Chemical shielding tensor and 13C–14N dipolar splitting in single crystals of <scp>L</scp>-alanine

Abstract

Chemical shielding tensors have been determined for three chemically distinct carbons in a single crystal of L-alanine using proton-enhanced 13C NMR. The results indicate that the most shielded direction is perpendicular to the sp2 plane for the carboxyl carbon and the C3 axis for the methyl carbon. 13C–14N dipolar splittings have been observed for the Ca carbon, causing further complication in spectral analysis. Since the 14N quadrupole coupling constant is of comparable order with the 14N Zeeman interaction, 14N quadrupole interaction is effective in the 13C–14N dipolar splittings. The 14N quadrupole effect is observed and demonstrated by calculation and the sign of the quadrupole coupling constant e2Qq has been determined to be positive. The chemical shielding tensor for the Ca carbon is determined by considering the 13C–14N dipolar interaction and the 14N quadrupole interaction. It is shown that the 14N quadrupole interaction causes the asymmetric doublet pattern in the 13C NMR signal for the Ca carbon in magic angle spinning (MAS) experiments.