Infrared Spectroscopy of Ar—CO

Abstract

We shall now investigate one prototype system for intermolecular interaction, the Ar—CO system, in more detail. In Chapter 1 of this book I described the different contributions to the intermolecular potential. I furthermore discussed one example, the Ar—HF complex, which has been studied by several groups. Complexes consisting of a noble gas and a diatomic molecule are the simplest systems in which anisotropic intermolecular forces play an important role. They are especially suitable for testing recent theoretical developments since a direct comparison with ab initio studies is possible. For experimental reasons, nearly all detailed studies of small complexes involve hydrogen-containing molecules; the complexes studied include Ar—HCl, Ar—HBr, Ar—HF, Ar—NH₃0 and Ar—H₂O. The molecules have large dipole and transition dipole moments and are therefore among the easiest to detect. For all these complexes the induction energy makes a considerable contribution to the overall binding energy, especially to the anisotropic part of the potential. The equilibrium structures of the complexes Ar—HBr, Ar—HCl and Ar—HF show a linear structure, corresponding to the maximum in the induction energy. However, the induction energy is the part of the intermolecular interaction (and in addition to the electrostatic part) which can be most easily predicted. Reliable estimates can be obtained from the known polarizibilities and electrostatic moments (μ and Q) of the monomers [90].