Residual dipolar couplings in MAS and MQMAS NMR spectra of quadrupolar nuclei

We report the first observation of residual dipolar couplings in magic-angle-spinning (MAS) and multiple quantum magic-angle-spinning (MQMAS) NMR spectra of half-integer quadrupolar nuclei. In particular, the fine spectral splittings observed in the 11B MAS spectrum of crystalline triethanolamine borate are attributed to the residual 11B–14N dipolar coupling, which is not completely averaged by MAS. This interpretation is corroborated by the observation of an asymmetrical doublet in the 11B MQMAS spectrum.