The effect of internal rotation on the methyl CH-stretching overtone spectra of toluene and the xylenes

Abstract

The structure of methyl CH-stretching overtone bands in the vibrational spectra of methylbenzenes was investigated theoretically. The anharmonic CH-stretching vibration, described by a Morse potential, was represented in terms of a harmonic basis while hindered internal rotation of the methyl group was represented by a rigid rotor attached to an infinitely massive frame. Relatively weak coupling between the anharmonic CH vibration and the hindered internal rotation is sufficient to shift the positions of rovibrational lines from a PQR-like rotational contour to patterns similar to those observed experimentally. For high rotational barriers, as in o-xylene, the rovibrational transitions form two bands associated with conformationally nonequivalent CH-bonds, consistent with the conformational preference established by microwave spectroscopy and molecular orbital calculations. For nearly free internal rotation, as in toluene, m-xylene and p-xylene, a prominent middle band is also present. This ‘‘free rotor’’ band corresponds to rotational transitions between states high above the barrier and disappears as the barrier height increases. The outer bands correspond to transitions for which either the initial or the final state is below or near the barrier height in energy. Contrary to earlier suggestions, the band structure is not indicative of the conformational preference of the methyl group in toluene. In fact, the calculated spectra of nearly free internal rotors are insensitive to this preference.