Special Pulse Sequences and Two-Dimensional NMR Spectroscopy

Abstract

One of the most dramatic advances in recent years has been the development of several new pulse sequences, made possible by the advent of dedicated microcomputers, which have allowed precise manipulations of pulse angles, introduction of delays between pulses, and rapid Fourier transformations. These have heralded the advent of a number of extremely powerful procedures among which may be mentioned: (a) new pulse sequences for the unambiguous assignments of methyl, methylene, methine, and quaternary carbon atoms, side—stepping the difficulties associated with the overlapping of multiplets in the normal off—resonances measurements. Examples are APT, DEPT, ADEPT, etc.; (b) two—dimensional NMR spectroscopy for observing couplings between protons, between protons and carbon atoms, and more recently between carbon atoms themselves; and (c) precise measurements of nuclear Overhauser enhancements by NOE difference measurements involving alternate recording of normal and NOE enhanced spectra, and automatic computer—assisted subtractions which result in measurements of even small enhancements, which were previously not possible.