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

FT-Raman spectroscopy was used to investigate lithium trifluoromethanesulfo nate (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 c haracteristics of the solvating molecules, allows calculation of the solvat ion numbers of Li+ at different molar concentrations (1-3.5 mol dm(-3)). Su ch number, likely four in the limit of infinite dilution in all the solvent s 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 high er 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 influ ence of both dielectric constants and donicity of the solvents studied. (C) 2000 Elsevier Science B.V. All rights reserved.