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 vibration al overtone spectra are recorded by conventional near-infrared spectro scopy and by intracavity titanium:sapphire and dye laser photoacoustic spectroscopy. Absolute oscillator strengths are obtained from the con ventional spectra and relative oscillator strengths within a given ove rtone from the photoacoustic spectra. Peaks corresponding to two noneq uivalent aryl local modes are consistently assigned. The structure of the methyl band is complicated by methyl torsion. We show a simple met hod of calculating the absolute intensity of this methyl band. Oscilla tor strengths for both the aryl and the methyl transitions are calcula ted with an anharmonic oscillator local mode model and ab initio dipol e moment functions. Our simple calculations show very good agreement b etween observed and calculated intensifies, both for absolute total in tensities and for relative intensities between the two aryl groups and between the aryl and methyl groups. This agreement indicates that mos t of the intensity in the methyl band arises from the CH or CD stretch ing transitions.