화학공학소재연구정보센터
Journal of Chemical Physics, Vol.113, No.10, 3980-3989, 2000
Can a local mode picture account for vibration-torsion coupling? Ab initio test based on torsional variation of methyl stretching and bending frequencies in methanol
The torsional dependence of the CH3 stretching and bending modes of methanol has been explored in terms of a local mode internal coordinate picture [X. Wang and D. S. Perry, J. Chem. Phys. 109, 10795 (1998)]. First, the torsional variations of the small-amplitude vibrational frequencies along the mass weighted intrinsic reaction coordinate from the top to the bottom of the torsional potential barrier were calculated by means of ab initio frequency projection utilizing GAUSSIAN 98. The resulting curves for the three C-H stretch ab initio frequencies as functions of the torsional angle cannot be reproduced by the original 3x3 local mode model incorporating stretch-torsion and stretch-stretch couplings at lowest-order only, but are well-fitted if the model is extended to include higher-order coupling terms. For the CH-bending modes, with internal coordinates chosen to give a high degree of localization, bend-torsion and bend-bend coupling parameters were determined from the ab initio projected frequencies, and were then used to predict torsional tunneling splittings. Just as observed for the C-H stretch modes, the two higher-frequency asymmetric CH-bend modes are predicted to have inverted tunneling splittings with reduced amplitudes, while the splitting pattern for the lower frequency symmetric-bend mode is predicted to be normal.