Preferential solvation and ionic association in lithium and silver trifluomethanesulfonate solutions in acrylonitrile/dimethylsulfoxide mixed solvent -: A Raman spectroscopic study

Solutions of lithium or silver trifluoromethanesulfonate (triflate) in a mixed solvent formed by acrylonitrile (ACN) and dimethylsulfoxide (DMSO) have been studied by FT-Raman spectroscopy. The composition of the solvation sphere in both ions is calculated through the study of the modifications induced by the presence of the salt in the nitrile Cequivalent toN stretching band at 2228 cm(-1) and in the (C-S-C) symmetric stretching band of DMSO at 669 cm(-1). DMSO preferentially solvates both cations, even though the preference is slightly less pronounced in the case of Ag-divided by. However, this preference is remarkable in Li-divided by which is almost exclusively solvated by DMSO in the equimolar mixed solvent. The total average solvation number in both cations decreases when the ACN concentration in the mixed solvent increases, indicating the presence of ionic association processes. These can be evaluated by studying the triflate anion CF3 symmetric bending at ca. 755 cm(-1). Lithium triflate is considerably associated in those samples rich in ACN, even showing the presence of ionic aggregates. However, spectroscopically free anions are a majority in the equimolar mixed solvent. Ionic association cannot be observed in silver triflate solutions and, therefore, the reported decrease in the total average solvation number must be promoted for other reasons, probably steric in origin, because in neat solvents, the stoichiometry of solvates are Ag-divided by(ACN)(3) and Ag-divided by(DMSO)(4). (C) 2003 Elsevier B.V. All rights reserved.