Abstract
Our recent Raman studies of cation and anion solvation and ion pairing in solutions of lithium salts in dimethyl sulfoxide, propylene carbonate, and dimethyl carbonate are briefly overviewed. Special attention is paid to differences in our and existing data and concepts. As follows from our results, cation solvation numbers in solutions are low (~2) and disagree with previous measurements. This discrepancy is shown to arise from correct accounting for dimerization, hydrogen bonding, and conformation equilibria in the solvents disregarded in early studies. Another disputable question touches upon the absence of free ions in solutions of lithium salts in carbonate solvents and the statement that the charge transfer in carbonate solutions is caused by SSIPs. Direct proofs of the nature of charge carriers in the solvents studied have been obtained by means of analyses of vibrational dynamics. It has been found that collision times for free anions are short and evidence weak interactions between anions and solvent molecules. In SSIPs, collision times are an order of magnitude longer thus signifying strong interactions between anions and cations. In CIPs, collision times become shorter than in SSIPs reflecting the transformation of the structure of concentrated solutions to that of molten salts.
Highlights
Due to its sensitivity to molecular interactions between functional groups of molecules and ions, vibration spectroscopy is perhaps the most powerful tool for studying speciation in nonaqueous solutions
The absence of free ions in solutions of lithium salts in carbonate solvents and to prove the statement that the charge transfer in carbonate solutions is caused by solvent separated ion pairs (SSIPs) is an analysis of vibrational dynamics in the systems studied
This discrepancy is shown to arise from correct accounting for possible equilibria in the solvents studied, like dimerization (DMSO and PC), hydrogen bonding (PC), and conformation equilibria (DMC), disregarded in early studies
Summary
Due to its sensitivity to molecular interactions between functional groups of molecules and ions, vibration spectroscopy is perhaps the most powerful tool for studying speciation in nonaqueous solutions. As far as speciation in DMSO solutions is concerned, signatures of cationic solvation have been characterized in precious studies [16,17,18,19] and up to four DMSO molecules have been found to form the solvation sphere of the Li+ ion interacting with it through the negatively charged oxygen atom of the S=O group. This solvation number might be corrected since DMSO is an ordered, associated liquid but dimerization equilibria have not been taken into account in solvation studies. We suggest that solvation equilibria in solutions are significantly determined by the donor numbers of solvents
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