THEORETICAL AND SPECTROSCOPIC STUDY OF GASEOUS MONOHYDROGENATED TOLUENE AND 4-METHYLPYRIDINE

The infrared and Raman spectra of gaseous monohydrogenated toluene and gamma-picoline are recorded. They are analyzed with a theoretical mod el that takes into account, in the adiabatic approximation, coupling b etween the internal rotation of the methyl group and the methyl CH str etching vibration. Ab initio calculations at the HF/6-31G* level of t heory are performed to determine the principal parameters used in this model and their variation with the internal rotation coordinate. Reco nstruction of the upsilon(CH) fundamental spectra of both compounds an d the two first upsilon(CH) overtone spectra of toluene present a good agreement with the experimental findings. Both molecules exhibit almo st the same spectral profiles, a signature of similar internal dynamic s of the methyl group essentially governed by the internal rotation po tential. Comparison of these results with those previously obtained fo r monohydrogenated nitromethane reveals that the molecular environment of the methyl group (NO2, C6D5, and C5ND4) is at the origin of some s pectral differences, essentially related to differences in electric an harmonicity.