Infrared spectroscopic study of ionic association of lithium trifluoromethanesulfonate in several solvents

The ionic pairing of lithium trifluoromethanesulfonate (triflate) in four solvents (water, acrylonitrile, acetonitrile and acetone) has been studied by dispersive infrared (IR) spectroscopy. Spectra were taken between 950 and 1400 cm(-1), where the anion internal stretching modes appear, and for a range of concentrations of 0.5-2.0 mol dm(-3) (0.5-1.25 mol dm(-3) for acetonitrile). The following bands have been analyzed: S-O symmetric stretching (nu(1)), C-F antisymmetric stretching (nu(7)) and S-O antisymmetric stretching (nu(10)) In the nu(1) envelope, three components are detected in all the solvents studied except water. These components are interpreted in terms of ionic pairs with different solvation degrees. The nu(7), band also shows two components in the organic solutions. Its absorption coefficient is higher in organic solvents than in water, where only one component is detected. The complex and broad nu(10) band is understood in terms of degeneracy lifting as an effect of the ionic pairing. (C) 1997 Elsevier Science B.V.