Methyl versus aryl CH and CD stretching overtone intensities in the vapor phase spectra of toluenes

Room temperature vapor phase overtone spectra of toluene-d(0) and -d(8) have been recorded in the CH and CD stretching regions corresponding to Delta upsilon(CH) = 2-7 and Delta upsilon(CD) = 2-6. The vibrational overtone spectra are recorded by conventional near-infrared spectroscopy and by intracavity titanium:sapphire and dye laser photoacoustic spectroscopy. Absolute oscillator strengths are obtained from the conventional spectra and relative oscillator strengths within a given overtone from the photoacoustic spectra. Peaks corresponding to two nonequivalent aryl local modes are consistently assigned. The structure of the methyl band is complicated by methyl torsion. We show a simple method of calculating the absolute intensity of this methyl band. Oscillator strengths for both the aryl and the methyl transitions are calculated with an anharmonic oscillator local mode model and ab initio dipole moment functions. Our simple calculations show very good agreement between observed and calculated intensifies, both for absolute total intensities and for relative intensities between the two aryl groups and between the aryl and methyl groups. This agreement indicates that most of the intensity in the methyl band arises from the CH or CD stretching transitions.