The influence of the radiofrequency excitation and conversion pulses on the lineshapes and intensities of the triple-quantum MAS NMR spectra of I=3/2nuclei

A rigorous examination of the various multiple-quantum magic angle spinning sequences is carried out with reference to sensitivity enhancement in the isotropic dimension and the lineshapes of the corresponding MAS peaks in th e anisotropic dimension. An echo efficiency parameter is defined here, whic h is shown to be an indicator of the performance aspects of the various seq uences. This can be used in the design of further new experiments in this f ield. A consequence of such a systematic analysis has been the combination of a spin-lock pulse for excitation of multiple-quantum coherences and an a mplitude-modulated pulse for their conversion to observable single-quantum coherences. This approach has resulted in an improved performance over othe r sequences with respect to both the anisotropic lineshapes and the isotrop ic intensities. (C) 2000 Academic Press.