Isotropic solid-state MQMAS NMR spectra for large quadrupolar interactions using satellite-transition selective inversion pulses and low rf fields

Multiple-quantum magic-angle spinning (MQMAS) pulse sequences are presented that are capable of obtaining isotropic NMR spectra for large quadrupolar interactions using lower rf fields. These experiments rely on rotor period long pulses applied at a large offset from the central-transition, making them selective to the satellite-transitions. Each such pulse gives rise to an anisotropic phase, which can be cancelled to obtain coherent signal evolution if a pair of pulses are applied in a symmetric manner. Thus, efficient excitation and conversion of triple-quantum coherences from and to the central-transition is achieved for MQMAS even for large quadrupolar couplings, by selective inversion of the satellite-transitions using such low-power pulses. Low-power multiple-quantum magic-angle spinning (lpMQMAS) pulse sequences are demonstrated on a model compound, RbNO3, and also applied on beta-Ga2O3, a sample with the largest quadrupolar interactions for which isotropic NMR spectra have been obtained to date. (C) 2021 Elsevier Inc. All rights reserved.

Automated summary

The multiple-quantum magic-angle spinning (MQMAS) pulse sequences allow for obtaining isotropic NMR spectra for large quadrupolar interactions using lower rf fields. By selectively inverting the satellite-transitions with low-power pulses, efficient excitation and conversion of triple-quantum coherences is achieved even for large quadrupolar couplings. The low-power multiple-quantum magic-angle spinning (lpMQMAS) pulse sequences have been demonstrated on samples with large quadrupolar interactions, showing promising results for isotropic NMR spectra.