Second-order quadrupolar-dipolar broadening in two-dimensional multiple-quantum MAS NMR

Second-order perturbation theory predicts the existence of a 'cross term' between the quadrupolar and dipolar interactions of two spin I = 3/2 nuclei. This cross term manifests itself as a broadening in solid-state NMR spectra of spin I = 3/2 nuclei which cannot be fully removed by magic angle spinning (MAS) and has on inverse dependence on the Larmor frequency, omega(0). In these attributes, the second-order quadrupolar-dipolar broadening does not differ from pure second-order quadrupolar broadening. However, we show here that the recently developed two-dimensional multiple-quantum (MQ) MAS NMR technique, designed originally to suppress second-order quadrupolar broadening, allows the two broadening interactions to be separated and quantified. (C) 1999 Elsevier Science B.V. All rights reserved.