A Raman spectral study of solvation and ion association in the systems LiAsF6/CH3CO2CH3 and LiAsF6/HCO2CH3

Abstract

The structure of the solvated lithium cation in methyl acetate (MA) solutions has been investigated using Raman spectroscopy. Two bands at 844 and 864 cm−1 have been assigned to two different types of MA: the former is from the bulk solvent and the latter arises from MA molecules solvating the lithium cation. From measurement of changes in intensity of these bands with increasing salt concentration a solvation number of four for Li+ in MA has been inferred. Changes in the Raman bands at ca. 1740 cm−1 suggest that solvation occurs through the carbonyl group. Evidence for contact ion pairing between Li+ and AsF6− is also presented. An equilibrium between solvent-shared ion pairs and contact ion pairs is proposed for which an equilibrium constant is estimated. The system LiAsF6/methyl formate (MF) is similar in structure. Key words: Raman, ion pair formation, lithium and hexafluoroarsenate ions, methyl acetate and formate, lithium ion solvation.