Internal Methyl Rotation in the CH Stretching Overtone Spectra of Toluene-#alpha#-d_2,-#alpha#-d_1,and -d_0

摘要

The room-temperature vapor phase spectrum of toluene-#alpha-d_2 has been recorded in the CH stretching region corresponging to triangle open#upsilon#_(CH) = 2-6 with conventional enar-infrared spectroscopy (triangle open#upsilon#_(CH) = 2-4) and with intracavity titanium:sapphire and dye laser photoacoustic spectroscopy (triangle open#upsilon#_(CH) = 2-6). Both absolute oscillator strengths (conventional spectra) and relative oscillator oscillator strengths within a given overtond (conventioal and photoacoustic spectra) have been measured. The aryl region of the spectrum is interpreted interms of two nonequivalent aryl local modes and is essectially identical to the aryl regions of the spectra of toluene-d_0 and toluene-#alpha#-d_1.However,the methyl band profile profile differs significantly in these three molecules.We use an anharmonc oscillator local mode model and an ab initio dipole moment function to calculate oscillator strengths for the aryl and methyl transitions.Parameters for this model come from a fit of the aryl taransition energies and from a fit of the methyl spectral profiles.These simple calculations give values that are in good agreement with observed absolute and relative intensities. Differences in the methyl profiles in toluene-d_0, -#alpha#-d_1,and -#alpha#-d_2 are ascribed to coupling between CH stretching and methyl torsional modes.The methyl profiles are simulatded on the basis of a simple adiabatic model that incorporates the harmonically coupledanharmonic oscillaror local model approach and our intensity calculations.The model successfully accounts for the change in methyl profiles between the three molecules,demonstrates the importance of torsional stretching coupling,and shows that coupling between the CH stretching oscillators is unimportant for higher overtones (triangle open#upsilon#_(CH) >= 4).