The far-infrared vapor phase spectra of the methylanilines

The potential energy barriers opposing coplanarity of the amino group and aromatic ring in 2-, 3-, and 4-methylaniline are found to be 558, 528, and 588 cm−1, respectively. The results show a reasonable correlation with experimental 15N NMR results and also (combined with barriers for halo-substituted anilines) with Hammett σ substituent constants. The trend in barrier heights is consistent with the concept that the methyl groups influence the electron density of the amino nitrogen more through the π system than through an inductive effect involving the σ bonds, as calculations reported elsewhere indicate.