Cx. Zhu et al., CH-STRETCHING OVERTONE SPECTRA AND INTERNAL METHYL ROTATION IN 2,6-DIFLUOROTOLUENE, The Journal of chemical physics, 107(3), 1997, pp. 691-701
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.