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

摘要

We report here torsional analysis of rotationally resolved spectra of the 3v_1, 5v_1, and 6v_1 (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 v_1 fundamental band. Detection of the overtone transitions (nv_1 <- v_1) 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 6v_1. For the available rotational levels at 3v_1 (K = 0-3) and 6v_1 (K =0,1), the pattern of torsional energies is approximately regular. To obtain the vibrational dependence of the torsional barrier V_3, 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 V_3 on v_1 accounts for the torsional tunneling splittings at #upsilon#_1 = 0, 3, and 6 and for the pattern of the torsional energies. V_3 increase by 40-45 cm~(-1) per quantum of OH excitation.