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

Abstract

The torsional tunneling splittings of the asymmetric C–H stretches (ν2 and ν9) in methanol are inverted with the E level lower in energy than the A level, whereas the symmetric C–H stretch (ν3) 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 ω=2934.0 cm−1, the direct local–local coupling λ=−42.2 cm−1, and a single stretch-torsion coupling parameter μ=12.9 cm−1. The torsion-vibration coupling is nonadiabatic in the sense that it is not consistent with a Born–Oppenheimer separation of the torsion from the other vibrations. The fact that the model is based largely on the G6 molecular symmetry suggests that tunneling inversion may be common in torsional molecules. The torsionally mediated couplings among the C–H stretches do not conserve symmetry in the Cs point group and are strong enough to contribute to rapid intramolecular vibrational redistribution (IVR).