It has been recently
shown that 2D multiple-quantum (MQ) isotropies can be bypassed to first order by focusing the spectroscopy in combination with conventional magic-angle spin-NMR observation on the central 0 1 2 } / 1 2 transition (8, 9), ning can provide high resolution NMR spectra from half-integer the resulting spectra may still lack chemical resolution due quadrupolar nuclei in powdered samples. In an effort to optimize to the presence of substantial second-order effects. Rapid signal-to-noise in this type of experiments, a systematic investigasample rotation can be employed toward the partial nartion on the relative efficiencies that single-and two-pulse excitarowing of these central transition quadrupolar powder pattion schemes exhibit toward the generation of MQ coherences was terns (10, 11), but the absence of a single ''magic'' angle carried out. Numerical simulations on spin-3 2 powders revealed capable of removing all second-order effects still leaves that x-y composite pulse excitations with nutation angles u y É 2 spectra thick with overlapping lines from inequivalent sites. u x maximize the excitation of triple-quantum coherences, yielding about 30% larger signals than those available from single-pulse
The overcoming of this important resolution limitation experiments. This behavior was corroborated experimentally, and was finally achieved thanks to a deeper theoretical underits origin elucidated with the aid of analytical derivations. Comstanding of second-order effects in NMR (12,13). These posite pulse schemes also yielded a superior excitation of MQ developments lead to two practical techniques, dynamiccoherences in the presence of large shielding offsets, although a angle spinning (DAS) and double rotation (DOR), capable simple recipe for optimizing such experiments was not apparent. of removing the central transition anisotropies via complex ᭧ 1998 Academic Press mechanical manipulations based on multiaxial sample rota-Key Words: solid state NMR; quadrupole nuclei; multiple quantions (14, 15). Recently we demonstrated that an averaging tum spectroscopy; composite pulses.
similar to that occurring in DAS can also be achieved using conventional MAS, if this method is combined with certain forms of 2D multiple-quantum (MQ) spectroscopy involvistry ( M / C 111 ) ,