Introduction and General Theory

Abstract

Nuclear magnetic resonance (NMR), which originated some 40 years ago primarily as a potentially accurate method for measuring nuclear magnetogyric ratios, turned out to be something of an embarrassment in that application when it transpired that the rf magnetic susceptibility it measured could be a quite complicated function, exhibiting many sharp, close-lying resonances. When it was realized however, that this complexity rather subtly reflected exceedingly fine characteristics of the electronic environment in which the nuclei were embedded, NMR began being developed as a high-resolution (HR) spectroscopic technique for the elucidation of molecular structure, dynamics, and, most recently, distribution (i.e., NMR imaging). Here again it soon became apparent that HR-NMR spectra were generally too complicated to admit of straightforward, unambiguous interpretations of molecular structures. Major effort, since then, has been spent on developing ever more powerful methods to help produce and interpret HR-NMR spectra. On the experimental side, this has, on the one hand, led to attempts (a) to develop NMR as a truly multinuclear technique, and (b) to improve the sensitivity or signal-to-noise ratio of NMR spectra as well as their resolution. On the other, people have sought to devise NMR experiments that can generate unambiguous, clearly recognizable features in the spectra by various means of selectively monitoring different kinds of nuclear magnetic interactions while suppressing others as required. Pulse Fourier transform (FT) NMR has emerged as the method of choice, allowing the spectroscopist maximum flexibility in the pursuit of practically any combination of these objectives. In conjunction with the rapid advances in the commercially available instrumentation, this situation has led to an explosion in the development of new techniques in NMR that shows no signs of letting up.