Nuclear magnetic resonance spectrometer applications
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The nuclear magnetic resonance (NMR) spectra of many compounds of a number of the more common elements have been studied during the past ten years. Among the nuclei employed have been those of H', F19, P31, B'l, N14, 0'7, and SiZ9. The NMR spectra of the first three of these, especially, have proved very useful in the solution of practical problems of structure determination and analysis. Some theoretical treatments of these spectra have also appeared, but except for some recent work on proton spectra they have had little success in actually calculating relative shifts. Despite the great practical and theoretical importance of carbon, it has not been the subject of similar studies because of the low abundance (1.1 per cent) of C13, the only stable isotope of carbon with nonzero spin. Aside from its low abundance, however, c'I3 is quite suitable for NMR spectroscopy. The magnetic moment is sufficiently large for the resonances to fall in a convenient frequency range, and the spin is one half, precluding quadrupole effects. has shown that, with proper apparatus and techniques, it is possible to obtain NMR spectra of C13 in natural abundance in a wide variety of compounds. The signal-to-noise ratios are sufficient to permit the study of fairly large molecules and of mixtures of two or more compounds. The chemical shifts are large and occur within fairly well-defined ranges for many types of compounds. They are also sensitive to small changes in structure so that even closely similar compounds can often be distinguished. The indirect spin-spin couplings between carbon and directly bonded hydrogen are also fairly large and give rise to characteristic multiplets that are very helpful in interpreting the spectra. The shifts also vary smoothly within series of related compounds and, therefore, often can be predicted with considerable accuracy. Although the signal-to-noise ratios are not sufficiently high at the present time to permit the use of the spectra in qualitative analysis as freely as proton, fluorine, and phosphorus spectra, a moderate improvement in this respect might lead to widespread practical applications. The theory of chemical shifts, on the other hand, may receive an immediate impetus from the availability of a large body of C13 data. Many types of compounds are being studied, and complete series of related simple compounds are readily available. Examples of these are the methyl derivatives of the elements, polysubstituted methanes, aromatic compounds, and heterocyclics. Many of these have undergone numerous investigations as to such characteristics as their chemical properties and absorption spectra, and have been considered in theoretical investigations of molecular structure.
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