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

Second-order perturbation theory predicts the existence of a `cross termʹ 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.