Intensity enhancements in the IR spectra of organic radical ions. A theoretical study

Abstract

The IR spectra of the methyl viologen dication and monocation have been calculated by molecular orbital (MO) methods in order to account for the considerable simplification observed in the IR spectra of viologens upon one-electron reduction to the corresponding radical monocations. Similar calculations on p-dicyanobenzene and its radical monoanion were carried out for comparison. The calculations show that a small number of fundamental modes of the reduced species show unusually large intensities and tend to dominate the spectrum, giving rise to the apparent simplicity of the observed spectra. The modes which show the greatest intensity enhancement are those which involve vibrations in the two halves of the molecule which are out of phase with each other. These modes produce a large charge oscillation and thus a large dipole change which gives rise to bands with high IR absorption intensity.