FT-Raman spectroscopic study of preferential solvation and ionic association in lithium and silver triflate solutions in acrylonitrile/N,N-dimethylformamide mixed solvent

Solutions of lithium or silver trifluoromethanesulfonate (triflate) in a mixed solvent formed by acrylonitrile (ACN) and N,N-dimethylformamide (DMF) 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 C=N stretching at 2228 cm(-1) and in the NC anti-symmetric stretching of DMF at 658 cm(-1). DMF preferentially solvates both cations. The preference is less pronounced in the case of Ag+, although it is very remarkable that Li+ is solvated exclusively by DMF molecules in the equimolar mixed solvent. The total average solvation number remains virtually constant in Ag+ (n = 3.5 +/- 0.1) and is augmented in Li+ when the molar fraction of DMF increases. This observation arises as a direct consequence of the degree of ionic association, which is evaluated through studying the components of the triflate anion SO symmetric stretching band at 1032 cm(-1). Lithium triflate is considerably associated in those samples rich in ACN. However, the situation changes completely in the equimolar samples, with a high concentration of spectroscopically free anions. These results are explained considering the corresponding dielectric constants of both co-solvents and their different donating ability. In silver triflate solutions, the degree of ionic association is low and remains approximately constant despite the changes in the composition of the mixed solvent, which indicates that the silver ion is solvated very effectively by both co-solvents. (C) 2001 Elsevier Science B.V. All rights reserved.