STRUCTURE AND IR-SPECTRUM CALCULATIONS FOR SMALL SF6 CLUSTERS

A new second order perturbation approach for evaluating the splittings and shifts of the vibrational bands of homogeneous molecular clusters, consistently treating degenerate normal modes, is described. The Hamiltonian of the system comprises harmonic and anharmonic intramolecular vibration terms, and the intermolecular potential. The anharmonic intramolecular contributions and the intermolecular potential are treated as a perturbation. A new site-site intermolecular potential model for SF6, featuring exchange, dispersion, electrostatic and induction terms, is presented. The new potential, with the parameters adjusted according to the observed monomer transition dipole moment and reproducing the experimental temperature dependence of the second virial coefficient, is used to determine SF6 cluster structures up to the hexamer and, by means of the new line shift formalism, to calculate the corresponding IR-spectra in the region of the nu(3) vibrational mode (at 947.968 cm(-1)). The contributions of the various potential terms to the frequency shifts are analyzed and the leading interaction mechanism is confirmed to be the electrostatic one (implicitly the resonant dipole-dipole coupling). The theoretical spectra are shown to fairly describe the experimental evidence when considering only exchange. dispersion and electrostatic interactions. With the available atomic polarizabilities. induction seems to lead to a systematic redshift of the entire spectrum for all cluster sizes. The structure of the cluster vibrations is investigated in terms of the individual monomer vibrations and is correlated with the found geometrical cluster configurations. (C) 1995 American Institute of Physics.