Raman spectral studies of ion association and solvation in solutions of LiAsF6–acetone

Abstract

The structure of the solvated lithium cation in acetone solutions has been investigated using Raman spectroscopy. Two bands at 1220 and 1238 cm^–1 have been assigned to two types of asymmetric carbon–carbon stretching vibration of acetone: the former is from the bulk solvent and the latter arises from acetone molecules solvating the lithium cation. Analogous assignments are made in the C—C stretch vibration region for two bands at 788 and 802 cm^–1 and in the CO stretch vibration region for bands at 1706 and 1720 cm^–1. From the measurement of changes in intensity of the two bands in the C—C stretching region of acetone with increasing salt concentration a solvation number of four for lithium cation in acetone has been inferred. Large changes in the Raman bands at ca.1720 cm^–1 suggest that solvation occurs through the carbonyl group. Evidence for contact ion pairing between Li⁺ and AsF^–₆ is also presented.