Intramolecular energy transfer in highly vibrationally excited methanol. III. Rotational and torsional analysis

We report here torsional analysis of rotationally resolved spectra of the 3ν1, 5ν1, and 6ν1 (OH stretch) bands of jet-cooled methanol. The upper states are reached by a double resonance excitation scheme involving the selection of single rotational states in the ν₁ fundamental band. Detection of the overtone transitions (nν₁←ν₁) is by infrared laser assisted photofragment spectroscopy (IRLAPS). The torsional tunneling frequency declines monotonically from 9.1 cm^-1 in the vibrational ground state to 1.6 cm^-1 at 6ν1. For the available rotational levels at 3ν1 (K=0-3) and 6ν1 (K=0,1), the pattern of torsional energies is approximately regular. To obtain the vibrational dependence of the torsional barrier V₃, it was necessary to use the OH radical and HOOH as models for the vibrational dependence of the torsional inertial constant F. The assumed linear dependence of V₃ on ν₁ accounts for the torsional tunneling splittings at v₁=0, 3, and 6 and for the pattern of the torsional energies. V₃ increases by 40-45 cm^-1 per quantum of OH excitation.