Features of H Spectra

Two-dimensional NMR provides powerful tecniques to aid interpretation, but the starting point is a simple, one-dimensional proton NMR spectrum, with careful integration to ascertain the relative numbers of protons in different lines or multiplets. In some instances one or two good 1H NMR spectra may be sufficient to solve the problem with little expenditure of instrument time. In other instances, where only minute amounts of sample are available, it may not be feasible to obtain any NMR data other than a simple H spectrum. However, as we pointed out in Chapter 3, with modern instrumentation and microprobes, it is usually possible to use indirect detection methods to obtain correlations with less sensitive nuclei, such as 13C and 15N, even with quite small amounts of sample. 

Before attempting to interpret an NMR spectrum, it is wise to ascertain whether it has been obtained under suitable experimental conditions so that it is meaningful. The symmetry and sharpness of the line due to TMS (or other reference) provides an indication of magnetic field homogeneity and general spectrometer performance. Most spectrometers designed for routine studies are sufficiently automated that reasonable sets of instrument parameters are selected, but it is possible to misset parameters for acquisition or data processing and obtain distorted spectra. 

Lines that are clearly due to impurities, to the solvent itself, or to a small amount of undeuterated solvent can usually be identified. Water is often present in 

An examination of the relative areas of the NMR lines or multiplets (resolved or unresolved) is often the best starting point for the interpretation of the spectrum. If the total number of protons in the molecule is known, the total area can be equated to it, and the number of hydrogen atoms in each portion of the spectrum established. The opposite procedure of assigning the smallest area to one or two protons and comparing other areas with this one is sometimes helpful, but it should be used with caution because appreciable error can be introduced in this way. Occasionally, lines so broad that they are unobservable in the spectrum itself can be detected in the integral trace. 

The positions of strong, relatively sharp lines can usually be correlated with expected chemical shifts. This correlation, together with the area measurements, frequendy permits the establishment of a number of methyl and methylene groups. In organic solvents such as CDC13 exchangeable protons can usually be observed as single lines and can often be identified by addition of a drop of D20 to the sample and resultant disappearance of peaks. 

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