Novel two-dimensional NMR methods that combine single-quantum cross-polarization and multiple-quantum MAS of quadrupolar nuclei

A new approach to combining cross-polarization with multiple-quantum MAS in a two-dimensional NMR experiment is demonstrated, involving cross-polarization from 1H to the single-quantum coherences of a quadrupolar nucleus. These coherences are then converted directly to multiple-quantum coherences rather than via a population state. In two separate methods, pure-absorption lineshapes are obtained using a z-filter and a shifted echo, respectively, with the latter, a `reversed split-t1ʹ method, demonstrated to be particularly effective. The efficiency of both sequences may be improved by incorporating `fast-amplitude-modulationʹ (FAM) pulses for both the conversion and excitation of triple-quantum coherences.