Vibrational spectroscopic studies of the chemical dynamics in charge transfer complexes of the type iodine-pyridine 2. Intermolecular dynamics from far infrared bands

Abstract

Detailed interpretation of the far infrared spectra of the pyridine–I₂ complex (in ternary mixtures with an inert solvent) has been attempted using models for elucidation of intermolecular dynamics between components of a liquid mixture. For the complex very diluted in an inert solvent, in the framework of the Robertson–Yarwood model, it is found that the bandshape of the υ_I-I mode originates from the phase relaxation of the I–I oscillator due to its anharmonic ‘indirect’ coupling with the υ_D-I mode. The frequency shifts and widths of the two bands observed at higher concentrations of pyridine in the mixture are then explored by applying the vibrational relaxation formalism of Kubo with the analytical expression of Fukuda, T., Ikawa, S., and Kimura, M., (1989, Chem. Phys., 133,137) to treat the ‘direct’ coupling of the transition dipole of the complex with the dipole moment of the solvent molecule. Both phase and energy relaxation contributions have been considered. It has been found that band intensities are intimately related to the extent of charge transfer between the complex components, and that the frequency shifts and widths are dependent on these intensities and on the signs of the transition dipole. At low concentrations of donor, the harmonic contribution ∂µ / ∂q totally dominates the bandwidths, but at higher concentrations ∂²µ/∂² q is needed. In addition, the υ_I-I band phase relaxation explains the bandwidth perfectly well, but for the (lower frequency) υ_D-I band there is a significant contribution from energy relaxation processes. A study of the effects of the diffusion coefficients on the bandwidths has shown that the reorientational motion of pyridine lies at the origin of the observed broadening.