Unusual observation of nitrogen chemical shift anisotropies in tetraalkylammonium halides by 14N MAS NMR spectroscopy.

14N Magic-angle spinning (MAS) NMR spectra for a number of polycrystalline, symmetrical tetraalkylammonium halides with short alkyl chains (C2H5– to n-C4H9–) have been recorded following a careful setup of the experimental conditions. Analysis of the spectra demonstrates the presence of 14N chemical shift anisotropies (CSAs) on the order of |δσ|=10–30ppm along with 14N quadrupole coupling constants in the range of 10–70kHz. The magnitude and sign of the CSAs determined from 14N MAS NMR are confirmed by recording and analysis of the corresponding slow-speed spinning (500–650Hz) 15N CP/MAS NMR spectra. Most interestingly, it is observed experimentally and demonstrated theoretically and by simulations, that these CSAs are reflected in the spinning sideband (ssb) intensities of the 14N MAS spectra at much higher spinning speeds than can be applied to retrieve the corresponding 15N CSAs from the ssb pattern in the 15N CP/MAS spectra.