>The health care team will give you oxygen, even if your blood oxygen levels are normal. This is done so that your body tissues have easy access to oxygen and your heart doesn't have to work as hard.</b><br />
<b>An intravenous line (IV) will be placed into one of your veins. Medicines and fluids pass through this IV. You may need a tube inserted into your bladder (urinary catheter) so that doctors can see how much fluid your body removes.</b><br />
<b>ANGIOPLASTY AND STENT PLACEMENT</b><br />
<b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/002953.htm">Angioplasty</a>, also called percutaneous coronary intervention (PCI), is the preferred emergency procedure for opening the arteries for some types of heart attacks. It should preferably be performed within 90 minutes of arriving at the hospital and no later than 12 hours after a heart attack.</b><br />
<b>Angioplasty is a procedure to open narrowed or blocked blood vessels that supply blood to the heart.</b><br />
<b>A coronary artery stent is a small, metal mesh tube that opens up (expands) inside a coronary artery. A stent is often placed after angioplasty. It helps prevent the artery from closing up again. A drug eluting stent has medicine in it that helps prevent the artery from closing.</b><br />
<b>THROMBOLYTIC THERAPY (CLOT-BUSTING DRUGS)</b><br />
<b>Depending on the results of the ECG, certain patients may be given drugs to break up the clot. It is best if these drugs are given within 3 hours of when the patient first felt the chest pain. This is called thrombolytic therapy. The medicine is first given through an IV. Blood thinners taken by mouth may be prescribed later to prevent clots from forming.</b><br />
<b>Thrombolytic therapy is not appropriate for people who have:</b><br />
<ul><li><b>Bleeding inside their head (intracranial hemorrhage)</b></li>
<li><b>Brain abnormalities such as tumors or blood vessel malformations</b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000726.htm">Stroke</a> within the past 3 months (or possibly longer)</b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000028.htm">Head injury</a> within the past 3 months</b></li>
</ul><b>Thrombolytic therapy is extremely dangerous in women who are pregnant or in people who have:</b><br />
<ul><li><b>A history of using blood thinners such as coumadin</b></li>
<li><b>Had major surgery or a major injury within the past 3 weeks</b></li>
<li><b>Had internal bleeding within the past 2-4 weeks</b></li>
<li><b>Peptic ulcer disease</b></li>
<li><b>Severe high blood pressure</b></li>
</ul><b>OTHER MEDICINES FOR HEART ATTACKS</b><br />
<b>Many different medicines are used to treat and prevent heart attacks. Nitroglycerin helps reduce chest pain. You may also receive strong medicines to relieve pain.</b><br />
<b>Antiplatelet medicines help prevent clots from forming. Aspirin is an antiplatelet drug. Another one is clopidogrel (Plavix). Ask your doctor which of these drugs you should be taking. Always talk to your health care provider before stopping either of these drugs.</b><br />
<ul><li><b>For the first year after a heart attack, you will likely take both aspirin and clopidogrel every day. After that, your health care provider may only prescribe aspirin.</b></li>
<li><b>If you had angioplasty and a coronary stent placed after your heart attack, you may need to take clopidogrel with your aspirin for longer than one year.</b></li>
</ul><b>Other medications you may receive during or after a heart attack include:</b><br />
<ul><li><b>Beta-blockers (such as metoprolol, atenolol, and propranolol) help reduce the strain on the heart and lower blood pressure.</b></li>
<li><b>ACE inhibitors (such as ramipril, lisinopril, enalapril, or captopril) are used to prevent heart failure and lower blood pressure.</b></li>
<li><b>Lipid-lowering medications, especially statins (such as lovastatin, pravastatin, simvastatin, atorvastatin, and rosuvastatin) reduce blood cholesterol levels to prevent plaque from increasing. They may reduce the risk of another heart attack or death.</b></li>
</ul><b>Always talk to your health care provider before stopping any medications, especially these drugs. Stopping or changing the amount of these medicines can be life threatening.</b><br />
<b>CORONARY ARTERY BYPASS SURGERY</b><br />
<b>Coronary angiography may reveal severe coronary artery disease in many vessels, or a narrowing of the left main coronary artery (the vessel supplying most of the blood to the heart). In these circumstances, the cardiologist may recommend emergency coronary artery bypass surgery (<a href="http://www.nlm.nih.gov/medlineplus/ency/article/002946.htm">CABG</a>). This procedure is also called "open heart surgery." The surgeon takes either a vein or artery from another location in your body and uses it to bypass the blocked coronary artery.</b><br />
<h2 class="subheading"><b>Support Groups</b></h2><b>See:<a href="http://www.nlm.nih.gov/medlineplus/ency/article/002203.htm">Heart disease -- resources</a></b><br />
<h2 class="subheading"><b>Outlook (Prognosis)</b></h2><b>How well you do after a heart attack depends on the amount and location of damaged tissue. Your outcome is worse if the heart attack caused damage to the signaling system that tells the heart to contract.</b><br />
<b>About a third of heart attacks are deadly. If you live 2 hours after an attack, you are likely to survive, but you may have complications. Those who do not have complications may fully recover.</b><br />
<b>Usually a person who has had a heart attack can slowly go back to normal activities, including sexual activity.</b><br />
<h2 class="subheading"><b>Possible Complications</b></h2><ul><li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000185.htm">Cardiogenic shock</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000158.htm">Congestive heart failure</a></b></li>
<li><b>Damage extending past heart tissue (infarct extension), possibly leading to rupture of the heart</b></li>
<li><b>Damage to heart valves or the wall between the two sides of the heart</b></li>
<li><b>Inflammation around the lining of the heart (<a href="http://www.nlm.nih.gov/medlineplus/ency/article/000182.htm">pericarditis</a>)</b></li>
<li><b>Irregular heartbeats, including <a href="http://www.nlm.nih.gov/medlineplus/ency/article/000187.htm">ventricular tachycardia</a> and ventricular fibrillation</b></li>
<li><b>Blood clot in the lungs (pulmonary embolism)</b></li>
<li><b>Blood clot to the brain (<a href="http://www.nlm.nih.gov/medlineplus/ency/article/000726.htm">stroke</a>)</b></li>
<li><b>Side effects of drug treatment</b></li>
</ul><h2 class="subheading"><b>When to Contact a Medical Professional</b></h2><b>Immediately call your local emergency number (such as 911) if you have symptoms of a heart attack.</b><br />
<h2 class="subheading"><b>Prevention</b></h2><b>To prevent a heart attack:</b><br />
<ul><li><b>Keep your blood pressure, blood sugar, and cholesterol under control.</b></li>
<li><b>Don't smoke.</b></li>
<li><b>Consider drinking 1 to 2 glasses of alcohol or wine each day. Moderate amounts of alcohol may reduce your risk of cardiovascular problems. However, drinking larger amounts does more harm than good.</b></li>
<li><b>Eat a low-fat diet rich in fruits and vegetables and low in animal fat.</b></li>
<li><b>Eat fish twice a week. Baked or grilled fish is better than fried fish. Frying can destroy some of the health benefits.</b></li>
<li><b>Exercise daily or several times a week. Walking is a good form of exercise. Talk to your doctor before starting an exercise routine.</b></li>
<li><b>Lose weight if you are overweight.</b></li>
</ul><b>If you have one or more risk factors for heart disease, talk to your doctor about possibly taking aspirin to help prevent a heart attack. Aspirin therapy (75 mg to 325 mg a day) or another drug such as prasugrel or clopidogrel may be prescribed.</b><br />
<b>New guidelines no longer recommend hormone replacement therapy, vitamins E or C, antioxidants, or folic acid to prevent heart disease.</b><br />
<b>After a heart attack, you will need regular follow-up care to reduce the risk of having a second heart attack. Often, a cardiac rehabilitation program is recommended to help you gradually return to a normal lifestyle. Always follow the exercise, diet, and medication plan prescribed by your doctor.</b><br />
<h2 class="subheading"><b>Alternative Names</b></h2><b>Myocardial infarction; MI; Acute MI; ST-elevation myocardial infarction; non-ST-elevation myocardial infarction</b><br />
<b> et al. Acute ST-segment elevation myocardial infarction: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th edition). <em>Chest</em>. 2008;133:708S-775S.</b><h2 class="subheading"><b><br />
</b></h2><b><br />
</b><div class="ency_image_column"><div class="ency_rdbox"><div class="tab blue"><b><span class="bluespan">MedlinePlus Topics</span></b></div></div><div class="ency_rdbox_c"><div class="ency_rdbox_c_c"><ul><li><b><a href="http://www.nlm.nih.gov/medlineplus/heartattack.html">Heart Attack</a></b></li>
</ul></div></div><div class="ency_rdbox"><div class="tab blue"><b><span class="bluespan">Images</span></b></div></div><div class="ency_rdbox_c"><div class="ency_rdbox_c_c group" id="img-box"><div class="f_0"><b><img alt="Heart, section through the middle" class="tnail" height="60" src="http://www.nlm.nih.gov/medlineplus/ency/images/ency/tnails/1056t.jpg" title="Heart, section through the middle" width="60" /><a href="http://www.nlm.nih.gov/medlineplus/ency/imagepages/1056.htm">Heart, section through the middle</a></b></div><div class="f_1"><b><img alt="Heart, front view" class="tnail" height="60" src="http://www.nlm.nih.gov/medlineplus/ency/images/ency/tnails/1097t.jpg" title="Heart, front view" width="60" /><a href="http://www.nlm.nih.gov/medlineplus/ency/imagepages/1097.htm">Heart, front view</a></b></div><div class="f_0"><b><img alt="Acute MI" class="tnail" height="60" src="http://www.nlm.nih.gov/medlineplus/ency/images/ency/tnails/17004t.jpg" title="Acute MI" width="60" /><a href="http://www.nlm.nih.gov/medlineplus/ency/imagepages/17004.htm">Acute MI</a></b></div><div class="f_1"><b><img alt="Post myocardial infarction ECG wave tracings" class="tnail" height="60" src="http://www.nlm.nih.gov/medlineplus/ency/images/ency/tnails/18030t.jpg" title="Post myocardial infarction ECG wave tracings" width="60" /><a href="http://www.nlm.nih.gov/medlineplus/ency/imagepages/18030.htm">Post myocardial infarction ECG wave tracings</a></b></div><div class="f_0"><b><img alt="Progressive build-up of plaque in coronary artery" class="tnail" height="60" src="http://www.nlm.nih.gov/medlineplus/ency/images/ency/tnails/18031t.jpg" title="Progressive build-up of plaque in coronary artery" width="60" /><a href="http://www.nlm.nih.gov/medlineplus/ency/imagepages/18031.htm">Progressive build-up of plaque in coronary artery</a></b></div><div class="f_1"><b><img alt="Posterior heart arteries" class="tnail" height="60" src="http://www.nlm.nih.gov/medlineplus/ency/images/ency/tnails/18037t.jpg" title="Posterior heart arteries" width="60" /><a href="http://www.nlm.nih.gov/medlineplus/ency/imagepages/18037.htm">Posterior heart arteries</a></b></div><div class="f_0"><b><img alt="Anterior heart arteries" class="tnail" height="60" src="http://www.nlm.nih.gov/medlineplus/ency/images/ency/tnails/9367t.jpg" title="Anterior heart arteries" width="60" /><a href="http://www.nlm.nih.gov/medlineplus/ency/imagepages/9367.htm">Anterior heart arteries</a></b></div><div class="f_1"><b><img alt="Heart attack symptoms" class="tnail" height="60" src="http://www.nlm.nih.gov/medlineplus/ency/images/ency/tnails/9807t.jpg" title="Heart attack symptoms" width="60" /><a href="http://www.nlm.nih.gov/medlineplus/ency/imagepages/9807.htm">Heart attack symptoms</a></b></div></div></div><div class="ency_rdbox"><div class="tab blue"><b><span class="bluespan">Read More</span></b></div></div><div class="ency_rdbox_c"><div class="ency_rdbox_c_c"><ul><li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/001101.htm">Arrhythmias</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000171.htm">Atherosclerosis</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000185.htm">Cardiogenic shock</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/001214.htm">Diabetes</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/002468.htm">Fat</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000158.htm">Heart failure</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000468.htm">Hypertension</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/007370.htm">Implantable cardioverter-defibrillator</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/007262.htm">Lipoprotein-a</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/002032.htm">Making the decision to quit tobacco</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/007290.htm">Metabolic syndrome</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000182.htm">Pericarditis</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/003211.htm">Stress and anxiety</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/article/000187.htm">Ventricular tachycardia</a></b></li>
</ul></div></div><div class="ency_rdbox"><div class="tab blue"><b><span class="bluespan">Patient Instructions</span></b></div></div><div class="ency_rdbox_c"><div class="ency_rdbox_c_c"><ul><li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000091.htm">Angioplasty and stent - heart - discharge </a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000314.htm">Cholesterol - drug treatment</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000211.htm">Cholesterol - what to ask your doctor </a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000090.htm">Heart attack - discharge</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000231.htm">Heart attack - what to ask your doctor</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000224.htm">Heart failure - what to ask your doctor </a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000226.htm">High blood pressure - what to ask your doctor </a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000292.htm">Taking warfarin (Coumadin)</a></b></li>
<li><b><a href="http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000255.htm">Taking warfarin (Coumadin) - what to ask your doctor </a></b></li>
</ul></div></div></div></div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com0tag:blogger.com,1999:blog-7953616238799155857.post-18886135636410298022011-06-18T09:28:00.000+05:302011-06-18T09:28:32.697+05:30Nuclear magnetic resonance spectroscopy<div dir="ltr" style="text-align: left;" trbidi="on"><form><a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr1"></a> <br />
<center><h2><span style="color: maroon;">Nuclear Magnetic Resonance Spectroscopy</span></h2></center> <b><span style="color: maroon; font-size: large;"> 1. Background</span></b><br />
Over the past fifty years nuclear magnetic resonance spectroscopy, commonly referred to as nmr, has become the preeminent technique for determining the structure of organic compounds. Of all the spectroscopic methods, it is the only one for which a complete analysis and interpretation of the entire spectrum is normally expected. Although larger amounts of sample are needed than for mass spectroscopy, nmr is non-destructive, and with modern instruments good data may be obtained from samples weighing less than a milligram. <b>To be successful in using nmr as an analytical tool, it is necessary to understand the physical principles on which the methods are based</b>.<br />
The nuclei of many elemental isotopes have a characteristic spin (<b>I</b>). Some nuclei have integral spins (e.g. I = 1, 2, 3 ....), some have fractional spins (e.g. I = 1/2, 3/2, 5/2 ....), and a few have no spin, I = 0 (e.g. <sup>12</sup>C, <sup>16</sup>O, <sup>32</sup>S, ....). Isotopes of particular interest and use to organic chemists are <sup>1</sup>H, <sup>13</sup>C, <sup>19</sup>F and <sup>31</sup>P, all of which have I = 1/2. Since the analysis of this spin state is fairly straightforward, our discussion of nmr will be limited to these and other I = 1/2 nuclei. <br />
<br />
<table align="center" border="1" cellpadding="6"><tbody>
<tr bgcolor="eeffdd"><th>For a table of nuclear spin characteristics <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr2.htm#nmr11">Click Here</a>.</th></tr>
</tbody></table><br />
<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr1b"></a> <b><span style="color: navy;">The following features lead to the nmr phenomenon:</span></b><br />
<br />
<center><table cellpadding="6" style="width: 760px;"><tbody>
<tr><td width="400"><b>1.</b> A spinning charge generates a magnetic field, as shown by the animation on the right.<br />
The resulting spin-magnet has a magnetic moment (<b>μ</b>) proportional to the spin.</td> <td valign="top"></td></tr>
<tr><td><div class="separator" style="clear: both; text-align: center;"><a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/nucspin1.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/nucspin1.gif" /></a></div><div class="separator" style="clear: both; text-align: center;"><a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/nucspin2.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/nucspin2.gif" /></a></div><br />
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<b>2.</b> In the presence of an external magnetic field (<b>B<sub>0</sub></b>), two spin states exist, <b>+1/2</b> and <b>-1/2</b>.<br />
The magnetic moment of the lower energy +1/2 state is aligned with the external field, but that of the higher energy -1/2 spin state is opposed to the external field. Note that the arrow representing the external field points North.</td> <td valign="top"></td></tr>
<tr><td colspan="2"><b>3.</b> The difference in energy between the two spin states is dependent on the external magnetic field strength, and is always very small. The following diagram illustrates that the two spin states have the same energy when the external field is zero, but diverge as the field increases. At a field equal to B<sub>x</sub> a formula for the energy difference is given (remember I = 1/2 and μ is the magnetic moment of the nucleus in the field).</td></tr>
<tr valign="top"><th colspan="2"><a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" name="enrg" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/enrgdia1.gif" /></a></th></tr>
<tr><td colspan="2">Strong magnetic fields are necessary for nmr spectroscopy. The international unit for magnetic flux is the tesla (<b>T</b>). The earth's magnetic field is not constant, but is approximately 10<sup>-4</sup> T at ground level. Modern nmr spectrometers use powerful magnets having fields of 1 to 20 T. Even with these high fields, the energy difference between the two spin states is less than 0.1 cal/mole. To put this in perspective, recall that infrared transitions involve 1 to 10 kcal/mole and electronic transitions are nearly 100 time greater.<br />
For nmr purposes, this small energy difference (ΔE) is usually given as a frequency in units of MHz (10<sup>6</sup> Hz), ranging from 20 to 900 Mz, depending on the magnetic field strength and the specific nucleus being studied. Irradiation of a sample with radio frequency (rf) energy corresponding exactly to the spin state separation of a specific set of nuclei will cause excitation of those nuclei in the +1/2 state to the higher -1/2 spin state. Note that this electromagnetic radiation falls in the <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UV-Vis/spectrum.htm#uv2">radio and television broadcast spectrum</a>. Nmr spectroscopy is therefore the energetically mildest probe used to examine the structure of molecules. <br />
The nucleus of a hydrogen atom (the proton) has a magnetic moment μ = 2.7927, and has been studied more than any other nucleus. <span style="color: brown;">The previous diagram may be changed to display energy differences for the proton spin states (as frequencies) by mouse clicking anywhere within it</span>. </td></tr>
<tr><td colspan="2"><b>4.</b> For spin 1/2 nuclei the energy difference between the two spin states at a given magnetic field strength will be proportional to their magnetic moments. For the four common nuclei noted above, the magnetic moments are: <sup>1</sup>H μ = 2.7927, <sup>19</sup>F μ = 2.6273, <sup>31</sup>P μ = 1.1305 & <sup>13</sup>C μ = 0.7022. These moments are in nuclear magnetons, which are 5.05078•10<sup>-27</sup> JT<sup>-1</sup>. The following diagram gives the approximate frequencies that correspond to the spin state energy separations for each of these nuclei in an external magnetic field of 2.35 T. The formula in the colored box shows the direct correlation of frequency (energy difference) with magnetic moment (h = Planck's constant = 6.626069•10<sup>-34</sup> Js).</td></tr>
<tr valign="top"><th colspan="2"><a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/nucfreq1.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/nucfreq1.gif" /></a></th></tr>
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<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr2"></a> <b><span style="color: maroon; font-size: large;"> 2. Proton NMR Spectroscopy</span></b><br />
This important and well-established application of nuclear magnetic resonance will serve to illustrate some of the novel aspects of this method. To begin with, the nmr spectrometer must be tuned to a specific nucleus, in this case the proton. The actual procedure for obtaining the spectrum varies, but the simplest is referred to as the <b>continuous wave</b> (CW) method. A typical CW-spectrometer is shown in the following diagram. A solution of the sample in a uniform 5 mm glass tube is oriented between the poles of a powerful magnet, and is spun to average any magnetic field variations, as well as tube imperfections. Radio frequency radiation of appropriate energy is broadcast into the sample from an antenna coil (colored red). A receiver coil surrounds the sample tube, and emission of absorbed rf energy is monitored by dedicated electronic devices and a computer. An nmr spectrum is acquired by varying or sweeping the magnetic field over a small range while observing the rf signal from the sample. An equally effective technique is to vary the frequency of the rf radiation while holding the external field constant.<br />
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<table align="center" border="1" cellpadding="6"><tbody>
<tr bgcolor="eeffdd"><th>For a description of the pulse Fourier transform technique, preferred by most spectroscopists over the older CW method, <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr2.htm#pulse">Click Here</a>.</th></tr>
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<div align="center"><a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/spctrmtr.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/spctrmtr.gif" /></a></div><br />
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As an example, consider a sample of water in a 2.3487 T external magnetic field, irradiated by 100 MHz radiation. If the magnetic field is smoothly increased to 2.3488 T, the hydrogen nuclei of the water molecules will at some point absorb rf energy and a resonance signal will appear. An animation showing this may be activated by clicking the <b>Show Field Sweep</b> button. The field sweep will be repeated three times, and the resulting resonance trace is colored red. For visibility, the water proton signal displayed in the animation is much broader than it would be in an actual experiment.<br />
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<div align="center"><img height="115" name="sweep" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/sweep1.gif" width="640" /></div><br />
<center> </center> <a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr2b"></a> Since protons all have the same magnetic moment, we might expect all hydrogen atoms to give resonance signals at the same field / frequency values. Fortunately for chemistry applications, this is not true. By clicking the <b>Show Different Protons</b> button under the diagram, a number of representative proton signals will be displayed over the same magnetic field range. It is not possible, of course, to examine isolated protons in the spectrometer described above; but from independent measurement and calculation it has been determined that a naked proton would resonate at a lower field strength than the nuclei of covalently bonded hydrogens. With the exception of water, chloroform and sulfuric acid, which are examined as liquids, all the other compounds are measured as gases.<br />
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<b>Why should the proton nuclei in different compounds behave differently in the nmr experiment ?</b> <br />
The answer to this question lies with the electron(s) surrounding the proton in covalent compounds and ions. Since electrons are charged particles, they move in response to the external magnetic field (B<sub>o</sub>) so as to generate a secondary field that opposes the much stronger applied field. This secondary field <b>shields</b> the nucleus from the applied field, so B<sub>o</sub> must be increased in order to achieve resonance (absorption of rf energy). As illustrated in the drawing on the right, B<sub>o</sub> must be increased to compensate for the induced shielding field. In the upper diagram, those compounds that give resonance signals at the higher field side of the diagram (CH<sub>4</sub>, HCl, HBr and HI) have proton nuclei that are more shielded than those on the lower field (left) side of the diagram.<br />
<div class="separator" style="clear: both; text-align: center;"><a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/shield1.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img align="right" border="0" hspace="7" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/shield1.gif" vspace="7" /></a></div><br />
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The magnetic field range displayed in the above diagram is very small compared with the actual field strength (only about 0.0042%). It is customary to refer to small increments such as this in units of <b>parts per million</b> (ppm). The difference between 2.3487 T and 2.3488 T is therefore about 42 ppm. Instead of designating a range of nmr signals in terms of magnetic field differences (as above), it is more common to use a frequency scale, even though the spectrometer may operate by sweeping the magnetic field. Using this terminology, we would find that at 2.34 T the proton signals shown above extend over a 4,200 Hz range (for a 100 MHz rf frequency, 42 ppm is 4,200 Hz). Most organic compounds exhibit proton resonances that fall within a 12 ppm range (the shaded area), and it is therefore necessary to use very sensitive and precise spectrometers to resolve structurally distinct sets of hydrogen atoms within this narrow range. <span style="color: green;">In this respect it might be noted that the detection of a part-per-million difference is equivalent to detecting a 1 millimeter difference in distances of 1 kilometer</span>. <br />
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<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr3"></a> <br />
<dir><h4><span style="color: maroon;">Chemical Shift</span></h4></dir> Unlike infrared and uv-visible spectroscopy, where absorption peaks are uniquely located by a frequency or wavelength, the location of different nmr resonance signals is dependent on both the external magnetic field strength and the rf frequency. Since no two magnets will have exactly the same field, resonance frequencies will vary accordingly and an alternative method for characterizing and specifying the location of nmr signals is needed. This problem is illustrated by the eleven different compounds shown in the following diagram. Although the eleven resonance signals are distinct and well separated, an unambiguous numerical locator cannot be directly assigned to each.<br />
<div align="center"><a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="222" name="shift" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/nmrtotl1.gif" width="400" /></a></div><br />
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One method of solving this problem is to report the location of an nmr signal in a spectrum relative to a reference signal from a standard compound added to the sample. Such a reference standard should be chemically unreactive, and easily removed from the sample after the measurement. Also, it should give a single sharp nmr signal that does not interfere with the resonances normally observed for organic compounds. <b>Tetramethylsilane</b>, (CH<sub>3</sub>)<sub>4</sub>Si, usually referred to as <b>TMS</b>, meets all these characteristics, and has become the reference compound of choice for proton and carbon nmr.<br />
Since the separation (or dispersion) of nmr signals is magnetic field dependent, one additional step must be taken in order to provide an unambiguous location unit. <span style="color: brown;">This is illustrated for the acetone, methylene chloride and benzene signals by clicking on the previous diagram</span>. To correct these frequency differences for their field dependence, we divide them by the spectrometer frequency (100 or 500 MHz in the example), <span style="color: brown;">as shown in a new display by again clicking on the diagram</span>. The resulting number would be very small, since we are dividing Hz by MHz, so it is multiplied by a million, as shown by the formula in the blue shaded box. Note that ν<sub>ref</sub> is the resonant frequency of the reference signal and ν<sub>samp</sub> is the frequency of the sample signal. This operation gives a locator number called the <b>Chemical Shift</b>, having units of parts-per-million (ppm), and designated by the symbol <b>δ</b> <span style="color: brown;">Chemical shifts for all the compounds in the original display will be presented by a third click on the diagram</span>.<br />
The compounds referred to above share two common characteristics:<br />
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<dir> <b>• </b> The hydrogen atoms in a given molecule are all <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/suppmnt1.htm#nom1">structurally equivalent</a>, averaged for fast conformational equilibria. <br />
<b>• </b> The compounds are all liquids, save for neopentane which boils at 9 °C and is a liquid in an ice bath.</dir> The first feature assures that each compound gives a single sharp resonance signal. The second allows the pure (neat) substance to be poured into a sample tube and examined in a nmr spectrometer. In order to take the nmr spectra of a solid, it is usually necessary to dissolve it in a suitable solvent. Early studies used carbon tetrachloride for this purpose, since it has no hydrogen that could introduce an interfering signal. Unfortunately, CCl<sub>4</sub> is a poor solvent for many polar compounds and is also toxic. Deuterium labeled compounds, such as deuterium oxide (D<sub>2</sub>O), chloroform-d (DCCl<sub>3</sub>), benzene-d<sub>6</sub> (C<sub>6</sub>D<sub>6</sub>), acetone-d<sub>6</sub> (CD<sub>3</sub>COCD<sub>3</sub>) and DMSO-d<sub>6</sub> (CD<sub>3</sub>SOCD<sub>3</sub>) are now widely used as nmr solvents. Since the deuterium isotope of hydrogen has a different magnetic moment and spin, it is invisible in a spectrometer tuned to protons.<table align="center" border="1" cellpadding="6"><tbody>
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From the previous discussion and examples we may deduce that one factor contributing to chemical shift differences in proton resonance is the <b>inductive effect</b>. If the electron density about a proton nucleus is relatively high, the induced field due to electron motions will be stronger than if the electron density is relatively low. The shielding effect in such high electron density cases will therefore be larger, and a higher external field (B<sub>o</sub>) will be needed for the rf energy to excite the nuclear spin. Since silicon is less electronegative than carbon, the electron density about the methyl hydrogens in tetramethylsilane is expected to be greater than the electron density about the methyl hydrogens in neopentane (2,2-dimethylpropane), and the characteristic resonance signal from the silane derivative does indeed lie at a higher magnetic field. Such nuclei are said to be <b>shielded</b>. Elements that are more electronegative than carbon should exert an opposite effect (reduce the electron density); and, as the data in the following tables show, methyl groups bonded to such elements display lower field signals (they are <b>deshielded</b>). The deshielding effect of electron withdrawing groups is roughly proportional to their electronegativity, as shown by the left table. Furthermore, if more than one such group is present, the deshielding is additive (table on the right), and proton resonance is shifted even further downfield. <br />
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<table align="center" cellpadding="9"><tbody>
<tr valign="top"> <th nowrap="nowrap"><h4>Proton Chemical Shifts of Methyl Derivatives</h4><table border="1" cellpadding="3" style="width: 360px;"><tbody>
<tr bgcolor="ddeeff"><th nowrap="nowrap">Compound</th> <th nowrap="nowrap">(C<span style="color: red;">H</span><sub>3</sub>)<sub>4</sub>C</th> <th nowrap="nowrap">(C<span style="color: red;">H</span><sub>3</sub>)<sub>3</sub>N</th> <th nowrap="nowrap">(C<span style="color: red;">H</span><sub>3</sub>)<sub>2</sub>O</th> <th nowrap="nowrap">C<span style="color: red;">H</span><sub>3</sub>F</th></tr>
<tr align="center"><th>δ</th><td>0.9</td><td>2.1</td><td>3.2</td><td>4.1</td></tr>
<tr bgcolor="ddeeff"><th>Compound</th><th>(C<span style="color: red;">H</span><sub>3</sub>)<sub>4</sub>Si</th> <th>(C<span style="color: red;">H</span><sub>3</sub>)<sub>3</sub>P</th> <th>(C<span style="color: red;">H</span><sub>3</sub>)<sub>2</sub>S</th> <th>C<span style="color: red;">H</span><sub>3</sub>Cl</th></tr>
<tr align="center"><th>δ</th><td>0.0</td><td>0.9</td><td>2.1</td><td>3.0 </td></tr>
</tbody></table></th> <th width="30"></th> <th nowrap="nowrap"><h4> Proton Chemical Shifts (ppm)</h4><table border="1"><tbody>
<tr align="CENTER" bgcolor="ddeeff"><th bgcolor="FFCC99" nowrap="nowrap" width="80">Cpd. / Sub.</th> <th width="50">X=Cl</th> <th width="50">X=Br</th> <th width="50">X=I</th> <th width="50">X=OR</th> <th width="50">X=SR</th></tr>
<tr align="CENTER"><td bgcolor="ddeeff"><b>C<span style="color: red;">H</span><sub>3</sub>X</b></td> <td>3.0</td> <td>2.7</td> <td>2.1</td> <td>3.1</td> <td>2.1</td></tr>
<tr align="CENTER"><td bgcolor="ddeeff"><b>C<span style="color: red;">H</span><sub>2</sub>X<sub>2</sub></b></td> <td>5.3</td> <td>5.0</td> <td>3.9</td> <td>4.4</td> <td>3.7</td></tr>
<tr align="CENTER"><td bgcolor="ddeeff"><b>C<span style="color: red;">H</span>X<sub>3</sub></b></td> <td>7.3</td> <td>6.8</td> <td>4.9</td> <td>5.0</td> <td></td></tr>
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<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr3bb"></a> The general distribution of proton chemical shifts associated with different functional groups is summarized in the following chart. Bear in mind that these ranges are approximate, and may not encompass all compounds of a given class. Note also that the ranges specified for OH and NH protons (colored orange) are wider than those for most CH protons. This is due to hydrogen bonding variations at different sample concentrations.<br />
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<tr><th colspan="3"><h3>Proton Chemical Shift Ranges*</h3></th></tr>
<tr><th><span style="color: purple;">Low Field<br />
Region</span></th><td><div align="center"><div style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img height="187" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/hnmr1.gif" width="400" /></div></div></td> <th><span style="color: purple;">High Field<br />
Region</span></th></tr>
<tr><td></td><td> <b>*</b> For samples in CDCl<sub>3</sub> solution. The δ scale is relative to TMS at δ = 0.</td><td></td></tr>
</tbody></table></center> <span style="color: green;"></span><br />
<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr3c"></a><br />
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<dir><h4><span style="color: maroon;">Signal Strength</span></h4></dir> The magnitude or intensity of nmr resonance signals is displayed along the vertical axis of a spectrum, and is proportional to the molar concentration of the sample. Thus, a small or dilute sample will give a weak signal, and doubling or tripling the sample concentration increases the signal strength proportionally. If we take the nmr spectrum of equal molar amounts of benzene and cyclohexane in carbon tetrachloride solution, the resonance signal from cyclohexane will be twice as intense as that from benzene because cyclohexane has twice as many hydrogens per molecule. This is an important relationship when samples incorporating two or more different sets of hydrogen atoms are examined, since it allows the ratio of hydrogen atoms in each distinct set to be determined. To this end it is necessary to measure the relative strength as well as the chemical shift of the resonance signals that comprise an nmr spectrum. Two common methods of displaying the integrated intensities associated with a spectrum are illustrated by the following examples. In the three spectra in the top row, a horizontal integrator trace (light green) rises as it crosses each signal by a distance proportional to the signal strength. Alternatively, an arbitrary number, selected by the instrument's computer to reflect the signal strength, is printed below each resonance peak, as shown in the three spectra in the lower row. From the relative intensities shown here, together with the previously noted chemical shift correlations, the reader should be able to assign the signals in these spectra to the set of hydrogens that generates each. <span style="color: brown;">If you click on one of the spectrum signals (colored red) or on hydrogen atom(s) in the structural formulas the spectrum will be enlarged and the relationship will be colored blue</span>.<br />
<b>Hint:</b> When evaluating relative signal strengths, it is useful to set the smallest integration to unity and convert the other values proportionally.<br />
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<dir><h4><span style="color: maroon;">Hydroxyl Proton Exchange and the Influence of Hydrogen Bonding</span></h4></dir> The last two compounds in the lower row are alcohols. The OH proton signal is seen at 2.37 δ in 2-methyl-3-butyne-2-ol, and at 3.87 δ in 4-hydroxy-4-methyl-2-pentanone, illustrating the wide range over which this chemical shift may be found. A six-membered ring intramolecular hydrogen bond in the latter compound is in part responsible for its low field shift, and will be shown by clicking on the hydroxyl proton. We can take advantage of <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alcohol1.htm#alcrx1">rapid OH exchange</a> with the deuterium of heavy water to assign hydroxyl proton resonance signals . As shown in the following equation, this removes the hydroxyl proton from the sample and its resonance signal in the nmr spectrum disappears. Experimentally, one simply adds a drop of heavy water to a chloroform-d solution of the compound and runs the spectrum again. The result of this exchange is displayed below. <br />
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<center><br />
<table cellpadding="6"><tbody>
<tr align="center"><td>R-O-<span style="color: blue;">H</span> + <span style="color: magenta;">D</span><sub>2</sub>O <img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Images/arroweq3.gif" /> R-O-<span style="color: magenta;">D</span> + <span style="color: magenta;">D</span>-O-<span style="color: blue;">H</span><br />
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<tr><td><img height="130" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/deutalc1.gif" width="400" /></td></tr>
</tbody></table></center> <b>Hydrogen bonding shifts the resonance signal of a proton to lower field ( higher frequency ).</b> Numerous experimental observations support this statement, and a few of these will be described here.<br />
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<table align="right"><tbody>
<tr><td><b>i) </b>The chemical shift of the hydroxyl hydrogen of an alcohol varies with concentration. Very dilute solutions of 2-methyl-2-propanol, (CH<sub>3</sub>)<sub>3</sub>COH, in carbon tetrachloride solution display a hydroxyl resonance signal having a relatively high-field chemical shift (< 1.0 δ ). In concentrated solution this signal shifts to a lower field, usually near 2.5 δ. </td></tr>
<tr><td><b>ii) </b>The more acidic hydroxyl group of phenol generates a lower-field resonance signal, which shows a similar concentration dependence to that of alcohols. OH resonance signals for different percent concentrations of phenol in chloroform-d are shown in the following diagram (C-H signals are not shown).</td></tr>
<tr valign="top"><th><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/phenol.gif" /></th></tr>
<tr><td><b>iii) </b>Because of their favored hydrogen-bonded dimeric association, the hydroxyl proton of carboxylic acids displays a resonance signal significantly down-field of other functions. For a typical acid it appears from 10.0 to 13.0 δ and is often broader than other signals. The spectra shown below for chloroacetic acid (left) and 3,5-dimethylbenzoic acid (right) are examples.</td></tr>
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<tr><td align="left"><img height="252" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/clacetac.gif" width="320" /></td><td align="right"><img align="right" height="253" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/2mebzacd.gif" width="320" /></td></tr>
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<tr><td><b>iv) </b>Intramolecular hydrogen bonds, especially those defining a six-membered ring, generally display a very low-field proton resonance. The case of 4-hydroxypent-3-ene-2-one (the enol tautomer of 2,4-pentanedione) not only illustrates this characteristic, but also provides an instructive example of the sensitivity of the nmr experiment to dynamic change. In the nmr spectrum of the pure liquid, sharp signals from both the keto and enol tautomers are seen, their mole ratio being 4 <b>:</b> 21 (keto tautomer signals are colored purple). Chemical shift assignments for these signals are shown in the shaded box above the spectrum. The chemical shift of the hydrogen-bonded hydroxyl proton is δ 14.5, exceptionally downfield. We conclude, therefore, that the rate at which these tautomers interconvert is slow compared with the inherent time scale of nmr spectroscopy.</td></tr>
<tr valign="top"><th><img height="192" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/acac2.gif" width="400" /></th></tr>
<tr><td>Two structurally equivalent structures may be drawn for the enol tautomer (in magenta brackets). If these enols were slow to interconvert, we would expect to see two methyl resonance signals associated with each, one from the allylic methyl and one from the methyl ketone. Since only one strong methyl signal is observed, we must conclude that the interconversion of the enols is very fast-so fast that the nmr experiment detects only a single time-averaged methyl group (50% α-keto and 50% allyl).</td></tr>
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Although hydroxyl protons have been the focus of this discussion, it should be noted that corresponding N-H groups in amines and amides also exhibit hydrogen bonding nmr shifts, although to a lesser degree. Furthermore, OH and NH groups can undergo rapid proton exchange with each other; so if two or more such groups are present in a molecule, the nmr spectrum will show a single signal at an average chemical shift. For example, 2-hydroxy-2-methylpropanoic acid, (CH<sub>3</sub>)<sub>2</sub>C(OH)CO<sub>2</sub>H, displays a strong methyl signal at δ 1.5 and a 1/3 weaker and broader OH signal at δ 7.3 ppm. Note that the average of the expected carboxylic acid signal (ca. 12 ) and the alcohol signal (ca. 2 ) is 7. Rapid exchange of these hydrogens with heavy water, as noted above, would cause the low field signal to disappear.<br />
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<b>π-Electron Functions </b><br />
An examination of the proton chemical shift chart (<a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr1.htm#nmr3bb">above</a>) makes it clear that the inductive effect of substituents cannot account for all the differences in proton signals. In particular the low field resonance of hydrogens bonded to double bond or aromatic ring carbons is puzzling, as is the very low field signal from aldehyde hydrogens. The hydrogen atom of a terminal alkyne, in contrast, appears at a relatively higher field. All these anomalous cases seem to involve hydrogens bonded to pi-electron systems, and an explanation may be found in the way these pi-electrons interact with the applied magnetic field.<br />
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Pi-electrons are more polarizable than are sigma-bond electrons, as addition reactions of electrophilic reagents to alkenes testify. Therefore, we should not be surprised to find that field induced pi-electron movement produces strong secondary fields that perturb nearby nuclei. The pi-electrons associated with a benzene ring provide a striking example of this phenomenon, as shown below. The electron cloud above and below the plane of the ring circulates in reaction to the external field so as to generate an opposing field at the center of the ring and a supporting field at the edge of the ring. This kind of spatial variation is called <b>anisotropy</b>, and it is common to nonspherical distributions of electrons, as are found in all the functions mentioned above. Regions in which the induced field supports or adds to the external field are said to be <b>deshielded</b>, because a slightly weaker external field will bring about resonance for nuclei in such areas. However, regions in which the induced field opposes the external field are termed <b>shielded</b> because an increase in the applied field is needed for resonance. Shielded regions are designated by a <b>plus sign</b>, and deshielded regions by a <b>negative sign</b>. <br />
<span style="color: brown;">The anisotropy of some important unsaturated functions will be displayed by clicking on the benzene diagram below</span>. Note that the anisotropy about the triple bond nicely accounts for the relatively high field chemical shift of ethynyl hydrogens. The shielding & deshielding regions about the carbonyl group have been described in two ways, which alternate in the display.<br />
<div align="center"><a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857"><img border="0" height="215" name="anisot" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/benzene.gif" width="320" /></a></div><table align="center" border="1" cellpadding="6"><tbody>
<tr bgcolor="eeffdd"><th nowrap="nowrap">For additional examples of chemical shift variation near strongly anisotropic groups Click Here.</th></tr>
</tbody></table>Sigma bonding electrons also have a less pronounced, but observable, anisotropic influence on nearby nuclei. This is seen in the small deshielding shift that occurs in the series C<span style="color: red;">H</span><sub>3</sub>–R, R–C<span style="color: red;">H</span><sub>2</sub>–R, R<sub>3</sub>C<span style="color: red;">H</span>; as well as the deshielding of equatorial versus axial protons on a fixed cyclohexane ring.<br />
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<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr3f"></a> <br />
<dir><h4><span style="color: maroon;">Solvent Effects</span></h4></dir> Chloroform-d (CDCl<sub>3</sub>) is the most common solvent for nmr measurements, thanks to its good solubilizing character and relative unreactive nature ( except for 1º and 2º-amines). As noted earlier, other deuterium labeled compounds, such as deuterium oxide (D<sub>2</sub>O), benzene-d6 (C<sub>6</sub>D<sub>6</sub>), acetone-d6 (CD<sub>3</sub>COCD<sub>3</sub>) and DMSO-d6 (CD<sub>3</sub>SOCD<sub>3</sub>) are also available for use as nmr solvents. Because some of these solvents have π-electron functions and/or may serve as hydrogen bonding partners, the chemical shifts of different groups of protons may change depending on the solvent being used. The following table gives a few examples, obtained with dilute solutions at 300 MHz.<br />
<table align="center" border="1" style="width: 680px;"><caption><b><span style="font-size: medium;">Some Typical <sup>1</sup>H Chemical Shifts (δ values) in Selected Solvents </span></b></caption> <tbody>
<tr bgcolor="bbddff"><td bgcolor="ffffff"><table><tbody>
<tr bgcolor="bbddff"><th><span style="font-size: medium;">Solvent</span></th></tr>
<tr bgcolor="ffeecc"><th><span style="font-size: medium;">Compound</span></th></tr>
</tbody></table></td> <th width="80">CDCl<sub>3</sub></th><th width="80">C<sub>6</sub>D<sub>6</sub></th><th width="90">CD<sub>3</sub>COCD<sub>3</sub></th><th width="90">CD<sub>3</sub>SOCD<sub>3</sub></th><th width="90">CD<sub>3</sub>C≡N</th><th width="80">D<sub>2</sub>O</th> </tr>
<tr align="center" valign="bottom"><th bgcolor=" ffeecc">(CH<sub>3</sub>)<sub>3</sub>C–O–CH<sub>3</sub><br />
<span style="color: teal;"> C–CH<sub>3</sub><br />
O–CH<sub>3</sub></span></th> <td>1.19<sub> </sub><br />
3.22</td><td>1.07<sub> </sub><br />
3.04</td><td>1.13<sub> </sub><br />
3.13</td><td><sub> </sub>1.11<br />
3.03</td><td>1.14<sub> </sub><br />
3.13</td><td>1.21<sub> </sub><br />
3.22</td></tr>
<tr align="center" valign="bottom"><th bgcolor=" ffeecc">(CH<sub>3</sub>)<sub>3</sub>C–O–H<br />
<span style="color: teal;"> C–CH<sub>3</sub><br />
O–H</span></th> <td>1.26<sub> </sub><br />
1.65</td><td>1.05<sub> </sub><br />
1.55</td><td>1.18<sub> </sub><br />
3.10</td><td>1.11<sub> </sub><br />
4.19</td><td>1.16<sub> </sub><br />
2.18</td><td>---<br />
---</td></tr>
<tr align="center" valign="bottom"><th bgcolor=" ffeecc">C<sub>6</sub>H<sub>5</sub>CH<sub>3</sub><br />
<span style="color: teal;">CH<sub>3</sub><br />
C<sub>6</sub>H<sub>5</sub></span></th> <td>2.36<sub> </sub><br />
7.15-7.20</td><td>2.11<sub> </sub><br />
7.00-7.10</td><td>2.32<sub> </sub><br />
7.10-7.20</td><td>2.30<sub> </sub><br />
7.10-7.15</td><td>2.33<sub> </sub><br />
7.15-7.30</td><td>---<br />
---</td></tr>
<tr align="center"><th bgcolor=" ffeecc" height="36">(CH<sub>3</sub>)<sub>2</sub>C=O </th><td>2.17</td><td>1.55</td><td>2.09</td><td>2.09</td><td>2.08</td><td>2.22</td></tr>
</tbody></table>For most of the above resonance signals and solvents the changes are minor, being on the order of ±0.1 ppm. However, two cases result in more extreme changes and these have provided useful applications in structure determination. First, spectra taken in benzene-d<sub>6</sub> generally show small upfield shifts of most C–H signals, but in the case of acetone this shift is about five times larger than normal. Further study has shown that carbonyl groups form weak π–π collision complexes with benzene rings, that persist long enough to exert a significant shielding influence on nearby groups. In the case of substituted cyclohexanones, axial α-methyl groups are shifted upfield by 0.2 to 0.3 ppm; whereas equatorial methyls are slightly deshielded (shift downfield by about 0.05 ppm). These changes are all relative to the corresponding chloroform spectra.<br />
The second noteworthy change is seen in the spectrum of tert-butanol in DMSO, where the hydroxyl proton is shifted 2.5 ppm down-field from where it is found in dilute chloroform solution. This is due to strong hydrogen bonding of the alcohol O–H to the sulfoxide oxygen, which not only de-shields the hydroxyl proton, but secures it from very rapid exchange reactions that prevent the display of spin-spin splitting. Similar but weaker hydrogen bonds are formed to the carbonyl oxygen of acetone and the nitrogen of acetonitrile. A useful application of this phenomenon is described elsewhere in this text.<br />
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<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr4"></a><br />
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<dir><h4><span style="color: maroon;">Spin-Spin Interactions</span></h4></dir> The nmr spectrum of 1,1-dichloroethane (below right) is more complicated than we might have expected from the previous examples. Unlike its 1,2-dichloro-isomer (below left), which displays a single resonance signal from the four structurally equivalent hydrogens, the two signals from the different hydrogens are split into close groupings of two or more resonances. This is a common feature in the spectra of compounds having different sets of hydrogen atoms bonded to adjacent carbon atoms. The signal splitting in proton spectra is usually small, ranging from fractions of a Hz to as much as 18 Hz, and is designated as <b>J</b> (referred to as the coupling constant). In the 1,1-dichloroethane example all the coupling constants are 6.0 Hz, <span style="color: brown;">as illustrated by clicking on the spectrum</span>.<br />
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<center><table><tbody>
<tr><td><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/12cleth.gif" /></td><td width="60"></td><td><a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857"><img border="0" name="dicle" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/11cleth1.gif" /></a></td></tr>
<tr><th>1,2-dichloroethane</th><th></th><th>1,1-dichloroethane</th></tr>
</tbody></table></center> <a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr4b"></a><br />
The splitting patterns found in various spectra are easily recognized, provided the chemical shifts of the different sets of hydrogen that generate the signals differ by two or more ppm. The patterns are symmetrically distributed on both sides of the proton chemical shift, and the central lines are always stronger than the outer lines. The most commonly observed patterns have been given descriptive names, such as <b>doublet</b> (two equal intensity signals), <b>triplet</b> (three signals with an intensity ratio of 1:2:1) and <b>quartet</b> (a set of four signals with intensities of 1:3:3:1). Four such patterns are displayed in the following illustration. The line separation is always constant within a given multiplet, and is called the <b>coupling constant (J)</b>. The magnitude of J, usually given in units of Hz, is magnetic field independent. <br />
<div align="center"><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/patterns.gif" /></div><a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr4bb"></a><br />
The splitting patterns shown above display the ideal or "<b>First-Order</b>" arrangement of lines. This is usually observed if the spin-coupled nuclei have very different chemical shifts (i.e. Δν is large compared to J). If the coupled nuclei have similar chemical shifts, the splitting patterns are distorted (second order behavior). In fact, signal splitting disappears if the chemical shifts are the same. Two examples that exhibit minor 2nd order distortion are shown below (both are taken at a frequency of 90 MHz). The ethyl acetate spectrum on the left displays the typical quartet and triplet of a substituted ethyl group. The spectrum of 1,3-dichloropropane on the right demonstrates that equivalent sets of hydrogens may combine their influence on a second, symmetrically located set. <br />
Even though the chemical shift difference between the A and B protons in the 1,3-dichloroethane spectrum is fairly large (140 Hz) compared with the coupling constant (6.2 Hz), some distortion of the splitting patterns is evident. The line intensities closest to the chemical shift of the coupled partner are enhanced. Thus the B set triplet lines closest to A are increased, and the A quintet lines nearest B are likewise stronger. A smaller distortion of this kind is visible for the A and C couplings in the ethyl acetate spectrum.<br />
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<center><div style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img height="343" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/13clprop.gif" width="400" /></div><div style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/etoac1.gif" /></div><br />
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<div id="para1"></div><table align="center" border="1" cellpadding="6"><tbody>
<tr bgcolor="eeffdd"><th>For additional examples of <b>Second Order</b> splitting patterns Click Here<b> </b></th></tr>
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<b>What causes this signal splitting, and what useful information can be obtained from it ?</b> <br />
If an atom under examination is perturbed or influenced by a nearby nuclear spin (or set of spins), the observed nucleus responds to such influences, and its response is manifested in its resonance signal. This spin-coupling is transmitted through the connecting bonds, and it functions in both directions. Thus, when the perturbing nucleus becomes the observed nucleus, it also exhibits signal splitting with the same J. For spin-coupling to be observed, the sets of interacting nuclei must be bonded in relatively close proximity (e.g. vicinal and geminal locations), or be oriented in certain optimal and rigid configurations. Some spectroscopists place a number before the symbol J to designate the number of bonds linking the coupled nuclei (colored orange below). Using this terminology, a vicinal coupling constant is <sup>3</sup>J and a geminal constant is <sup>2</sup>J.<br />
<div align="center"><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/gemvic.gif" /></div><br />
<b>The following general rules summarize important requirements and characteristics for spin 1/2 nuclei :</b><br />
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<dir> <b>1)</b> Nuclei having the same chemical shift (called <b>isochronous</b>) do not exhibit spin-splitting. They may actually be spin-coupled, but the splitting cannot be observed directly.<br />
<b>2)</b> Nuclei separated by three or fewer bonds (e.g. vicinal and geminal nuclei ) will usually be spin-coupled and will show mutual spin-splitting of the resonance signals (same J's), provided they have different chemical shifts. Longer-range coupling may be observed in molecules having rigid configurations of atoms.<br />
<b>3)</b> The magnitude of the observed spin-splitting depends on many factors and is given by the coupling constant <b>J</b> (units of Hz). J is the same for both partners in a spin-splitting interaction and is independent of the external magnetic field strength.<br />
<b>4)</b> The splitting pattern of a given nucleus (or set of equivalent nuclei) can be predicted by the <b>n+1 rule</b>, where n is the number of neighboring spin-coupled nuclei with the same (or very similar) Js. If there are 2 neighboring, spin-coupled, nuclei the observed signal is a triplet ( 2+1=3 ); if there are three spin-coupled neighbors the signal is a quartet ( 3+1=4 ). In all cases the central line(s) of the splitting pattern are stronger than those on the periphery. The intensity ratio of these lines is given by the numbers in Pascal's triangle. Thus a doublet has 1:1 or equal intensities, a triplet has an intensity ratio of 1:2:1, a quartet 1:3:3:1 etc. To see how the numbers in Pascal's triangle are related to the Fibonacci series <span style="color: brown;">click on the diagram</span>.</dir> <br />
<table><tbody>
<tr valign="top"><th><img height="220" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/splitting.gif" width="320" /></th> <th><a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857"><img border="0" name="pascal" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/pascltri1.gif" /></a></th></tr>
<tr><td colspan="2">If a given nucleus is spin-coupled to two or more sets of neighboring nuclei by different J values, the n+1 rule does not predict the entire splitting pattern. Instead, the splitting due to one J set is added to that expected from the other J sets. Bear in mind that there may be fortuitous coincidence of some lines if a smaller J is a factor of a larger J.</td></tr>
<tr valign="top"><th colspan="2"><img height="190" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/splitting2.gif" width="320" /></th></tr>
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<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr4d"></a> <br />
<table align="center"><caption><h4>Magnitude of Some Typical Coupling Constants</h4></caption> <tbody>
<tr valign="top"><th><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/jconstnt.gif" /><</th></tr>
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Spin 1/2 nuclei include <sup>1</sup>H, <sup>13</sup>C, <sup>19</sup>F & <sup>31</sup>P. The spin-coupling interactions described above may occur between similar or dissimilar nuclei. If, for example, a <sup>19</sup>F is spin-coupled to a <sup>1</sup>H, both nuclei will appear as doublets having the same J constant. <sup> </sup> Spin coupling with nuclei having spin other than 1/2 is more complex and will not be discussed here.<br />
<span style="color: green;">To make use of a calculator that predicts first order splitting patterns <a href="http://www.colby.edu/chemistry/NMR/jmmset.html">Click Here</a>. This application was developed at Colby College.</span><br />
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<table align="center" border="1" cellpadding="6"><tbody>
<tr bgcolor="eeffdd"><th>For additional information about spin-spin coupling <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr2.htm#nmr16">Click Here</a>.</th></tr>
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<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="nmr5"></a><br />
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<center><h3><span style="color: maroon;">Some Examples</span></h3></center> Test your ability to interpret <sup>1</sup>H nmr spectra by analyzing the seven examples presented below. The seven spectra may be examined in turn by clicking the "<u>Toggle Spectra</u>" button. Try to associate each spectrum with a plausible structural formula. <br />
Although the first four cases are relatively simple, keep in mind that the integration values provide ratios, not absolute numbers. In two cases additional information from infrared spectroscopy is provided. When you have made an assignment you may check your answer by clicking on the spectrum itself. In the sixth example, a similar constitutional isomer cannot be ruled out by the data given. <br />
<div align="center"><a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857"><img border="0" name="prob" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/nmrspc11.gif" /></a></div><br />
<center></center> <br />
<table align="center" border="1" cellpadding="6"><tbody>
<tr bgcolor="#ffebcf"><th>For a challenging problem having many spin couplings <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr2.htm#nmr18">Click Here</a>.</th></tr>
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<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="cnmr1"></a> <b><span style="color: maroon; font-size: large;"> 3. Carbon NMR Spectroscopy</span></b><br />
The power and usefulness of <sup>1</sup>H nmr spectroscopy as a tool for structural analysis should be evident from the past discussion. Unfortunately, when significant portions of a molecule lack C-H bonds, no information is forthcoming. Examples include polychlorinated compounds such as chlordane, polycarbonyl compounds such as croconic acid, and compounds incorporating triple bonds (structures below, orange colored carbons).<br />
<div align="center"><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/c-struc1.gif" /></div>Even when numerous C-H groups are present, an unambiguous interpretation of a proton nmr spectrum may not be possible. The following diagram depicts three pairs of isomers (A & B) which display similar proton nmr spectra. Although a careful determination of chemical shifts should permit the first pair of compounds (blue box) to be distinguished, the second and third cases (red & green boxes) might be difficult to identify by proton nmr alone.<br />
<div align="center"><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/c-struc2.gif" /></div>These difficulties would be largely resolved if the carbon atoms of a molecule could be probed by nmr in the same fashion as the hydrogen atoms. Since the major isotope of carbon (<sup>12</sup>C) has no spin, this option seems unrealistic. Fortunately, 1.1% of elemental carbon is the <sup>13</sup>C isotope, which has a spin I = 1/2, so in principle it should be possible to conduct a carbon nmr experiment. <span style="color: green;">It is worth noting here, that if much higher abundances of <sup>13</sup>C were naturally present in all carbon compounds, proton nmr would become much more complicated due to large one-bond coupling of <sup>13</sup>C and <sup>1</sup>H.</span><br />
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<table align="center" bgcolor="#ffebcf" border="1" cellpadding="9"><tbody>
<tr><td nowrap="nowrap"><b>Many obstacles needed to be overcome before carbon nmr emerged as a routine tool :</b><br />
<b>i)</b> As noted, the abundance of <sup>13</sup>C in a sample is very low (1.1%), so higher sample concentrations are needed.<br />
<b>ii)</b> The <sup>13</sup>C nucleus is over fifty times less sensitive than a proton in the nmr experiment, adding to the previous difficulty.<br />
<b>iii)</b> Hydrogen atoms bonded to a <sup>13</sup>C atom split its nmr signal by 130 to 270 Hz, further complicating the nmr spectrum.</td></tr>
</tbody></table><br />
The most important operational technique that has led to successful and routine <sup>13</sup>C nmr spectroscopy is the use of high-field <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr2.htm#pulse">pulse technology</a> coupled with broad-band <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr2.htm#decoupl">heteronuclear decoupling</a> of all protons. The results of repeated pulse sequences are accumulated to provide improved signal strength. Also, for reasons that go beyond the present treatment, the decoupling irradiation enhances the sensitivity of carbon nuclei bonded to hydrogen. <br />
When acquired in this manner, the carbon nmr spectrum of a compound displays a single sharp signal for each structurally distinct carbon atom in a molecule (remember, the proton couplings have been removed). The spectrum of camphor, shown on the left below, is typical. Furthermore, a comparison with the <sup>1</sup>H nmr spectrum on the right illustrates some of the advantageous characteristics of carbon nmr. The dispersion of <sup>13</sup>C chemical shifts is nearly twenty times greater than that for protons, and this together with the lack of signal splitting makes it more likely that every structurally distinct carbon atom will produce a separate signal. The only clearly identifiable signals in the proton spectrum are those from the methyl groups. The remaining protons have resonance signals between 1.0 and 2.8 ppm from TMS, and they overlap badly thanks to spin-spin splitting.<br />
<br />
<table cellpadding="7"><tbody>
<tr><th><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/ccamphor.gif" /></th><th><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/hcamphor.gif" /></th></tr>
</tbody></table><br />
<a href="http://www.blogger.com/post-create.g?blogID=7953616238799155857" name="cnmr2"></a> Unlike proton nmr spectroscopy, <b>the relative strength of carbon nmr signals are not normally proportional to the number of atoms generating each one</b>. Because of this, the number of discrete signals and their chemical shifts are the most important pieces of evidence delivered by a carbon spectrum. The general distribution of carbon chemical shifts associated with different functional groups is summarized in the following chart. Bear in mind that these ranges are approximate, and may not encompass all compounds of a given class. Note also that the over 200 ppm range of chemical shifts shown here is much greater than that observed for <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr1.htm#nmr3bb">proton chemical shifts</a>. <br />
<br />
<center><br />
<table><tbody>
<tr><th colspan="3"><h3><sup>13</sup>C Chemical Shift Ranges<sup>*</sup></h3></th></tr>
<tr><th><span style="color: purple;">Low Field<br />
Region</span></th><td><div align="center"><img height="198" src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/cnmr1.gif" width="400" /></div></td> <th><span style="color: purple;">High Field<br />
Region</span></th></tr>
<tr><td></td><td><sup>*</sup> For samples in CDCl<sub>3</sub> solution. The δ scale is relative to TMS at δ=0. </td><td></td></tr>
</tbody></table></center> The isomeric pairs previously cited as giving very similar proton nmr spectra are now seen to be distinguished by carbon nmr. In the example on the left below (blue box), cyclohexane and 2,3-dimethyl-2-butene both give a single sharp resonance signal in the proton nmr spectrum (the former at δ 1.43 ppm and the latter at 1.64 ppm). However, in its carbon nmr spectrum cyclohexane displays a single signal at δ 27.1 ppm, generated by the equivalent ring carbon atoms (colored blue); whereas the isomeric alkene shows two signals, one at δ 20.4 ppm from the methyl carbons (colored brown), and the other at 123.5 ppm (typical of the green colored sp<sup>2</sup> hybrid carbon atoms).<br />
<div align="center"><img src="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/Images/c-struc3.gif" /></div>The C<sub>8</sub>H<sub>10</sub> isomers in the center (red) box have pairs of <a href="http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/suppmnt1.htm#nom1">homotopic</a> carbons and hydrogens, so symmetry should simplify their nmr spectra. The fulvene (isomer A) has five structurally different groups of carbon atoms (colored brown, magenta, orange, blue and green respectively) and should display five <sup>13</sup>C nmr signals (one near 20 ppm and the other four greater than 100 ppm). Although ortho-xylene (isomer B) will have a proton nmr very similar to isomer A, it should only display four <sup>13</sup>C nmr signals, originating from the four different groups of carbon atoms (colored brown, blue, orange and green). The methyl carbon signal will appear at high field (near 20 ppm), and the aromatic ring carbons will all give signals having δ > 100 ppm. Finally, the last isomeric pair, quinones A & B in the green box, are easily distinguished by carbon nmr. Isomer A displays only four carbon nmr signals (δ 15.4, 133.4, 145.8 & 187.9 ppm); whereas, isomer B displays five signals (δ 15.9, 133.3, 145.8, 187.5 & 188.1 ppm), the additional signal coming from the non-identity of the two carbonyl carbon atoms (one colored orange and the other magenta).</form></div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com0tag:blogger.com,1999:blog-7953616238799155857.post-38754962192342172552011-06-18T09:09:00.000+05:302011-06-18T09:09:45.681+05:30Organometallic Compounds<div dir="ltr" style="text-align: left;" trbidi="on"> Organometallic compounds have at least one carbon to metal bond, according to most definitions. This bond can be either a direct carbon to metal bond ( <i> σ </i> bond or sigma bond) or a metal complex bond ( <i> π </i> bond or pi bond). Compounds containing metal to <b> hydrogen bonds </b> as well as some compounds containing nonmetallic ( <b> metalloid </b> ) elements bonded to carbon are sometimes included in this class of compounds. Some common properties of organometallic compounds are relatively low melting points, insolubility in water, solubility in ether and related solvents, toxicity, oxidizability, and high reactivity. <br />
An example of an organometallic compound of importance years ago is tetraethyllead (Et <sub> 4 </sub> 4Pb) which is an antiknock agent for gasoline. It is presently banned from use in the United States. <br />
The first metal complex identified as an organometallic compound was a salt, K(C <sub> 2 </sub> H <sub> 4 </sub> )PtCl <sub> 3 </sub> , obtained from reaction of ethylene with platinum (II) chloride by William Zeise in 1825. It was not until much later (1951–1952) that the correct structure of Zeise's compound (see Figure 1) was reported in connection with the structure of a metallocene compound known as ferrocene (see Figure 2). <br />
<br />
<div class="gale_imggroup"> <img alt="Figure 1. Anion of Zeise's compound" height="87" src="http://www.chemistryexplained.com/images/chfa_03_img0680.jpg" width="166" /> <div class="caption"> Figure 1. Anion of Zeise's compound </div></div><br />
<div class="gale_imggroup"> <img alt="Figure 2. Ferrocene" height="130" src="http://www.chemistryexplained.com/images/chfa_03_img0681.jpg" width="165" /> <div class="caption"> Figure 2. Ferrocene </div></div>Preparation of ferrocene was reported at about the same time by two research groups, and a sandwich structure was proposed, based on ferrocene's physical properties (Kauffman, pp. 185–186). The sandwich structure was confirmed by x-ray diffraction studies. Since then, other metallocenes composed of other metals and other carbon ring molecules, such as dibenzenechromium (see Figure 3) and uranocene (see Figure 4), have been prepared. <br />
Possibly the first scientist to synthesize an organometallic compound was Edward Frankland, who prepared diethylzinc by reaction of ethyl iodide with zinc metal in 1849 (Thayer 1969b, pp. 764–765). <br />
2 CH <sub> 3 </sub> CH <sub> 2 </sub> I + 2 Zn → CH <sub> 3 </sub> CH <sub> 2 </sub> ZnCH <sub> 2 </sub> CH <sub> 3 </sub> + ZnI <sub> 2 </sub> <br />
In organometallic compounds, most p-electrons of <b> transition metals </b> conform to an empirical rule called the 18-electron rule. This rule assumes that the metal atom accepts from its <b> ligands </b> the number of electrons needed in order for it to attain the electronic configuration of the next <b> noble gas </b> . It assumes that the <b> valence </b> shells of the metal atom will contain 18 electrons. Thus, the sum of the number of d electrons plus the number of electrons supplied by the ligands will be 18. Ferrocene, for example, has 6 d electrons from Fe(II), plus 2 × 6 electrons from the two 5-membered rings, for a total of 18. (There are exceptions to this rule, however.) <br />
<br />
<div class="gale_imggroup"> <img alt="Figure 3. Dibenzenechromium" height="121" src="http://www.chemistryexplained.com/images/chfa_03_img0682.jpg" width="166" /> <div class="caption"> Figure 3. Dibenzenechromium </div></div>Possibly the earliest biomedical application of an organometallic compound was the discovery, by Paul Ehrlich, of the organoarsenical Salvarsan, the first antisyphilitic agent. Salvarsan and other organoarsenicals are sometimes listed as organometallics even though <b> arsenic </b> is not a true metal. <b> Vitamin </b> B <sub> 12 </sub> is an organocobalt complex essential to the diet of human beings. Absence of or deficiency of B <sub> 12 </sub> in the diet (or a body's inability to absorb it) is the cause of pernicious anemia. <br />
<h2> Use as Reagents or Catalysts </h2>Organometallic compounds are very useful as catalysts or reagents in the <b> synthesis </b> of organic compounds, such as pharmaceutical products. One of the major advantages of organometallic compounds, as compared with organic or inorganic compounds, is their high reactivity. Reactions that cannot be carried out with the usual types of organic reagents can sometimes be easily carried out using one of a wide variety of available organometallics. A second advantage is the high reaction selectivity that is often achieved via the use of organometallic catalysts. For example, ordinary free-radical polymerization of ethylene yields a waxy low-density polyethylene, but use of a special organometallic <b> catalyst </b> produces a more ordered linear polyethylene with a higher density, a higher <b> melting point </b> , and a greater strength. A third advantage is that many in this wide range of compounds are stable, and many of these have found uses as medicinals and pesticides. A fourth advantage is the case of recovery of pure metals. Isolation of a pure sample of an organometallic compound containing a desired metal can be readily accomplished, and the pure metal can then be easily obtained from the compound. (This is generally done via preparation of a pure metal carbonyl, such as Fe[CO] <sub> 5 </sub> or Ni[CO] <sub> 4 </sub> , followed by thermal decomposition.) Other commonly used organometallic compounds are organolithium, organozinc, and organocuprates (sometimes called Gilman reagents). <br />
<div> The name "ferrocene" was coined by one of Harvard University professor R. B. Woodward's postdoctoral students, Mark Whiting. The entire class of transitional metal dicyclopentadienyl compounds quickly became known as "metallocenes" and this has since been expanded for compounds [(H <sub> 5 </sub> ‒C <sub> 5 </sub> H <sub> 5 </sub> ) 2M] in general. G. Wilkinson and Woodward published their results on ferrocene in 1952. <br />
</div><br />
<div class="gale_imggroup"> <img alt="Figure 4. Uranocene" height="135" src="http://www.chemistryexplained.com/images/chfa_03_img0683.jpg" width="166" /> <div class="caption"> Figure 4. Uranocene </div></div><h2> Grignard Reagents </h2>One of the most commonly used classes of organometallic compounds is the organomagnesium halides, or Grignard reagents (generally RMgX or ArMgX, where R and Ar are alkyl and aryl groups, respectively, and X is a <b> halogen </b> atom), used extensively in synthetic organic chemistry. Organomagnesium halides were discovered by Philippe Barbier in 1899 and subsequently developed by Victor Grignard. They are usually prepared by reaction of magnesium metal with alkyl or aryl halides. Other commonly used organometallic compounds are the organolithium and organozinc compounds. <br />
<h2> Carbenes </h2>Carbenes are the electrons of free carbenes that have two spin states, singlet and triplet. The electrons are paired as a sp <sup> 2 </sup> lone pair in the singlet (:CH <sub> 2 </sub> ); there is one electron in each of the sp <sup> 2 </sup> and p orbitals in the triplet (·CH <sub> 2 </sub> ). Carbenes are generally unstable in the free state, but are stable when bonded to metal atoms. Metal-carbene complexes have the general structure L <sub> <i> n </i> </sub> M=CXY, where L <sub> <i> n </i> </sub> M is the metal fragment with <i> n </i> ligands, and X and Y are alkyl groups, aryl groups, hydrogen atoms, or heteroatoms (O, N, S, or halogens). The first carbene complex [(CO) <sub> 5 </sub> W = CPh(OMe)] was reported by E. O. Fischer and A. Maasbol in 1964 (Dunitz, Orgel, and Rich, pp. 373–375). In 1974 Richard R. Schrock prepared compounds in which the substituents attached to carbon were hydrogen atoms or alkyl groups; these complexes are known as Schrock-type carbene complexes. The two types of carbene complexes differ in their reactivities. Fischer-type complexes tend to undergo attack at carbon atoms by nucleophiles (negatively charged species) and are electrophilic (electron-attracting). Schrock-type complexes undergo attack at carbon atoms by electrophiles and are considered to be nucleophilic species. </div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com0tag:blogger.com,1999:blog-7953616238799155857.post-72000224110159073262011-06-16T17:34:00.000+05:302011-06-16T17:34:38.722+05:30TETRACYCLINE SYNTHESIS<div dir="ltr" style="text-align: left;" trbidi="on"><h2><span style="color: #c1ab00; font-family: Bookman;">Chemical syntheses</span> </h2><span style="color: black; font-family: Arial;"> The tetracycline molecule has offered a splendid challenge to a synthetic organic chemist, a mojor obstacle in its total synthesis being the stereospecific introduction of many functional groups into the basic carbon nucleus. Of even greater concern is the extreme chemical sensitivity of this molecule, particularly its lability in acidic and basic media [29].<br />
<br />
The first synthesis of a tetracycline-like molecule with the functionality necessary for antimicrobial properties was accomplished by the legendary</span> <b><span style="color: #c15195;"> Robert B. Woodward</span></b> <span style="color: black; font-family: Arial;"> and a group at Pfizer [12] (Some earlier work utilized completely aromatic precursors in conventional acylation reaction to form the C(11), C(12)</span> <span style="color: black; font-family: Symbol;">b</span><span style="color: black; font-family: Arial;">-diketone system [14,15]).<br />
The product, sancycline (6-demethyl-6-deoxytetracycline), is an active antibiotic, but not used as often as aureomycin or terramycin. The Woodward's synthesis is completely linear and involves incorporation of the 4-dimethylamino- group and the 12a-hydroxy- group into the final structure. The synthesis provides a dramatic illustration of the importance and creativity of organic chemistry [13].</span> <br />
<div align="center"><img src="http://www.chm.bris.ac.uk/motm/tetracycline/tetracycline_files/image002.gif" /></div><span style="color: black; font-family: Arial;"> The following synthesis was reported by</span> <b><span style="color: #c15195;">Shemyatkin</span></b><span style="color: black; font-family: Arial;"><i> et al.</i> [16]. The precursor had been prepared in six stages from juglone (a naturally occurring quinone, the active staining principle of black walnut hulls) [17], and the product was obtained by degradation of the naturally occurring tetracycline according to the procedure of Green and Boothe [18]. Since the latter is a degradation product of tetracycline, and has been reconverted by 12a-hydroxylation followed by photo-addition etc., according to the Scott procedure [19], into tetracycline, a formal total synthesis of the latter had been accomplished [8].</span><br />
<span style="color: black; font-family: Arial;"> </span><br />
<div align="center"><span style="color: black; font-family: Arial;"><img src="http://www.chm.bris.ac.uk/motm/tetracycline/tetracycline_files/image008.gif" /></span></div><span style="color: black; font-family: Arial;"><span style="color: black; font-family: Arial;">The next synthesis of a tetracycline-like structure by</span> <b><span style="color: #c15195;">Woodward</span></b> <span style="color: black; font-family: Arial;">was reported in 1968 [5]:</span> <br />
<div align="center"><br />
<img src="http://www.chm.bris.ac.uk/motm/tetracycline/tetracycline_files/image012.gif" /> </div><b><span style="color: #c15195;">Muxfeldt</span></b> <span style="color: black; font-family: Arial;"><i>et al</i>. presented a simple route to tetracycline system, using brilliant condensation and oxazolone moiety cleavage to produce a sensitive group in A ring [29]:</span> <br />
<div align="center"><img src="http://www.chm.bris.ac.uk/motm/tetracycline/tetracycline_files/image014.gif" /><br />
<img src="http://www.chm.bris.ac.uk/motm/tetracycline/tetracycline_files/image016.gif" /> </div><span style="color: black; font-family: Arial;">The improved method of the latter, with a controlled introduction of stereocenter at C4a, has been presented by <b><span style="color: #c15195;">Stork</span></b><span style="color: black; font-family: Arial;"> <i>et al</i>. [30]:</span> </span><br />
<span style="color: black; font-family: Arial;"> <div align="center"><img src="http://www.chm.bris.ac.uk/motm/tetracycline/tetracycline_files/image006.gif" /></div></span></span></div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com0tag:blogger.com,1999:blog-7953616238799155857.post-30982231836710716852011-06-15T09:42:00.000+05:302011-06-15T09:42:46.248+05:30Electronic Configuration of the Elements<div dir="ltr" style="text-align: left;" trbidi="on"><span style="font-family: verdana,geneva,helvetica; font-size: x-small;"><span style="font-size: small;"><b>H</b></span>ere's a useful table for your chemistry homework or general use! This is a compilation of the electron configurations of the elements up through number 104, broken into three pages (the table was too large for anything less). To arrive at the electron configurations of atoms, you must know the order in which the different sublevels are filled. Electrons enter available sublevels in order of their increasing energy. A sublevel is filled or half-filled before the next sublevel is entered. For example, the <i>s</i> sublevel can only hold two electrons, so the 1<i>s</i> is filled at helium (1<i>s</i><sup>2</sup>). The <i>p</i> sublevel can hold six electrons, the <i>d</i> sublevel can hold 10 electrons, and the <i>f</i> sublevel can hold 14 electrons. Common shorthand notation is to refer to the noble gas core, rather than write out the entire configuration. For example, the configuration of magnesium could be written [Ne]3s<sup>2</sup>, rather than writing out 1s<sup>2</sup>2s<sup>2</sup>2p<sup>6</sup>3s<sup>2</sup>.</span><br />
<br />
<table border="1" cellpadding="5"><tbody>
<tr><th><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">No.</span></th> <th><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Element</span></th> <th><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">K</span></th> <th><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">L</span></th> <th><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">M</span></th> <th><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">N</span></th> <th><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">O</span></th> <th><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">P</span></th> <th><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Q</span></th> </tr>
<tr> <td> </td> <td> </td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">3</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">4</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">5</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">7</span></td> </tr>
<tr> <td> </td> <td> </td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">s</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">s p</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">s p d</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">s p d f</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">s p d f</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">s p d f</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">s</span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">H</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">He</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">3</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Li</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">4</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Be</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">5</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">B</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">C</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">7</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">N</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 3</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">8</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">O</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 4</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">9</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">F</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 5</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">10</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Ne</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">11</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Na</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">12</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Mg</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">13</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Al</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">14</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Si</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">15</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">P</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 3</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">16</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">S</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 4</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">17</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Cl</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 5</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">18</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Ar</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">19</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">K</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 - </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">20</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Ca</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 - </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">21</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Sc</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 1 </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">22</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Ti</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 2 </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">23</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">V</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 3 </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">24</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Cr</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 5*</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">25</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Mn</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 5</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">26</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Fe</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">27</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Co</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 7</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">28</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Ni</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 8</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">29</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Cu</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 10</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">1*</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">30</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Zn</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 10</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">31</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Ga</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 10</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 1</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">32</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Ge</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 10</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">33</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">As</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 10</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 3</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">34</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Se</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 10</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 4</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">35</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Br</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 10</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 5</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
<tr> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">36</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">Kr</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6 10</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;">2 6</span></td> <td><span style="font-family: verdana,geneva,helvetica; font-size: xx-small;"> </span></td> </tr>
</tbody></table></div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com0tag:blogger.com,1999:blog-7953616238799155857.post-88576488688640111012011-06-15T09:36:00.000+05:302011-06-15T09:36:19.530+05:30Valence Bond Theory and Hybrid Atomic Orbitals<div dir="ltr" style="text-align: left;" trbidi="on"> <img align="LEFT" src="http://www.science.uwaterloo.ca/%7Ecchieh/cact/fig/sphinx.gif" /> This picture is an image of a Centaur from Sphinx Stargate. The Centaur is a race of monsters in Greek mythology, <b>hybrid</b> animal having the head, arms and torso of a man united to the body and legs of a horse. <b>Mixing a number of atomic orbitals to form the same number of hybrid orbitals</b> to explain chemical bonding and shapes and molecular structures is a rather recent myth. The most significant development in the first half of the 20th century is the human's ability to understand the structure of atoms and molecules. Computation has made mathematical concepts visible to the extent that we now can <i>see</i> the atomic and molecular orbitals. On the other hand, Using everyday encountered materials or toys can also generate beautiful illustrations of hybrid atomic orbitals. <br />
The valence bond (VB) approach is different from the molecular orbital (MO) theory. Despite their differences, most of their results are the same, and they are interesting. <br />
<h2> The valence bond (VB) theory </h2>The <b>valence-bond approach</b> considers the overlap of the atomic orbitals (AO) of the participation atoms to form a chemical bond. Due to the overlapping, electrons are localized in the bond region. The overlapping AOs can be of different types, for example, a sigma bond may be formed by the overlapping the following AOs. <br />
<br />
<table align="center" bgcolor="#DDEEFF"><tbody>
<tr bgcolor="#FFEEDD"><th colspan="6">Chemical bonds formed due to overlap of atomic orbitals </th></tr>
<tr><th><i>s-s </i></th><th><i>s-p </i></th><th><i>s-d </i></th><th><i>p-p </i></th><th><i>p-d </i></th><th><i><b><i>d-d </i></b></i></th></tr>
<tr valign="top"><th>H-H<br />
Li-H </th><th>H-C<br />
H-N<br />
H-F </th><th>H-Pd in<br />
palladium<br />
hydride </th><th>C-C<br />
P-P<br />
S-S </th><th>F-S<br />
in SF<sub>6</sub> </th><th>Fe-Fe </th></tr>
</tbody></table><br />
However, the atomic orbitals for bonding may not be "pure" atomic orbitals directly from the solution of the Schrodinger Equation. Often, the bonding atomic orbitals have a character of several possible types of orbitals. The methods to get an AO with the proper character for the bonding is called <b>hybridization</b>. The resulting atomic orbitals are called <b>hybridized atomic orbitals</b> or simply <b>hybrid orbitals</b>. <br />
We shall look at the shapes of some hybrid orbitals first, because these shapes determine the shapes of the molecules. <br />
<h2> Hybridization of atomic orbitals</h2>The solution to the Schrodinger Equation provides the wavefunctions for the following atomic orbitals: <br />
<dir> 1<i>s</i>, 2<i>s</i>, 2<i>p</i>, 3<i>s</i>, 3<i>p</i>, 3<i>d</i>, 4<i>s</i>, 4<i>p</i>, 4<i>d</i>, 4<i>f</i>, etc. </dir> For atoms containing two or more electrons, the energy levels are shifted with respect to those of the H atom. An atomic orbital is really the energy state of an electron bound to an atomic nucleus. The energy state changes when one atom is bonded to another atom. Quantum mechanical approaches by combining the wave functions to give new wavefunctions are called <b>hybridization</b> of atomic orbitals. Hybridization has a sound mathematical fundation, but it is a little too complicated to show the details here. Leaving out the jargons, we can say that an imaginary mixing process converts a set of atomic orbitals to a new set of <b>hybrid atomic orbitals</b> or <b>hybrid orbitals</b>. <br />
At this level, we consider the following hybrid orbitals: <br />
<dir> <i>sp<br />
sp</i><sup>2</sup><br />
<i>sp</i><sup>3</sup><br />
<i>sp</i><sup>3</sup><i>d</i><br />
<i>sp</i><sup>3</sup><i>d</i><sup>2</sup><br />
</dir> <h3> The <i>sp</i> hybrid atomic orbitals </h3><dir><img align="right" src="http://www.science.uwaterloo.ca/%7Ecchieh/cact/fig/sp1.jpg" /> The <b><i>sp</i> hybrid atomic orbitals</b> are possible states of electron in an atom, especially when it is bonded to others. These electron states have half 2<i>s</i> and half 2<i>p</i> characters. From a mathematical view point, there are two ways to combine the 2<i>s</i> and 2<i>p</i> atomic orbitals: <dir> <i>sp</i><sub>1</sub> = 2<i>s</i> + 2<i>p</i><br />
<i>sp</i><sub>2</sub> = 2<i>s</i> - 2<i>p</i><br />
</dir> These energy states (<i>sp</i><sub>1</sub> and <i>sp</i><sub>2</sub>) have a region of high electron probability each, and the two atomic orbitals are located opposite to each other, centered on the atom. The sp hybrid orbitals are represented by this photograph. <br />
<table align="right" bgcolor="#DDDDFF"><tbody>
<tr><th>H-Be-H </th><th>1<i>s</i> 1<i>s</i><br />
H <i>sp</i><sub>1</sub> <b>Be</b> <i>sp</i><sub>2</sub> H <br />
1<i>s</i> 1<i>s</i><br />
</th></tr>
</tbody></table>For example, the molecule H-Be-H is formed due to the overlapping of two 1<i>s</i> orbitals of 2 H atoms and the two <i>sp</i> hybridized orbitals of Be. Thus, the H-Be-H molecule is linear. The diagram here shows the overlapping of AOs in the molecule H-Be-H. <br />
The ground state electronic configuration of Be is 1<i>s</i><sup>2</sup>2<i>s</i><sup>2</sup>, and one may think of the electronic configuration "before" bonding as 1<i>s</i><sup>2</sup><i>sp</i><sup>2</sup>. The two electrons in the <i>sp</i> hybrid orbitals have the same energy. <br />
<br />
<table align="right" bgcolor="#DDDDFF"><tbody>
<tr><th bgcolor="#FFEEDD">Linear molecules </th></tr>
<tr><th>ClBeCl HCCH<br />
HCN<br />
O=C=O </th></tr>
</tbody></table>You may say that the concept of hybridizing AOs for the bonding is just a story made up to explain the molecular shape of Cl-Be-Cl. You are right! The story is lovely and interesting, though. <br />
In general, when two and only two atoms bond to a third atom and the third atom makes use of the <i>sp</i> hybridized orbitals, the three atoms are on a straight line. For example, <i>sp</i> hybrid orbitals are used in the central atoms in the molecules shown on the right. </dir> <h3> The <i>sp</i><sup>2</sup> hybrid orbitals </h3><dir> <img align="right" src="http://www.science.uwaterloo.ca/%7Ecchieh/cact/fig/sp2.jpg" /> The energy states of the valence electrons in atoms of the second period are in the 2<i>s</i> and 2<i>p</i> orbitals. If we mix two of the 2<i>p</i> orbitals with a 2<i>s</i> orbital, we end up with <b>three <i>sp</i><sup>2</sup> hybridized orbitals</b>. These three orbitals lie on a plane, and they point to the vertices of a equilateral triangle as shown here. When the central atom makes use of <i>sp</i><sup>2</sup> hybridized orbitals, the compound so formed has a trigonal shape. BF<sub>3</sub> is such a molecule: <br />
<br />
<table align="center" bgcolor="#ddDDFF"><tbody>
<tr><th bgcolor="#FFFFDD" colspan="5">Molecules with <i>sp</i><sup>2</sup> Hybrid orbitals </th></tr>
<tr><th>F<br />
|<br />
B<br />
/ \<br />
F F </th><th> . . -2<br />
:O:<br />
|<br />
C<br />
/ \\ <br />
:O:: O </th><th><br />
.<br />
N<br />
// \\<br />
O O </th><th><br />
. .<br />
O<br />
// \\<br />
O O </th><th><br />
. .<br />
S<br />
// \\<br />
O O </th></tr>
</tbody></table><br />
Not all three <i>sp</i><sup>2</sup> hybridized orbitals have to be used in bonding. One of the orbitals may be occupied by a pair or a single electron. If we do not count the unshared electrons, these molecules are bent, rather than linear. The three molecules shown together with the BF<sub>3</sub> molecule are such molecules. <br />
Carbon atoms also makes use of the <i>sp</i><sup>2</sup> hybrid orbitals in the compound H<sub>2</sub>C=CH<sub>2</sub>. In this molecule, the remaining <i>p</i> orbital from each of the carbon overlap to form the additional pi, <span style="font-family: SYMBOL;">p</span>, bond. <br />
<br />
<table align="center" bgcolor="#ddDDFF"><tbody>
<tr><th bgcolor="#FFFFDD" colspan="3">Planar molecules with <i>sp</i><sup>2</sup> Hybrid orbitals </th></tr>
<tr> <th>H H<br />
\ /<br />
C = C<br />
/ \<br />
H H<br />
</th><th>O 2-<br />
\ <br />
C = O<br />
/ <br />
O <br />
</th><th>O 1-<br />
\ <br />
N = O<br />
/ <br />
O <br />
</th></tr>
</tbody></table><br />
Other ions such as CO<sub>3</sub><sup>2-</sup>, and NO<sub>3</sub><sup>-</sup>, can also be explained in the same way. </dir> <h3> The <i>sp</i><sup>3</sup> hybrid orbitals </h3><dir> <img align="right" src="http://www.science.uwaterloo.ca/%7Ecchieh/cact/fig/sp3.jpg" /> Mixing one <i>s</i> and all three <i>p</i> atomic orbitals produces a set of four equivalent <i>sp</i><sup>3</sup> hybrid atomic orbitals. The four <i>sp</i><sup>3</sup> hybrid orbitals points towards the vertices of a tetrahedron, as shown here in this photograph. When <i>sp</i><sup>3</sup> hybrid orbitals are used for the central atom in the formation of molecule, the molecule is said to have the shape of a tetrahedron. <br />
The typical molecule is CH<sub>4</sub>, in which the 1<i>s</i> orbital of a H atom overlap with one of the <i>sp</i><sup>3</sup> hybrid orbitals to form a C-H bond. Four H atoms form four such bonds, and they are all equivalent. The CH<sub>4</sub> molecule is the most cited molecule to have a tetrahedral shape. Other molecules and ions having tetrahedral shapes are SiO<sub>4</sub><sup>4-</sup>, SO<sub>4</sub><sup>2-</sup>, <br />
As are the cases with <i>sp</i><sup>2</sup>, hybrid orbitals, one or two of the <i>sp</i><sup>3</sup> hybrid orbitals may be occupied by non-bonding electrons. Water and ammonia are such molecules. <br />
<br />
<table align="right" bgcolor="#DDEEFF"><tbody>
<tr><th bgcolor="#FFEEDD" colspan="3">Tetrahedral arrangements of <br />
CH<sub>4</sub>, NH<sub>3</sub>E and OH<sub>2</sub>E<sub>2</sub> </th></tr>
<tr> <td align="center">H H<br />
\ / <br />
C<br />
<b>/</b> \<br />
<b>H</b> H </td><td align="center">H H<br />
\ / <br />
N<br />
<b>/</b> \<br />
<b>:</b> H </td><td align="center">H H<br />
\ / <br />
O<br />
<b>/</b> \<br />
<b>:</b> : </td></tr>
</tbody></table>The C, N and O atoms in CH<sub>4</sub>, NH<sub>3</sub>, OH<sub>2</sub> (or H<sub>2</sub>O) molecules use the <i>sp</i><sup>3</sup> hybrid orbitals, however, a lone pair occupy one of the orbitals in NH<sub>3</sub>, and two lone pairs occupy two of the <i>sp</i><sup>3</sup> hybrid orbitals in OH<sub>2</sub>. The lone pairs must be considered in the VSEPR model, and we can represent a lone pair by E, and two lone pairs by E<sub>2</sub>. Thus, we have NH<sub>3</sub>E and OH<sub>2</sub>E<sub>2</sub> respectively. <br />
The <i>VSEPR number</i> is equal to the number of bonds plus the number of lone pair electrons. Does not matter what is the order of the bond, any bonded pair is considered on bond. Thus, the <i>VSEPR number</i> is 4 for all of CH<sub>4</sub>, :NH<sub>3</sub>, ::OH<sub>2</sub>. <br />
According the the VSEPR theory, the lone electron pairs require more space, and the H-O-H angle is 105 deegrees, less than the ideal tetrahedral angle of 109.5 degrees. </dir> <h3> The <i>dsp</i><sup>3</sup> hybrid orbitals </h3><dir> <img align="RIGHT" src="http://www.science.uwaterloo.ca/%7Ecchieh/cact/fig/pclf4.gif" width="300" /> The five <i>dsp</i><sup>3</sup> hybrid orbitals resulted when one 3<i>d</i>, one 3<i>s</i>, and three 3<i>p</i> atomic orbitals are mixed. When an atom makes use of fice <i>dsp</i><sup>3</sup> hybrid orbitals to bond to five other atoms, the geometry of the molecule is often a <b>trigonalbipyramidal</b>. For example, The molecule PClF<sub>4</sub> displayed here forms such a structure. In this diagram, the Cl atom takes up an axial position of the trigonalbipyramid. There are structures in which the Cl atom may take up the equatorial position. The change in arrangement is accomplished by simply change the bond angles. This link discusses this type of configuration changes of this molecule. Some of the <i>dsp</i><sup>3</sup> hybrid orbitals may be occupied by electron pairs. The shapes of these molecules are interesting. In TeCl<sub>4</sub>, only one of the hybrid <i>dsp</i><sup>3</sup> orbitals is occupied by a lone pair. This structure may be represented by TeCl<sub>4</sub>E, where E represents a lone pair of electrons. Two lone pairs occupy two such orbitals in the molecule BrF<sub>3</sub>, or BrF<sub>3</sub>E<sub>2</sub>. These structures are given in a VSEPR table of 5 and 6 coordinations. <br />
<dir> <img src="http://www.science.uwaterloo.ca/%7Ecchieh/cact/fig/tecl4v.gif" /> <img src="http://www.science.uwaterloo.ca/%7Ecchieh/cact/fig/brf3v.gif" /> </dir> The compound SF<sub>4</sub> is another AX<sub>4</sub>E type, and many interhalogen compounds ClF<sub>3</sub> and IF<sub>3</sub> are AX<sub>3</sub>E<sub>2</sub> type. The ion I<sub>3</sub><sup>-</sup> is of the type AX<sub>2</sub>E<sub>3</sub>. </dir> <h3> The <i>d<sup>2</sup>sp</i><sup>3</sup> hybrid orbitals </h3><dir> <img align="RIGHT" src="http://www.science.uwaterloo.ca/%7Ecchieh/cact/fig/octa.jpg" /> The six <i>d<sup>2</sup>sp</i><sup>3</sup> hybrid orbitals resulted when two 3<i>d</i>, one 3<i>s</i>, and three 3<i>p</i> atomic orbitals are mixed. When an atom makes use of six <i>d<sup>2</sup>sp</i><sup>3</sup> hybrid orbitals to bond to six other atoms, the molecule takes the shape of an octahedron, in terms of molecular geometry. The gas compound SF<sub>6</sub> is a typical such structure. This link provides other shapes as well. There are also cases that some of the <i>d<sup>2</sup>sp</i><sup>3</sup> hybrid orbitals are occupied by lone pair electrons leading to the structures of the following types: <br />
<br />
<dir> AX<sub>6</sub>, AX<sub>5</sub>E, AX<sub>4</sub>E<sub>2</sub> AX<sub>3</sub>E<sub>3</sub> and AX<sub>2</sub>E<sub>4</sub><br />
IOF<sub>5</sub>, IF<sub>5</sub>E, XeF<sub>4</sub>E<sub>2</sub> </dir> No known compounds of AX<sub>3</sub>E<sub>3</sub> and AX<sub>2</sub>E<sub>4</sub> are known or recognized, because they are predicted to have a T shape and linear shape respectively when the lone pairs of electrons are ignored. </dir> <h2> Molecular shapes of compounds </h2>While the hybridized orbitals were introduced, in the foregoing discussion, Valence-shell Electron-pair Repulsion (VSEPR) Model were included to suggest the shapes of various molecules. Specifically, the VSEPR model counts unshared electron pairs and the bonded atoms as the <i>VSEPR number</i>. A single-, double- and tripple-bond is considered as 1. After having considered the hybridized orbitals and the VSEPR model, we can not take a systematic approach to rationalize the shapes of many molecules based on the number of valence electrons. A summary in the form of a table is given here to account for the concepts of <b>hybrid orbitals, valence bond theory, VSEPR, resonance structures</b>, and <b>octet rule</b>. In this table, the geometric shapes of the molecules are described by <b>linear, trigonal planar, tetrahedral, trigonal bypyramidal, and octahedral</b>. The hybrid orbitals use are <i>sp, sp<sup>2</sup>, sp<sup>3</sup>, dsp<sup>3</sup></i>, and <i>d<sup>2</sup>sp<sup>3</sup></i>. <br />
The <i>VSEPR number</i> is the same for all molecules of each group. Instead of using NH<sub>3</sub>E, and OH<sub>2</sub>E<sub>2</sub>, we use :NH<sub>3</sub>, ::OH<sub>2</sub> to emphasize the unshared (or lone) electron pairs. <br />
<dir> <table bgcolor="#DDDDFF"><tbody>
<tr bgcolor="#FFEEDD"><th colspan="5">A summary of hybrid orbitals, valence bond theory, VSEPR, <br />
resonance structures, and octet rule. </th></tr>
<tr><th>Linear </th><th>Trigonal<br />
planar </th><th>Tetrahedral </th><th>Trigonal<br />
bipyramidal </th><th>Octahedral </th></tr>
<tr><th><i>sp </i></th><th><i>sp</i><sup>2</sup></th><th><i>sp</i><sup>3</sup> </th><th><i>dsp</i><sup>3</sup> </th><th><i>d</i><sup>2</sup><i>sp</i><sup>3</sup> </th></tr>
<tr valign="top"><th>BeH<sub>2</sub><br />
BeF<sub>2</sub><br />
CO<sub>2</sub><br />
HCN<br />
HC<span style="font-family: symbol;">º</span>CH<br />
</th><th>BH<sub>3</sub><br />
BF<sub>3</sub><br />
CH<sub>2</sub>O<br />
(>C=O)<br />
>C=C<<br />
CO<sub>3</sub><sup>2-</sup><br />
benzene<br />
graphite<br />
fullerenes<br />
•NO<sub>2</sub><br />
N<sub>3</sub><sup>-</sup><br />
:OO<sub>2</sub> (O<sub>3</sub>)<br />
:SO<sub>2</sub><br />
SO<sub>3</sub> </th><th>CH<sub>4</sub><br />
CF<sub>4</sub><br />
CCl<sub>4</sub><br />
CH<sub>3</sub>Cl<br />
NH<sub>4</sub><sup>+</sup><br />
:NH<sub>3</sub><br />
:PF<sub>3</sub><br />
:SOF<sub>2</sub><br />
::OH<sub>2</sub><br />
::SF<sub>2</sub><br />
<br />
SiO<sub>4</sub><sup>4-</sup><br />
PO<sub>4</sub><sup>3-</sup><br />
SO<sub>4</sub><sup>2-</sup><br />
ClO<sub>4</sub><sup>-</sup> </th><th>PF<sub>5</sub><br />
PCl<sub>5</sub><br />
PFCl<sub>4</sub><br />
:SF<sub>4</sub><br />
:TeF<sub>4</sub><br />
::ClF<sub>3</sub><br />
::BrF<sub>3</sub><br />
:::XeF<sub>2</sub><br />
:::I<sub>3</sub><sup>-</sup><br />
(:::I I<sub>2</sub><sup>-</sup>)<br />
:::ICl<sub>2</sub><sup>-</sup> </th><th>SF<sub>6</sub><br />
IOF<sub>5</sub><br />
PF<sub>6</sub><sup>-</sup><br />
SiF<sub>6</sub><sup>2-</sup><br />
:BrF<sub>5</sub><br />
:IF<sub>5</sub><br />
::XeF<sub>4</sub><br />
</th></tr>
<tr bgcolor="#FFEEDD"> <td colspan="5">• a lone odd electron <b>:</b> a lone electron pair </td></tr>
</tbody></table></dir> This table correlates a lot of interesting chemical concepts in order to understand the molecular structures of these compounds or ions. There are some intriqueing chemical relationships among the molecules in each column for you to ponder. Only Be and C atoms are involved in linear molecules. In gas phase, BeH<sub>2</sub> and BeF<sub>2</sub> are stable, and these molecules do not satisfy the octet rule. The element C makes use of <i>sp</i> hybridized orbitals and it has the ability to form double and triple bonds in these linear molecules. <br />
Carbon compounds are present in trigonal planar and tetrahedral molecules, using different hybrid orbitals. The extra electron in nitrogen for its compounds in these groups appear as lone unpaired electron or lone electron pairs. More electrons in O and S lead to compounds with lone electron pairs. The five-atom anions are tetrahedral, and many resonance structures can be written for them. <br />
Trigonal bipyramidal and octahedral molecules have 5 and 6 VSEPR pairs. When the central atoms contain more than 5 or 6 electrons, the extra electrons form lone pairs. The number of lone pairs can easily be derived using Lewis dot structures for the valence electrons. <br />
In describing the shapes of these molecules, we often ignore the lone pairs. Thus, •NO<sub>2</sub>, N<sub>3</sub><sup>-</sup>, :OO<sub>2</sub> (O<sub>3</sub>), and :SO<sub>2</sub> are <b>bent molecules</b> whereas :NH<sub>3</sub>, :PF<sub>3</sub>, and :SOF<sub>2</sub> are pyramidal. You already know that ::OH<sub>2</sub> (water) and ::SF<sub>2</sub> are bent molecules. <br />
The lone electron pair takes up the equatorial location in :SF<sub>4</sub>, which has the same structure as :TeF<sub>4</sub> described earlier. If you lay a model of this molecule on the side, it looks like a <i>butterfly</i>. By the same reason, ::ClF<sub>3</sub> and ::BrF<sub>3</sub> have a <i>T</i> shape, and :::XeF<sub>2</sub>, :::I<sub>3</sub><sup>-</sup>, and :::ICl<sub>2</sub><sup>-</sup> are linear. <br />
Similarly, :BrF<sub>5</sub> and :IF<sub>5</sub> are square pyramidal whereas ::XeF<sub>4</sub> is square planar. <br />
<h3> The Center Atom </h3>A nice student asked a brilliant question. <span style="color: red;">Which atom in the formula is usually the center atom?</span> <br />
Usually, the atom in the center is more electropositive than the terminal atoms. However, the H and halogen atoms are usually at the terminal positions because they form only one bond. <br />
Take a look at the chemical formulas in the table, and see if the above statement is true. <br />
However, the application of VSEPR theory can be expanded to complicated molecules such as <br />
<table align="center" bgcolor="##FFEEDD"><tbody>
<tr><td> <pre>H H H O
| | | //
H-C-C=C=C-C=C-C-C
| | \
H N O-H
/ \
H H
</pre></td></tr>
</tbody></table>By applying the VSEPR theory, one deduces the following results: <br />
<ul><li>H-C-C bond angle = 109<sup>o</sup> </li>
<li>H-C=C bond angle = 120<sup>o</sup>, geometry around C trigonal planar </li>
<li>C=C=C bond angle = 180<sup>o</sup>, in other words linear </li>
<li>H-N-C bond angle = 109<sup>o</sup>, tetrahedral around N </li>
<li>C-O-H bond angle = 105 or 109<sup>o</sup>, 2 lone electron pairs around O </li>
</ul></div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com0tag:blogger.com,1999:blog-7953616238799155857.post-35162646579100771882011-06-15T09:12:00.000+05:302011-06-15T09:12:17.725+05:30ATOMIC ORBITAL<div dir="ltr" style="text-align: left;" trbidi="on">An <b>atomic orbital</b> is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom.<sup class="reference" id="cite_ref-0"><a href="http://en.wikipedia.org/wiki/Atomic_orbital#cite_note-0"><span></span><span></span></a></sup> This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus. The term may also refer to the physical region defined by the function where the electron is likely to be.<sup class="reference" id="cite_ref-1"><a href="http://en.wikipedia.org/wiki/Atomic_orbital#cite_note-1"><span></span><span></span></a></sup><br />
Within a physical context atomic orbitals are the basic building blocks of the <b>electron cloud model</b> (alternatively referred to as the wave mechanics model or atomic orbital model), a modern framework for describing the placement of electrons in an atom. In this model, the atom consists of a nucleus surrounded by orbiting electrons. These electrons exist in atomic orbitals, which are a set of quantum states of the negatively charged electrons trapped in the electrical field generated by the positively charged nucleus. The electron cloud model can only be described by quantum mechanics, in which the electrons are most accurately described as standing waves surrounding the nucleus.<br />
<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjeDSSO4Av1AUMTh7aElMFEhkBviahQjPADzc37uJifLkEPgkOdKwkPTWEMd-p5JVHv-ElxRRf37FLqA4o3ppups4Xq4iXNTSs73iuY5ZupAHOj5zUrcgnXJpgOutCMCfvvE-vk7r1QCR_f/s1600/ch9orbitals1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="215" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjeDSSO4Av1AUMTh7aElMFEhkBviahQjPADzc37uJifLkEPgkOdKwkPTWEMd-p5JVHv-ElxRRf37FLqA4o3ppups4Xq4iXNTSs73iuY5ZupAHOj5zUrcgnXJpgOutCMCfvvE-vk7r1QCR_f/s320/ch9orbitals1.jpg" width="320" /></a></div><br />
Atomic orbitals are typically described as “hydrogen-like” (meaning one-electron) wave functions over space, categorized by <i>n</i>, <i>l</i>, and <i>m</i> quantum numbers, which correspond to the electrons' energy, angular momentum, and an angular momentum direction, respectively. Each orbital is defined by a different set of quantum numbers and contains a maximum of two electrons. The simple names <b>s orbital</b>, <b>p orbital</b>, <b>d orbital</b> and <b>f orbital</b> refer to orbitals with angular momentum quantum number <i>l</i> = 0, 1, 2 and 3 respectively. These names indicate the orbital shape and are used to describe the electron configurations. They are derived from the characteristics of their spectroscopic lines: <b>s</b>harp, <b>p</b>rincipal, <b>d</b>iffuse, and <b>f</b>undamental, the rest being named in alphabetical order (omitting j).<br />
<br />
<br />
<sup class="reference" id="cite_ref-2"><a href="http://en.wikipedia.org/wiki/Atomic_orbital#cite_note-2"><span></span><span></span></a></sup><sup class="reference" id="cite_ref-3"><a href="http://en.wikipedia.org/wiki/Atomic_orbital#cite_note-3"><span></span><span></span></a></sup><br />
<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhlrNYNNBbG6KvP4aS5_XuZnmeAkKmrS_K7JT9vpDZC3qeJbcoE9XYXM1onlmW6wY3sz4AHh5UW-5cIQ0MqCYbGg5v29T4_frpfWgzLb-06p33af2ksjxEH3VJXqiYTE-TM-prVigGIf36C/s1600/Neon_orbitals.JPG" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="87" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhlrNYNNBbG6KvP4aS5_XuZnmeAkKmrS_K7JT9vpDZC3qeJbcoE9XYXM1onlmW6wY3sz4AHh5UW-5cIQ0MqCYbGg5v29T4_frpfWgzLb-06p33af2ksjxEH3VJXqiYTE-TM-prVigGIf36C/s320/Neon_orbitals.JPG" width="320" /></a></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiSM4KFz426TMUDY99-QtuRuPyW13NiVi8y2IfGrndeSjeaN5kleK4pbRtKiWr8KHGEGmMWohwHXRJvyGSOylz8tW0u20-SBOyPHm4u06U2F3RKCMDqEB7b9w6OODMj3p5G3a8aNoSWQ-MC/s1600/atom-quantum.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="240" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiSM4KFz426TMUDY99-QtuRuPyW13NiVi8y2IfGrndeSjeaN5kleK4pbRtKiWr8KHGEGmMWohwHXRJvyGSOylz8tW0u20-SBOyPHm4u06U2F3RKCMDqEB7b9w6OODMj3p5G3a8aNoSWQ-MC/s320/atom-quantum.jpg" width="320" /></a></div><br />
<br />
The wavefunction for the electron cloud of a complex atom may be seen as being built up (in approximation) in an electron configuration that is a sum of simpler hydrogen-like atomic orbitals. In such a configuration, pairs of electrons are arranged in simple repeating patterns of increasing odd numbers (1,3,5,7..), each of which represents a set of electron pairs with a given energy and angular momentum. The repeating <i>periodicity</i> of the blocks of 2, 6, 10, and 14 elements in the periodic table arises naturally from the total number of electrons which occupy a complete set of <b>s</b>, <b>p</b>, <b>d</b> and <b>f</b> atomic orbitals, respectively.</div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com1tag:blogger.com,1999:blog-7953616238799155857.post-74829964314271507282011-06-14T08:12:00.001+05:302011-06-15T08:20:25.991+05:30Cholesterol Synthesis<div dir="ltr" style="text-align: left;" trbidi="on"><div id="the-entry"><h1>Statins <span class="alias">HMG-CoA reductase inhibitors</span></h1><a href="http://medlibes.com/uploads/HMG-CoA_reductase_pathway.png" rel="lightbox[entry]" title="Statins"><img alt="Statins" id="large-no-text" src="http://medlibes.com/uploads/HMG-CoA_reductase_pathway.png" width="534" /></a><br />
<dl><ul id="entry-extras"><li><br />
<dt>Facts<b>:</b></dt><br />
<br />
<br />
<dd>Lower cholesterol by inhibiting the enzyme HMG-CoA reductase (<span class="underline"><b>rate-limiting enzyme of the mevalonate pathway</b></span> of cholesterol synthesis) <br />
<br />
Inhibition of this enzyme in the liver results in decreased cholesterol synthesis and increased synthesis of LDL receptors, resulting in an increased clearance of LDL</dd> </li>
<li><br />
<dt>Side Effects<b>:</b></dt><br />
<br />
<br />
<dd>Myalgias, muscle cramps <br />
Rhabdomyolysis </dd> </li>
<li><br />
<dt>Notes<b>:</b></dt><br />
<br />
<br />
<dd>Mevalonate is also used for the production of dolichol and CoQ10 <br />
<span class="underline"><b>Reduced CoQ10</b></span> is implicated in the pathogenesis of statin-induced myopathy </dd> </li>
</ul></dl></div></div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com0tag:blogger.com,1999:blog-7953616238799155857.post-77926625022136727422011-06-14T08:03:00.000+05:302011-06-14T08:03:12.756+05:30Aufbau Rule for filling electrons in orbitals<div dir="ltr" style="text-align: left;" trbidi="on">The <b>Aufbau principle</b> or <b>building-up principle</b> is used to determine the electron configuration of an atom, molecule or ion. The principle postulates a hypothetical process in which an atom is "built up" by progressively adding electrons. As they are added, they assume their most stable conditions with respect to the nucleus and those electrons already there.<br />
According to the principle, electrons fill orbitals starting at the lowest available (possible) energy states before filling higher states (e.g. 1s before 2s). The number of electrons that can occupy each orbital is limited by the Pauli exclusion principle.<br />
<br />
Unoccupied orbitals will be filled before occupied orbitals are reused.<br />
<h2><span class="mw-headline" id="The_Madelung_energy_ordering_rule">The Madelung energy ordering rule</span></h2><div class="thumb tright"><div class="separator" style="clear: both; text-align: center;"><a href="http://upload.wikimedia.org/wikipedia/commons/thumb/9/95/Klechkovski_rule.svg/330px-Klechkovski_rule.svg.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="275" src="http://upload.wikimedia.org/wikipedia/commons/thumb/9/95/Klechkovski_rule.svg/330px-Klechkovski_rule.svg.png" width="320" /></a></div><div class="thumbinner" style="width: 332px;"> Order in which orbitals are arranged by increasing energy according to the Madelung rule. Each diagonal red arrow corresponds to a different value of <i>n+l</i>. </div></div> The rule is based on the total number of nodes in the atomic orbital, <i>n+l</i>, which is related to the energy.<sup class="reference" id="cite_ref-0"><a href="http://en.wikipedia.org/wiki/Aufbau_principle#cite_note-0"><span></span></a></sup> In the case of equal <i>n+l</i> values, the orbital with a lower <i>n</i> value is filled first. The fact that most of the ground state configurations of neutral atoms fill orbitals following this <i>n+l,n</i> pattern was obtained experimentally, by reference to the spectroscopic characteristics of the elements.<sup class="reference" id="cite_ref-1"><a href="http://en.wikipedia.org/wiki/Aufbau_principle#cite_note-1"><span></span><span></span></a></sup><br />
The Madelung energy ordering rule applies only to neutral atoms in their ground state, and even in that case, there are several elements for which it predicts configurations that differ from those determined experimentally. Copper and chromium are common examples of this property. According to the Madelung rule, the 4s orbital (<i>n+l</i> = 4+0 = 4) is occupied before the 3d orbital (<i>n+l</i> = 3+2 = 5). The rule then predicts the configuration of <sub>29</sub>Cu to be 1s<sup>2</sup>2s<sup>2</sup>2p<sup>6</sup>3s<sup>2</sup> 3p<sup>6</sup>4s<sup>2</sup>3d<sup>9</sup>, abbreviated [Ar]4s<sup>2</sup>3d<sup>9</sup> where [Ar] denotes the configuration of Ar (the preceding noble gas). However the experimental electronic configuration of the copper atom is [Ar]4s<sup>1</sup>3d<sup>10</sup>. By filling the 3d orbital, copper can be in a lower energy state. Similarly, chromium takes the electronic configuration of [Ar]4s<sup>1</sup>3d<sup>5</sup> instead of [Ar]4s<sup>2</sup>3d<sup>4</sup>. In this case, chromium has a half-full 3d shell.<br />
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<br />
<br />
</div>Shilpihttp://www.blogger.com/profile/08600850374860787635noreply@blogger.com0