THE VIBRATIONAL MECHANISM OF THE FUNDAMENTAL NH2 STRETCHING VIBRATIONS IN ANILINES

Abstract

The integrated intensities, frequencies, and half-band widths of the fundamental symmetric and asymmetric NH₂ stretching vibrations in 33 ortho-substituted anilines, measured in dilute carbon tetrachloride solution, have been examined in relation to the corresponding absorption band parameters for 31 meta- and para-substituted anilines, taking into consideration the electronic effects of the substituents. From an almost tetrahedral configuration in p-phenylenediamine, the calculated s-character of the nitrogen atom gradually increases as the substituent groups become more electron withdrawing, with a resultant increase in the apparent HNH angle and the NH force constant. Ortho substitution in general leads to enhanced HNH angle opening, probably because of intramolecular hydrogen bonding. The decrease in half-band width for both vibrational modes in ortho-substituted anilines with respect to corresponding values in meta and para compounds is ascribed to steric hindrance to solvation of the amino group.The asymmetric intensities in ortho-substituted anilines are generally increased over corresponding values in meta and para compounds, unlike the behavior of the symmetric mode. These results are consistent with a vibrational mechanism taking into account the following factors for each mode: (i) the direction of the transition moment, (ii) the extent of nitrogen lone pair and aromatic π-electron participation, and (iii) the direct field effect of the ortho substituent.