Amplitude-Modulated Pulse Shapes for the Elimination of Cross Relaxation during Multiple-Pulse NMR Experiments

Abstract

New amplitude-modulated windowless multiple-pulse sequences for the elimination of cross-relaxation effects in 2D NMR spectroscopy of macromolecules are described. The sequences remove cross-relaxation responses near resonance by continuous rapid averaging of the transverse and longitudinal cross-relaxation rates, which have opposite signs for large molecules. Experimental results for an application to chemical-exchange spectroscopy, in which both cross-relaxation and J-coupling responses are removed, are presented. Within the working bandwidth, the spins are effectively spin locked along the y axis of the rotating frame, resulting in 2D spectra that are easily phased. For strictly positive wave forms, variational methods are used to obtain analytical solutions for minimum energy deposition per pulse under the additional constraints of minimum dynamic range, specified minimum field, or specified 90° pulse length. © 1994 by Academic Press, Inc.