Requirements for High Resolution NMR

The components of an NMR system involve apparatus that is mechanically and electronically quite sophisticated. Particularly for the study of the narrow lines 

As we saw in Section 2.5, NMR lines from small molecules in the liquid phase often have a natural line width of less than 1 Hz (frequently a small fraction of a hertz), and as will be seen later, lines separated by 1 Hz or less must sometimes be resolved to obtain information of chemical interest. For proton NMR, rf frequencies of 200—800 MHz are usually used hence a resolution of the order of 10 9 is often required. In addition to careful fabrication of the magnet, three approaches are used to obtain adequate homogeneity over the sample volume, as follows  

A considerable improvement in effective homogeneity across the sample is often achieved by spinning the sample tube about its axis. If the field gradient across the sample in a direction transverse to the spinning axis is given by AB, 

Restriction of the sample volume can reduce substantially the overall inhomogeneity across the sample. In most instruments the effective volume of the sample i.e., that within the rf receiver coil) is restricted to about 0.1 ml, when the sample is placed in a 5-mm-diameter cylindical tube—a size that is regarded as the standard NMR tube. However, larger diameter tubes (8—15 mm) are sometimes preferred for studying dilute solutions or for observing nuclei of low sensitivity, even with some loss of homogeneity across the sample. (As we see in Section 3.11, much smaller diameter tubes are also valuable when the total amount of sample is limited.) 

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The anisotropic nature of the dipolar, quadrupolar and chemical shift anisotropy interactions requires that the isotropy of molecular orientation relative to the applied magnetic field be broken in order to allow their direct observation in terms of shifts in the frequencies of resonances.20,32,38 40 For high resolution NMR studies this has meant, thus far, that some degree of alignment of the molecule needs to be established. Almost all molecules will align to a small extent due to the anisotropy of their magnetic... 

The incorporation of carbon black into elastomeric systems is a process of significant commercial importance. However, the additional stiffness of the sample imparted by the reinforcement effect of fillers is not favourable in terms of the experimental conditions for high-resolution NMR spectroscopy. Electric conductivity of the carbon black may also interfere to some extent. Under these circumstances, filled formulations are not widely used for the study of elastomer vulcanisations where high resolution and signal-to-noise ratios are required to detect small amounts of vulcanisation products. 

Combined rotation and multiple-pulse spectroscopy (CRAMPS). A special pulse sequence, in addition to MAS, is required for high-resolution proton NMR in solids. This technique is known as CRAMPS. 

Consider the frequency domain again. The requirement for a sharp cutoff arises because we want to cut off as much noise as possible and the sharper the filter cutoff the closer it can be set to the spectral feature to be saved. It is clear that the filter cutoff needs to be only as sharp as the spectral feature. Therefore, a careful consideration of filter characteristics is most important for high resolution NMR, whereas the RC filter is tolerably good for very broad lines such as those in solids observed by normal NMR. But since the opportunity for some oversampling is much greater for high resolution NMR than in wideline NMR, the absolute necessity for an "ultrasharp" filter is not as great as it is supposed in either case. 

It is probably more difficult to prepare a polymer sample for high-resolution NMR than for infrared spectroscopy, since generally a liquid is required for NMR spectroscopy. I use the word generally here because recent work was reported where a solid polymer was spun at a high speed and chemical shifts were observed that were not found on the sample that was not spun. 

A coirplete understanding of the role of carbohydrates in biological systems requires knowledge of the distribution at equilibrium of the various conformers in aqueous solution. The conformational behavior of carbohydrates in solution can be examined from different vantage points (1,), but the most relevant approach is, no doubt, study of dilute solutions themselves. At present, high resolution NMR spectroscopy is the primary tool for determination of three-dimensional structure of oligosaccharides in solution. Optical rotation is also very sensitive to conformation (2) and there is a new, semi-enqpirical theory of optical rotation of oligosaccharides ( ). 

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