Analysis of the vibrational spectrum of symmetrically solvated alkali metal cations

Abstract

This paper presents the results of an analysis of the vibrational spectrum of the solvated alkali metal cations in dimethyl sulphoxide (DMSO). The purpose of this study is to determine more about the structure of the primary solvation shell and to formulate a clearer picture of the vibrational states of these species. The ionic systems are investigated with the use of a potential energy function which consists basically of an ionic term and an exponential repulsion. Dispersion interactions, where they apply, are accounted for with the use of the Morse function. For systems with cubicsymmetry there is no dependence in the force constant upon electrostatic terms. Thus, the far infrared spectrum provides a probe of the repulsion interaction which operates between the ion and the solvent. On the other hand, the breathing modes depend upon all contributions in the potential function. This study shows that the ions of lithium, sodium and potassium can be regarded as classical point charges. Dispersion effects are not important for these ions. The far infrared spectra of the ions of rubidium and cesium, on the other hand, depend upon dispersion forces. Finally, the ions of lithium and sodium may be regarded as atomic masses trapped inside an infinitely massive solvent cage. For the remaining alkali metal ions, reduced mass effects are important in the determination of the vibrational frequencies.