Multiple-pulse sequences for homonuclear decoupling. Experimental verification

Abstract

The manipulation of a spin system to isolate or enhance one interaction while removing the effects of all other interactions is an ability that is used constantly in solid-state NMR (l-3). An increasing number of experimental approaches are being developed to enhance this ability and probe interactions which are present in a spin system in even greater detail. One fruitful area has been in the arena of homonuclear decoupling in the solid state. In this instance, the line-broadening effects of homonuclear dipolar couplings are removed, revealing the underlying finer details, e.g., the chemicalshift interaction. Beginning with the pioneering WAHUHA pulse sequence (4), a number of pulse sequences have been developed for the task of homonuclear decoupling (2, 5-7). Much of the recent work requiring the need for homonuclear decoupling has involved the use of MREV-8 (8) or BR-24 (9) for the observation of ‘H or 19F in hydrogenor fluorine-rich solid materials. When combined with magic-angle spinning (CRAMPS), it is possible, in some cases, to obtain highly resolved solid-state ‘H NMR spectra (6 ) . Many of these homonuclear-decoupling multiple-pulse sequences owe their improved decoupling performance to supercycling of the basic WAHUHA sequence. In the WAHUHA sequence, homonuclear decoupling is achieved by applying a train of 7r/2 pulses, phase shifted by 90” from one another, and sampling the magnetization at specific intervals. At these specific times, the spin system appears to evolve in the absence of homonuclear dipolar couplings. The improved line-narrowing performance of many of these multiple-pulse sequences has come at the expense of longer cycle times and, in the case of MREV-8 and BR-24, a greater scaling of those interactions linear in I,. What one must keep in mind is that these multiple-pulse sequences are but one family in a larger set. The WAHUHA sequence and pulse sequences built upon it have tip angles near 7r/2, identical pulse widths, constant pulse delays, and phase shifts of 90”. Other homonuclear decoupling pulse sequences might be found by removing constraints on tip angles, pulse widths, pulse delays, and phase shifts. Recently Liu et al. (IO) introduced a class of semiwindowless sequences with nonquadrature