NMR and Conformations

It isn t often that someone goes to the emergency room because of a headache, and when the staff discovered that the man who did was due in court for sentencing the next day, some of them felt that there might not be anything wrong with him at all. There vras. 

The man s behavior toward the staff soon convinced them that he shouid be admitted, kept overnight, and seen by a neu-roiogist the next day. After a preliminary examination, a magnetic resonance image, or MRI, was ordered which revealed a brain tumor. The tumor was located in the right frontal cortex, a portion of the brain known to be involved in controlling impulsive behavior. 

Once the tumor was found, it was surgically removed. The man s ability to curb his impulses was restored, his mental, graphical, and writing skills improved to the normal range, and he successfully completed a rehabilitation prc am. About a year later though, the headaches and some of the earlier behaviors returned. When a new MRI showed that the tumor had regrown, it was removed and again the symptoms disappeared. 

An MRI scanner is an NMR machine large enough to accommodate a human being, has a powerful magnet, operates in the pulse-FT mode, and detects protons—usually the protons in water and, to a lesser extent, lipids. The principles are 

A selective pulse is used in order to excite protons in a particular slice of the object to be imaged. 

We know from Chapter 3 that the protons in cyclohexane exist in two different environments axial and equatorial. The NMR spectrum of cyclohexane, however, shows only a single sharp peak at 8 1.4. All the protons of cyclohexane appear to be equivalent in the NMR spectrum. Why  

The answer is related to the very rapid rate of chair-chair interconversion in cyclohexane. 

One property of NMR spectroscopy is that it is too slow a technique to see the individual conformations of cyclohexane. What NMR sees is the average enviromnent of the protons. Since chair-chair interconversion in cyclohexane converts each axial proton to an equatorial one and vice versa, the average environments of aU the protons are the same. A single peak is observed that has a chemical shift midway between the true chemical shifts of the axial and the equatorial protons. 

The rate of ring flipping can be slowed down by lowering the temperature. At temperatures on the order of -100°C, separate signals are seen for the axial and equatorial protons of cyclohexane. 

We pointed out in Section 13.3 that both H and are nuclei that can provide useful structural information when studied by NMR. Although a H NMR spectrum helps us infer much about the carbon skeleton of a molecule, a NMR spectrum has the obvious advantage of probing the carbon skeleton directly. NMR spectroscopy is analo- 

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The structure, NMR and conformational properties of various tetroxocanes are discussed in Section 5.19.9. 

The polysaccharides from proteoglycans have been very extensively analyzed by means of X-ray and CD spectroscopy [209, 210]. This section will review only the newer developments based on NMR and conformational calculations. 

N. E. Nifantiev, A. S. Shashkov, G. M. Lipkind, and N. K. Kochetkov, Synthesis, NMR, and conformational studies of branched oligosaccharides. 7. Synthesis and l3C NMR spectra of 2,3-di-O-glycosyl derivatives of methyl a-L-rhamnopyranoside and methyl a-D-mannopyrano-side, Carbohydr. Res., 237 (1992)95-113. 

Perry NB, Aiyaz M, Kerr DS, Lake RJ, Leach MT (2001) NOESY on Neurotoxins NMR and Conformational Assignment of Picrotoxins. Phytochem Anal 12 69... 

L. A. Mulard, M.-J. Clement, A. Imberty, and M. Delepierre, Synthesis of ligands related to the O-specific polysaccharides of Shigella flexneri serotype 2a and Shigella flexneri serotype 5a. Part 7. Convergent synthesis, NMR and conformational analysis of tetra- and pentasaccharide haptens of the Shigella flexneri serotype 5a O-specific polysaccharide, Eur. J. Org. Chem, (2002) 2486-2498. 

In the following sections emphasis will be laid upon some practical aspects of NMR and conformational notation as an aid in interpretation of the tables rather than upon the underlying theoretical concepts. 

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