Internal Methyl Rotation in the CH Stretching Overtone Spectra of ortho-, meta-, and para-Xylene

Abstract

The room-temperature vapor phase overtone spectra of o-, m-, and p-xylene have been recorded in the CH stretching region corresponding to ΔvCH = 2−6 with conventional near-infrared spectroscopy (ΔvCH = 2 and 3), intracavity titanium:sapphire (ΔvCH = 4 and 5) and dye laser photoacoustic spectroscopy (ΔvCH = 6). Absolute oscillator strengths have been measured from the conventional spectra and relative oscillator strengths within a given overtone from the conventional and photoacoustic spectra. The aryl region of the spectra can be interpreted simply in terms of a number of nonequivalent and independent local modes. The methyl region of the spectra is more complex. The methyl band profiles in the overtone spectra of m-xylene and p-xylene are very similar to each other and to that of toluene and differ significantly from the methyl band profiles in the spectra of o-xylene. We use a simple anharmonic oscillator local mode model with ab initio calculated dipole moment functions to calculate oscillator strength...