Composite pulse excitation schemes for MQMAS NMR of half-integer quadrupolar spins

It has been recently shown that 2D multiple-quantum (MQ) spectroscopy in combination with conventional magic-angle spinning can provide high resolution NMR spectra from half-integer quadrupolar nuclei in powdered samples. In an effort to optimize signal-to-noise in this type of experiments, a systematic investigation on the relative efficiencies that single-and two-pulse excitation schemes exhibit toward the generation of MQ coherences was carried out. Numerical simulations on spin-3/2 powders revealed that x-y composite pulse excitations with nutation angles theta(y) approximate to 2 theta(x) maximize the excitation of triple-quantum coherences, yielding about 30% larger signals than those available from single-pulse experiments. This behavior was corroborated experimentally, and its origin elucidated with the aid of analytical derivations. Composite pulse schemes also yielded a superior excitation of MQ coherences in the presence of large shielding offsets, although a simple recipe for optimizing such experiments was not apparent. (C) 1998 Academic Press.