Deuterium isotope effects on the CH stretching overtone spectrum of toluene-α-d1

The room-temperature vapor phase overtone spectrum of toluene-a-dl has been recorded in the CH stretching regions corresponding to Delta upsilon(CH) = 2-7. 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 both the conventional and photoacoustic spectra. The aryl region of the spectrum is nearly identical to the aryl region of the spectrum of toluene-d(0) and can be understood on the basis of two nonequivalent aryl local modes. The methyl band differs markedly from the methyl band in toluene-d(0) in relative intensity, in line width, and in structure. We use an anharmonic oscillator local mode model and an nb initio dipole moment function to calculate oscillator strengths for the aryl and methyl transitions. As was the case for toluene-d(0), these simple calculations show good agreement between observed and calculated intensities both in absolute total intensities and for relative intensities between the two aryl groups and between the aryl and methyl groups. We explain the differences between the methyl bands in the -alpha-d(1) and -d(0) spectra on the basis of our intensity calculations and on the basis of coupling between CH and CD stretching vibrations and methyl torsions.