Torsion–rotation analysis of OH stretch overtone–torsion combination bands in methanol

Abstract

We report rotationally resolved spectra of jet-cooled methanol for the OH stretch overtones, 2v1 and 3v1, and for the torsional combinations, 2v1+v12, 2v1+2v12, 3v1+v12, and 3v1+2v12. The spectra are obtained by direct excitation from the vibrational ground state with an infrared laser pulse. Population in the resulting upper state levels is detected by infrared laser assisted photofragment spectroscopy (IRLAPS). Global fits of the spectra to the Herbst Hamiltonian yield the torsional and rotational parameters, including F, ρ, V3, and V6, for each OH stretch excited state. For each quantum of OH stretch excitation, we find that the torsional barrier height V3 increases by 40.9±1.9 cm−1 and the torsional inertial F decreases by 0.89±0.02 cm−1. With reference to ab initio calculations, we explain the increase in V3 in terms of changes in the electronic structure of methanol as the OH bond is elongated. For Δv12=1 we observe only transitions with ΔK=±1, and for Δv12=2 we observe only ΔK=0. We present a Franck–Condon model to explain these apparent selection rules and the overall pattern of intensity.