A PERTURBATION APPROACH TO PREDICT INFRARED-SPECTRA OF SMALL MOLECULAR CLUSTERS APPLIED TO METHANOL

A method for predicting splittings and shifts of bands in infrared spe ctra of small clusters of polyatomic molecules is presented. Based on an approach of early publications of Buckingham, the influence of the intermolecular forces on the vibrational energy levels of the constitu ent molecules is calculated using perturbation theory to second order. In order to describe the interaction of identical molecules, this ans atz is extended to also cover degenerate systems. In first order, a co upling of the vibrational modes of the interacting molecules occurs wh ich leads to delocalized vibrations of all the molecules in the cluste r. The second order correction of the vibrational excitation frequenci es are found to be dominated by the intramolecular couplings Of the no rmal modes due to the cubic anharmonicity-of the force field. The proc edures developed here are applied for the interpretation of vibrationa l photodissociation spectra of small methanol clusters in the region o f the fundamental excitation frequency of the OH stretching mode (nu1, 3681.5 cm-1), the CH3 rocking mode (nu7, 1074.5 cm-1), and the CO str etching mode (nu8, 1033.5 cm-1). Using semiempirical models for the in termolecular potential functions, splittings and positions of the expe rimental bands can well be explained. The nonequivalent positions of t he two molecules in the linear dimer structure give rise to two differ ent absorption frequencies for each of the three modes of the donor an d the acceptor molecule, respectively. The trimer and tetramer spectru m with only one absorption band are in agreement with the existence of symmetric planar ring structures (C3h and C4h) for these species. The pentamer spectrum which also consists of one band is explained by the occurrence of three closely spaced frequencies of an asymmetric ring. The double peak structure in the hexamer spectra can be attributed to a distorted ring structure of S6 symmetry, while the occurrence of ot her energetically near-degenerate isomers can be ruled out by means of their spectra.