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. Figure 1 shows the C-NH₂ signal of 2-methyl-4-nitroaniline. 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.

Fig. 1. Temperature dependence of the C-NH₂ signal in ¹³C CP/MAS NMR spectra of 2-methyl-4-nitroaniline, measured at 50.32 MHz.