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

N-14 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 N-14 chemical shift anisotropies (CSAs) on the order of \delta(sigma)\ = 10-30 ppm along with N-14 quadrupole coupling constants in the range of 10-70 kHz. The magnitude and sign of the CSAs determined from N-14 MAS NMR are confirmed by recording and, analysis of the corresponding slow-speed spinning (500-650Hz) N-15 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 N-14 MAS spectra at much higher spinning speeds than can be applied to retrieve the corresponding N-15 CSAs from the ssb pattern in the N-15 CP/MAS spectra. (C) 2003 Elsevier Science (USA). All rights reserved.