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 of concentrations between 0.5 and 4 mol-kg(-1), have been studied by Fourier-transform Raman spectroscopy. The spectral regions studied include the solvent v(C drop N) fundamental and the anion B-F symmetric stretching band. In AgBF4 solutions the absence of ionic pairing was demonstrated and the anion v(1)(A(1)) remains as single narrow band located at 764.7+/-0.1 cm(-1). Consequently, the silver 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 pairing was observed in the corresponding solutions of LiBF4. Three components were detected in the tetrafluoroborate v(1)(A(1)) 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 should be three in the limit of infinite dilution, decreases as the salt concentration increases as a result of the ionic pairing. However, the ionic pairing of LiBF4 in acrylonitrile is less than that previously observed in lithium trifluoromethanesulfonate (triflate) or lithium perchlorate.