Differential broadening between the signals split by the residual dipolar interaction with 14N in 13C CP/MAS NMR spectra of nitroanilines

The 13C CP/MAS NMR spectra of various nitroanilines have been measured, and the origin of the differential broadening observed between the signals split by the residual dipolar interaction with 14N has been clarified experimentally. The dipolar interaction between 1H and 14N is responsible for the 13C differential broadening, while the direct dipolar interaction with 1H is neglected. A molecular motion can eliminate the differential broadening, which is presumably the 180° flip of the NH2 group around the C–N axis. Therefore, the differential broadening is highly correlated to the molecular motion and the hydrogen bonding.