Studies of ion–ion and ion–solvent interactions using far-infrared spectroscopy. Part 3. Comparison of observed spectra with results of simple model calculations

Abstract

The principal features of the far-infrared spectra of tetra-n-alkylammonium salts in non-polar solvents have been compared with the results of some simple model calculations for the motions of, and interactions between, ion-ion and ion-solvent “aggregates.” It has been shown that the spectral features, and their variation with solute, solvent, temperature and concentration, are consistent with the absorptions arising from ion–ion vibrations, ion aggregate “librational” motions and fluctuating electrical fields due to “effective” dipole-induced interactions. The observed intensities are reproduced for model geometries and effective aggregate dipoles which agree very well with those obtained using dielectric relaxation measurements, and it is concluded that the high degree of ion “clustering” in the solutions leads to severe restrictions on the movement of solute and solvent species. It is also thought likely that some solvent molecules may be trapped within the ion “cluster”. Extension of previous measurements down to concentrations of 0.01 mol dm^–3 show no obvious changes in band shape or structure and it seems clear that extensive aggregation occurs even in relatively dilute solutions.