FT-Raman Study of Ionic Interactions in Lithium and Silver Tetrafluoroborate Solutions in Acrylonitrile

Solutions of silver and lithium tetrafluoroborate in acrylonitrile, over a range ofconcentrations between 0.5 and 4 mol-kg−1, have been studied byFourier-transform Raman spectroscopy. The spectral regions studied include the solventν(C=dN) fundamental and the anion B-F symmetric stretching band. In AgBF4solutions the absence of ionic pairing was demonstrated and the anion ν1(A1)remains as a single narrow band located at 764.7±0.1 cm−1. Consequently, thesilver ion solvation number does not change in the range of concentrations studied,having a constant value of 3.54±0.10. However, a high level of ionic pairingwas observed in the corresponding solutions of LiBF4. Three components weredetected in the tetrafluoroborate ν1(A1) band located at 766.0±0.4, 773.4±1.1,and 782.7±0.9 cm−1, and assigned to spectroscopically free anions, ion pairs,and dimers, respectively. The solvation number of the lithium ion, which shouldbe three in the limit of infinite dilution, decreases as the salt concentrationincreases as a result of the ionic pairing. However, the ionic pairing of LiBF4 inacrylonitrile is less than that previously observed in lithiumtrifluoromethanesulfonate (triflate) or lithium perchlorate.