INTENSITY ENHANCEMENTS IN THE IR-SPECTRA OF ORGANIC RADICAL IONS - A THEORETICAL-STUDY

The IR spectra of the methyl viologen dication and monocation have bee n calculated by molecular orbital (MO) methods in order to account for the considerable simplification observed in the IR spectra of viologe ns upon one-electron reduction to the corresponding radical monocation s. Similar calculations on p-dicyanobenzene and its radical monoanion were carried out for comparison. The calculations show that a small nu mber of fundamental modes of the reduced species show unusually large intensities and tend to dominate the spectrum, giving rise to the appa rent simplicity of the observed spectra. The modes which show the grea test intensity enhancement are those which involve vibrations in the t wo halves of the molecule which are out of phase with each other. Thes e modes produce a large charge oscillation and thus a large dipole cha nge which gives rise to bands with high IR absorption intensity.