Potential energy surface, van der Waals motions, and vibronic transitions in phenol-argon complex

The structure and intermolecular vibrational energy levels of the phenol-Ar complex are calculated from its potential energy surface. This surface is constructed from a large set of the interaction energy values computed using second-order Moller-Plesset perturbation theory with the augmented correlation consistent polarized valence double-zeta basis set. The global minimum in the potential energy surface corresponds to a cluster structure with Ar located over the geometric center of the phenol ring at a distance of 3.510 A and shifted by 0.1355 A towards oxygen. The calculated dissociation energy of 371 cm(-1) is in accordance with the experiment. Additional local minima higher in energy are with Ar placed in the phenol plane. However, they are too shallow to form the bound states corresponding to planar isomers. The deformation of the potential energy surface shape, created by the interaction of Ar with the phenolic oxygen, is responsible for a pronounced intermode mixing. As a result, a set of hybrid stretching-bending states appears which cannot be described in terms of the standard models. The intermode coupling is reflected in the vibronic structure of the S-1-S-0 electronic transition. The intensities of the vibronic bands are calculated from the electronic transition dipole moment surfaces determined using the ab initio single-excitation configuration interaction method. They allow us to correct and complete the assignment of the spectra observed in phenol-Ar, as well as in the analogous complexes of phenol with Kr and Xe.