Ion solvation and ion association in lithium trifluoromethanesulfonate solutions in three aprotic solvents. An FT-Raman spectroscopic study

Abstract

FT-Raman spectroscopy was used to investigate lithium trifluoromethanesulfonate (triflate) solutions in three dipolar aprotic, electron donor solvents, namely acetone, N, N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO). The rising of new bands in the Raman spectra of the solutions, which are characteristics of the solvating molecules, allows calculation of the solvation numbers of Li + at different molar concentrations (1–3.5 mol dm −3). Such number, likely four in the limit of infinite dilution in all the solvents studied, significantly decreases when the salt concentration is increased as an effect of the ionic association. The deconvolution and band fitting of the triflate anion S–O symmetric stretching band permits detailed assess of the ionic pairing. Bands assigned to either ionic pairs as well as higher aggregates are observed in acetone and DMF solutions. Ionic association in DMSO solutions is limited to the formation of ionic pairs. The observed association degrees can be explained taking into account the combined influence of both dielectric constants and donicity of the solvents studied.