Vibronic spectra and normal coordinate analysis of substituted anilines—I. 2,3-, 2,4-, and 2,5-dimethylanilines

Abstract

Vibronic spectra of 2,3-, 2,4-, and 2,5-dimethylanilines (DMA) have been recorded in the vapor phase and analysed primarily for ascertaining the frequencies of certain low lying fundamentals (< 1000 cm−1) which were observed earlier in infrared and Raman spectra with rather poor intensities. In most cases, the investigation of vibronic spectra has supported the data observed in infrared and Raman spectra. However, a new frequency has been observed at ∼120 cm−1 in each molecule and has been assigned to the CNH2 out-of-plane wag. A detailed analysis of the vibronic spectra of these three molecules revealed that an interaction-induced blue-shift of nearly 1700 cm−1 is caused in the (0, 0) band of all the three DMAs due to an electrostatic interaction between the NH2 and CH3 groups. The normal coordinate analyses (NCA) of the above three molecules have been carried out using a general valence force field (GVFF) with 21 principal and 30 interaction force constants, assuming an averaged structure, a planar geometry, and a Cs symmetry in each case. The fundamental frequencies were mostly taken from an earlier work reported from this laboratory. Some frequencies were also picked up from the investigation of the vibronic spectra reported herein. The force field calculations were carried out using the least-squares iterative technique. During final calculations, all the 51 force constants were allowed to iterate simultaneously, which reproduced the 52 experimentally observed fundamentals out of the 54 (35a′+ 19a″) expected ones within an average error of ± 1.0% with a reasonable potential energy distribution (PED) among the various normal modes.