Multiple-quantum magic-angle spinning NMR: A new method for the study of quadrupolar nuclei in solids

Whereas solid state isotropic spectra can be obtained from spin-1/2 nuclei by fast magic-angle spinning (MAS), this methodology fails when applied on half-integer quadrupoles due to the presence of non-negligible second-order anisotropic effects. Very recently, however, we have shown that the combined use of MAS and bidimensional multiple-quantum (MQ) spectroscopy can refocus these anisotropies; the present paper discusses theoretical and experimental aspects of this novel MQMAS methodology and illustrates its application on a series of sodium salts. It is shown that even under fixed magnetic field operation, a simple model-free inspection of the peaks in a bidimensional MQMAS NMR spectrum can separate the contributions of isotropic chemical and isotropic quadrupolar shifts for different chemical sites. Moreover the anisotropic line shapes that can be resolved from these spectra are almost unaffected by excitation distortions and can thus be used to discern the values of a site's quadrupolar coupling constant and asymmetry parameter. The conditions that maximize the MQMAS signal-to-noise ratio for a spin-3/2 are then explored with the aid of a simple analytical model, which can also be used to explain the absence of distortions in the anisotropic line shapes. The MQMAS method thus optimized was applied to the high-resolution Na-23 NMR analysis of the multi-site ionic compounds Na2TeO3, Na2SO3, Na3P5O10, and Na2HPO4; extensions of the MQMAS NMR methodology to the quantitative analysis of inequivalent sites are also discussed and demonstrated.