Studies of ion-ion and ion-molecule interactions using far-infrared interferometry

Abstract

The far-infrared spectra of tetra-n-butylammonium halides in benzene, chloroform and carbon tetrachloride are interpreted with the aid of a dynamic model based on the stochastically modulated oscillator theory of Kubo. Although the model is not necessarily unique for this particular (and complicated) system, it does enable a reasonable interpretation of the observed band frequencies, widths and intensities and their variation (or otherwise) with changes of salt, solvent and temperature. All the data are consistent with situations in which the ion-pair (or aggregate) vibrations are stochastically but relatively slowly, modulated by interaction with the surrounding solvent molecules. The perturbation of the solvent molecules, mainly by dipole-induced dipole interaction, is shown to be severe. There is also evidence of strong coupling of the solvent “collision mode” and the ion-pair vibration and that the latter is strongly overdamped by a high Langevin friction constant. The implications of this work for the interpretation of data obtained at lower frequencies are considered to centre on the large solvent-solute interaction and this large microscopic viscosity coefficient.