Vibrational dependence of the torsional tunneling splitting and the a to b intensity evolution in the OH overtone spectra of CH3OH

An ab initio investigation of the OH-stretching overtone of methanol is presented aimed at interpretation of two of the interesting features reported in recent supersonic-jet infrared-laser-assisted photofragment spectroscopy (IRLAPS) studies, namely, (i) a monotonic decrease of the torsional E - A splitting as uOH increases, (ii) an evolution of the a-type transition moment towards dominance at higher excitations of the OHstretching vibration. At the MP2 level with a 6-311+G(3df,2p) basis set, onedimensional functions for the potential energy, dipole moment (a and b directions), barrier height V3, and torsional constant F have been mapped out as a function of the OH-bond length with the remaining structure relaxed. All ab initio quantities have been expressed as Taylor-series expansions in the dimensionless OH normal coordinate, q. Calculations have been carried out in the harmonic basis set to yield vibrational energies and eigenfunctions. The latter have been used to evaluate the torsional parameters (V3, F) and the resulting E - A torsional splittings at each OH excitation, and to compute the evolution of the infrared transition intensity ratio a/b. The ab initio results reproduce the reported decrease in E - A splitting by a factor of 5 in going from uOH = 0 to uOH = 6, with a maximum deviation of 19% at the highest excitation. The calculated variation in a/b is in qualitative agreement with the experimental trend, showing the observed sharp rise at high uOH.