CH-stretching overtone spectra and internal methyl rotation in 2,6-difluorotoluene

Vapor phase overtone spectra of 2,6-difluorotoluene are recorded in the $ΔvCH=2$ and 3 regions by conventional near-infrared spectroscopy and in the $ΔvCH=4–6$ regions by intracavity dye/titanium: sapphire laser photoacoustic spectroscopy. The spectra are interpreted on the basis of ab initio calculations at the $HF/6-31G*$ level. The methyl regions of the spectra are complex due to coupling between 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 the rigid rotor model for stretching and torsion, respectively. A dipole moment function is formulated which combines a Taylor series for CH stretching and a Fourier series for torsion. The dipole moment function includes both angular dependence and higher order expansion terms in the CH-stretching coordinate. The model is successful in predicting the methyl overtone spectral profiles and attributes these profiles to a very large number of transitions that arise from terms involving torsion–stretching coupling, both in the Hamiltonian and in the dipole moment function.