CH-STRETCHING OVERTONE SPECTRA AND INTERNAL METHYL ROTATION IN 2,6-DIFLUOROTOLUENE

Vapor phase overtone spectra of 2,6-difluorotoluene are recorded in th e Delta upsilon(CH)=2 and 3 regions by conventional near-infrared spec troscopy and in the Delta upsilon(CH)=4-6 regions by intracavity dye/t itanium: sapphire laser photoacoustic spectroscopy. The spectra are in terpreted on the basis of nb initio calculations at the HF/6-31G leve l. The methyl regions of the spectra are complex due to coupling betwe en the nearly freely rotating methyl rotor and CH stretching. A model has been developed to predict the methyl spectral profiles, which uses the harmonically coupled anharmonic oscillator local mode model and t he rigid rotor model for stretching and torsion, respectively. A dipol e moment function is formulated which combines a Taylor series for CH stretching and a Fourier series for torsion. The dipole moment functio n includes both angular dependence and higher order expansion terms in the CH-stretching coordinate. The model is successful in predicting t he methyl overtone spectral profiles and attributes these profiles to a very large number of transitions that arise from terms involving tor sion-stretching coupling, both in the Hamiltonian and in the dipole mo ment function. (C) 1997 American Institute of Physics.