Knowledge Bank

Methanol CH- and OH-stretching overtones have been the subject of numerous studies (a) and in 1998 a full session of this Conference was dedicated to various aspects of the spectroscopy of this molecules (b). We focused on the interaction of CH- and OH-stretching with the large amplitude hindered rotation/torsion of $CH3$ with respect to OH groups. Allowing for a dependence of the torsional kinetic energy terms on the stretching coordinates, by numerical solution of the classical equations of motion, we found that CH- and OH-stretching resontant states exist corresponding to the following ratios of the stretching frequency to the hindered rotation frequency: 3/1, 6/1, 9/1, 9/2. The width of such resonances is observed to depend mainly on the height of the hindering potential barrier, which is also assumed to depend on the stretching overtone order (a). Applying Lie series perturbation theory (c), we were able to account for the resonances observed numerically.