An internal coordinate model of coupling between the torsion and C-H vibrations in methanol

The torsional tunneling splittings of the asymmetric C-H stretches (nu(2) a nd nu(9)) in methanol are inverted with the E level lower in energy than th e A level, whereas the symmetric C-H stretch (nu(3)) is normal with A below E. An internal coordinate model, which treats the torsion and the three C- H stretches simultaneously, accounts for the observed tunneling splittings. The model parameters are the local stretching frequency omega = 2934.0 cm( -1), the direct local-local coupling lambda = -42.2 cm(-1) and a single str etch-torsion coupling parameter mu = 12.9 cm(-1). The torsion-vibration cou pling is nonadiabatic in the sense that it is not consistent with a Born-Op penheimer separation of the torsion from the other vibrations. The fact tha t the model is based largely on the G(6) molecular symmetry suggests that t unneling inversion may be common in torsional molecules. The torsionally me diated couplings among the C-H stretches do not conserve symmetry in the C- s point group and are strong enough to contribute to rapid intramolecular v ibrational redistribution (IVR). (C) 1998 American Institute of Physics. [S 0021-9606(98)00248-7].