Theoretical and Spectroscopic Study of Gaseous Monohydrogenated Toluene and 4-Methylpyridine

Abstract

The infrared and Raman spectra of gaseous monohydrogenated toluene and γ-picoline are recorded. They are analyzed with a theoretical model that takes into account, in the adiabatic approximation, coupling between the internal rotation of the methyl group and the methyl CH stretching vibration. Ab initio calculations at the HF/6-31G** level of theory are performed to determine the principal parameters used in this model and their variation with the internal rotation coordinate. Reconstruction of the ν(CH) fundamental spectra of both compounds and the two first ν(CH) overtone spectra of toluene present a good agreement with the experimental findings. Both molecules exhibit almost the same spectral profiles, a signature of similar internal dynamics of the methyl group essentially governed by the internal rotation potential. Comparison of these results with those previously obtained for monohydrogenated nitromethane reveals that the molecular environment of the methyl group (NO2, C6D5, and C5ND4) is at the origin of some spectral differences, essentially related to differences in electric anharmonicity.