Internal Methyl Rotation in the CH Stretching Overtone Spectra of Toluene-α-d2, -α-d1, and -d0

Abstract

The room-temperature vapor phase overtone spectrum of toluene-α-d2 has been recorded in the CH stretching region corresponding to ΔvCH = 2−6 with conventional near-infrared spectroscopy (ΔvCH = 2−4) and with intracavity titanium:sapphire and dye laser photoacoustic spectroscopy (ΔvCH = 4−6). Both absolute oscillator strengths (conventional spectra) and relative oscillator strengths within a given overtone (conventional and photoacoustic spectra) have been measured. The aryl region of the spectrum is interpreted in terms of two nonequivalent aryl local modes and is essentially identical to the aryl regions of the spectra of toluene-d0 and toluene-α-d1. However, the methyl band profile differs significantly in these three molecules. We use an anharmonic oscillator local mode model and an ab initio dipole moment function to calculate oscillator strengths for the aryl and methyl transitions. Parameters for this model come from a fit of the aryl transition energies and from a fit of the methyl spectral profiles. These simple calculations give values that are in good agreement with observed absolute and relative intensities. Differences in the methyl profiles in toluene-d0, -α-d1, and -α-d2 are ascribed to coupling between CH stretching and methyl torsional modes. The methyl profiles are simulated on the basis of a simple adiabatic model that incorporates the harmonically coupled anharmonic oscillator local mode approach and our intensity calculations. The model successfully accounts for the change in methyl profiles between the three molecules, demonstrates the importance of torsional stretching coupling, and shows that coupling between the CH stretching oscillators is unimportant for higher overtones (ΔvCH ≥ 4).