Internal methyl rotation in the CH stretching overtone spectra of toluene-alpha-d(2), -alpha-d(1), and -d(0)

The room-temperature vapor phase overtone spectrum of toluene-alpha-d(2) ha s been recorded in the CH stretching region corresponding to Delta v(CH) = 2-6 With conventional near-infrared spectroscopy (Delta v(CH) = 2-4) and wi th intracavity titanium:sapphire and dye laser photoacoustic spectroscopy ( Delta v(CH) = 4-6). Both absolute oscillator strengths (conventional spectr a) and relative oscillator strengths within a given overtone (conventional and photoacoustic spectra) have been measured. The aryl region of the spect rum is interpreted in terms of two nonequivalent aryl local modes and is es sentially identical to the aryl regions of the spectra of toluene-d(0) and toluene-alpha-d(1). 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 f or the aryl and methyl transitions. Parameters for this model come from a f it of the aryl transition energies and from a fit of the methyl spectral pr ofiles. These simple calculations give values that are in good agreement wi th observed absolute and relative intensities. Differences in the methyl pr ofiles in toluens-d(0), -alpha-d(1), and -alpha-d(2) are ascribed to coupli ng between CH stretching and methyl torsional modes. The methyl profiles ar e simulated on the basis of a simple adiabatic model that incorporates the harmonically coupled anharmonic oscillator local mode approach and our inte nsity calculations. The model successfully accounts for the change in methy l profiles between the three molecules, demonstrates the importance of tors ional stretching coupling, and shows that coupling between the CH stretchin g oscillators is unimportant fur higher overtones (Delta v(CH) greater than or equal to 4).