Second-Order Quadrupolar-Dipolar Broadening in Two-Dimensional Multiple-Quantum MAS NMR

Abstract 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 an inverse dependence on the Larmor frequency, ω0. 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.

Two-dimensional 23Na triple-quantum MAS NMR spectra of (a-c) NaClO3 and (d-f) NaBrO3.