Knowledge Bank

Rizzo and co-$workersa$ have used supersonic-jet infrared-laser-assisted photofragment spectroscopy (IRLAPS) to record the O---H stretching overtone spectra of $CH3OH$. Their analysis of the rotation-torsion structures revealed the following interesting features: (i) the torsional A-E splitting decreases monotonically as $\nu OH$ increases, indicating increase of the torsional barrier height $V3$, (ii) a-type transitions become dominant at higher excitations of the OH stretching vibration, (iii) a 1:1 anharmonic resonance occurs between the OH stretch and CH stretch vibrations, reaching its maximum in the $5\nu 1$ region. The third observation has been recently studied by Quack and $Willekeb$ for the case of $CD2HOH$, by means of ab initio five-dimensional potential energy and dipole moment surfaces. The present contribution explores possible ab initio explanations for the first two observations. At the MP2 level with 6-311G+(3df,2p) basis set, effective one-dimensional functions for the potential energy, dipole moment ($a$ and $b$ directions), barrier height and torsional constant $F$ have been obtained by scanning the O---H bond length in order to take into account the mechanical and electrical anharmonicities. All ab initio quantities have been expressed as Taylor expansions in the dimensionless coordinate, q. Calculations have been carried out in the harmonic basis set to yield vibrational energies and eigenfunctions. The latter have been used to compute the patterns of the barrier height $V3$, the torsional constant F, and the evolution of the infrared intensity ratio $Ia/Ib$, as functions of the OH vibrational quantum number. All our ab initio results agree with the experimental observations in points (i) and (ii) above. Details of the calculations, the corresponding results and the comparison to experimental data will be presented.