文档介绍：8. Infrared Spectroscopy of Ar–CO
Motivation
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 dis-
cussed one example, the Ar–plex, which has been studied by sev-
eral 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 de-
velopments since a parison with ab initio studies is possible. For
experimental reasons, nearly all detailed studies of plexes involve
hydrogen-containing molecules; plexes studied include Ar–HCl, Ar–
HBr, Ar–HF, Ar–NH30andAr–H2O. The molecules have large dipole and
transition dipole moments and are therefore among the easiest to detect. For
all plexes the induction energy makes a considerable contribution to
the overall binding energy, especially to the anisotropic part of the potential.
The equilibrium structures of plexes 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 in-
teraction (and in addition to the electrostatic part) which can be most easily
predicted. Reliable estimates can be obtained from the known polarizibili-
ties and electrostatic moments (µ and Q) of the monomers [90]. An accurate
description of the parts involving correlation effects, such as the dispersion
interaction, is theoretically more challenging. The estimates are based on
C6
knowledge of the 0 coefficients [203] and on the measured anisotropic po-
larizibilities of the molecules [44]. For Ar–CO the induction energy is nearly
negligible (less than 2% of the overall binding energy). The potential and es-
pecially the anisotropic part of the potential surface are therefore determined
e