The geometry and vibrational frequency shift of CO molecules in an argon matrix studied by force-field calculations

Abstract

In this paper, the CO·Ar interaction potential including pairwise and three-body contributions which reproduces the experimental CO vibrational frequency shift, Δω = −4.7 cm −1 in a static force-field approximation, is presented. The geometry of the COAr solid system is described in detail. The frequency shift is obtained when the CO molecule is oriented along the (0, 0, 1) crystal axis, with its center of mass shifted by 0.25 Å from the center of interaction, and with the surrounding first shell argon atoms relaxed into an approximately ellipsoidal cage. The semi-axes of the ellipsoid deviate from the radius of the undistorted first shell, 3.756 Å, by only 0.06 Å. The second shell of the argon atoms is also distorted into an ellipsoidal cage but the deviations from the sphere in this case are an order of magnitude smaller than for the first shell. Distortions of more distant crystal sites are negligible. Substitution of an Ar atom by a CO molecule with subsequent lattice relaxations into the minimum potential energy configuration results in a decrease in potential energy of 0.16 kJ mol −1. The three-body contributions to the CO·Ar potential have only a negligible influence on the geometry of the COAr system (including the lattice relaxations) but a strong influence on the vibrational frequency shift.