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I. GENERAL When a sample containing hydrogen is placed in the Although it is assumed that the reader has been exposed static magnetic field, each hydrogen nucleus will precess to the elementary theory of NMR and to the operation at a frequency determined by the magnetic field it of an NMR spectrometer, a brief review of some of the actually experiences. This field, in turn, is determined by basic concepts and definitions will indicate the point of the electronic, and therefore the chemical, environment view used in this book and clarify some of the defini- of the nucleus. Thus the variety of chemical environ- tions. The discussion is confined to the hydrogen-l iso- ments that exist in a molecule will produce a spectrum tope because this is by far the most generally used and, of precession frequencies that will indicate the chemical consequently, far more data are available for it than for nature of the various parts of the molecule. The remain- any other isotope. This wealth of data, in turn, leads to ing problem is to observe this spectrum of frequencies. the most accurate and comprehensive set of spectra- There are two general methods of observing the structure correlations. spectrum.