Infrared spectroscopy of weak charge-transfer complexes

Central Radio Propagation Laboratory, National Bureau of Standards, Boulder, Colorado, U.S.A..

Abstract

A comprehensive theory of the infrared spectra of weak charge-transfer complexes is given. The mechanism required for increased variation of dipole moment with molecular vibration due to complexing, and hence infrared enhancement, is a variation in the dative coefficient of the complex. In addition to the previously discussed mechanisms for dative coefficient variation, namely ionization potential and electron affinity variation, it is proposed that a variation in donor and acceptor wave function overlap due to molecular vibration is required to explain the benzene-iodine complex infrared spectrum.

This latter mechanism is consistent with either a non-symmetric geometry for the complex or an « Axial » model in which the very low frequency vibrations associated with motion of the benzene with respect to the iodine cause the near iodine atom to spend a large fraction of time appreciably off of the C₆ axis. The infrared spectrum of the benzene-iodine complex is explained in detail. Infrared selection rules for aromatic-halogen complexes are formulated and spectra of a number of methylated benzene-iodine complexes are explained very well.