Far-infrared optical properties and molecular interactions

Abstract

The far-infrared absorption spectra of solutions of tetra-n-alkylammonium salts have been studied in benzene, chloroform, carbon tetrachloride and tetrahydrofuran, as a function of concentration, temperature, solute and solvent. The absorption profile has been computer fitted to the sum of 3 component bands, and the variation of band centre, half-band width, area and intensity with concentration, temperature, solute and solvent has been studied. The spectral features, and their variation with the parameters above, have been shown to be consistent with the absorptions arising from ion-ion vibrations, ion aggregate 'librational' absorptions and fluctuating electrical fields, due to 'effective' dipole-induced dipole interactions. The observed intensities were seen to be reproduced for model geometries and effective aggregate dipoles, which agreed well with those obtained from dielectric relaxation measurements. It was concluded that the higher degree of ion-clustering in the solutions leads to severe restrictions on the movement of both solute and solvent species. It is proposed that some solvent molecules may be trapped within the ionic cluster. The absorptions of more polar systems have been compared with the non-polar systems. Computer programmes have been developed to compute refractive index data from interferograms obtained with a Michelson interferometer operated in the dispersive mode. The experimental and computational procedures have been tested by determining the refractive index of polytetrafluoroethylene sheets in the far-infrared. Methods for the computation of intensities from refractive index values have been developed, and the intensities for the 202 cm(^-1) band of polytetrafuoroethylene obtained by both dispersive and non-dispersive methods have been compared. A cell has been designed, constructed and tested, which can be used to measure the refractive index of liquids and solutions in the far-infrared region. This cell has been incorporated into a Beckman-RIIC Ltd. FS-720 interferometer, as an extra (dispersive) module.