Differential broadening caused by dipolar interaction with1H in13C MAS NMR signals split by residual dipolar interaction with14N

Abstract

The 13C cross‐polarization magic angle spinning NMR spectra of various nitroanilines and some other N‐containing compounds were measured and the differential broadening between the signals split by the residual dipolar interaction with 14N was investigated. The dipolar interaction between 14N and 1H produces 13C differential broadening as well as the 13C–14N interaction does, whereas the dipolar interaction between 13C and 1H is discounted as its origin. The differential broadening caused by 1H is observed in the C*—NHR (R = H and CH3) groups at the para and ortho positions to the NO2 group in nitroanilines, C—NH2 in m‐nitroaniline and 13C bonded to the amide NH group, where 14N is directly bonded to 1H and molecular motions are restricted by hydrogen bondings. At higher temperatures the differential broadening decreases in the para‐type nitroanilines. The 1H‐caused differential broadening is not observed for the C—NH2 group in the meta‐type nitroanilines except for m‐nitroaniline, C—NO2, C*—N(CH3)2 and C—NH3 groups. Molecular motions can reduce the differential broadening, which are presumably the 180° flip of the NH2 group and the rotation of the NH3 group around the C—N axis. Copyright © 1999 John Wiley & Sons, Ltd.