Far-infrared interaction-induced spectra of the halogens

Abstract

We have investigated the far-infrared spectrum (3–125 cm^–1) of iodine in benzene solutions over a range of concentrations. The data have been successfully analysed in terms of a quadrupole-polarisability interaction mechanism which leads to fluctuating dipoles when the molecules rotate and translate relative to one another. Bandshape analysis, via the corresponding correlation functions, indicates that at least two relaxation times are involved in the description of the interaction processes. These times appear to get slower when Bz–Bz interactions are replaced by Bz–I₂ interactions. This analysis is supported by fitting the data to two different solutions of the generalised Langevin equation (GLE). These show up changes in interaction dynamics very well, but are less good at predicting the extent of the intermolecular forces. This may be due to the relative inaccuracy of the measured higher spectral moments. It is found that the rate of torque fluctuations for Bz–I₂ interactions is slower than that in the Bz–Bz case, probably reflecting more specifically directed and stronger forces between the molecules. There is little evidence from these data that the short-range potentials contain any significant ‘donor–acceptor’ contribution.