Vibrational spectrum of alkali metal cations in distorted solvation shells. A prediction

Abstract

The purpose of this paper is to examine the effect of the distortion of the primary solvation shell upon the vibrations of an ion within this shell. In addition, the effect of an external charge (or image charge) on the ionic vibrations is also investigated. An objective of this work is to see whether it is possible to use predicted changes in the far infrared vibrational spectrum to probe the alterations in the solvation structure which take place when an ion is incorporated into the metal/solution interface. In the solution phase the vibrations of the ion within the solvation shell yield a single band, as was discussed in the previous paper. This single band can split into two or three new bands due to the presence of distortion of the solvation shell and the effect of an external charge. The case of the lithium cation dissolved in dimethylsulphoxide with four-coordination is examined. An external charge or an image charge in the metal has a pronounced effect upon the extent of the splitting of the far infrared band. An image charge in the metal, for example, yields a splitting of ≈ 16 cm–1 when the lithium ion is 2 A from the metal surface. Distortion of the solvation shell, for moderate deviations from uniform spherical symmetry, has a relatively small effect upon the system. Nevertheless, these distortions may have an important effect in the activation process of the electron transfer reaction and other transport processes. Far infrared vibrational spectroscopy of the interface may be able to probe the structure of adsorbed, solvated ions.