Speciation in Solutions of Lithium Salts in Dimethyl Sulfoxide, Propylene Carbonate, and Dimethyl Carbonate from Raman Data: A Minireview

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.

Picosecond Dynamics of Molecular Entities in Lithium Salt Solutions in Dimethyl Sulfoxide, Propylene Carbonate, and Dimethyl Carbonate

“…Like in DMSO, the line at ∼2960 сm −1 corresponding to the symmetric CH 3 stretch 2 ( 1 ) of the ciscis conformer of the neat DMC [29] shows no evidence of the hydrogen bonding . The behavior of the 2 ( 1 ) vibration in the salt solutions in DMC is similar to that of CH-vibrations in DMSO solutions [25]. The split lines can be assigned to the nonbonded DMC molecules (∼2919 cm −1 ) and the solvent molecules entering the solvation sphere of the anion and hydrogen-bonded to it (∼2913 cm −1 ) [25,34,35].…”

“…From the point of view of dimerization processes, the absence of its splitting means that the perturbations of CH 3 -groups in the monomers and dimers are of the same value, and, in this spectral region, the monomers and dimers are indistinguishable in Raman spectra. The analysis of the spectra of lithium salt solutions in DMSO in the region shows the presence of the nonbonded DMSO molecules (∼2919 cm −1 ) and the solvent molecules entering the solvation sphere of an anion and hydrogen-bonded to it (∼2913 cm −1 ) [25].…”

“…The line at ∼2930 cm −1 corresponding to the symmetric CH 3 stretch 14 ( 1 ) in the neat PC is split into two components at 2930 and 2941 cm −1 corresponding to hydrogen-bond-free molecules and to molecules hydrogen-bonded to anions [31]. Upon adding the salt, the intensity of the component at ∼2941 cm −1 increases, and the intensity of the component at ∼2930 cm −1 decreases [25,32].…”

“…When lithium salts are added, the Raman spectra of DMSO undergo changes. From our recent studies [25], a composite line in the region of the 10 ( ′ ) vibration can be decomposed into three components, the first two corresponding to the vibrations of monomeric ( = 663-670 cm −1 ) DMSO molecules and DMSO dimers ( = 667-669 cm −1 ), and the new component at higher wavenumbers ( = 674-676 cm −1 ) corresponding to particles entering the solvation shell of cations [36][37][38][39].…”

“…A method to obtain Raman spectra is described in Refs. [18,25] in detail. Since all the lines studied are sharply polarized, only isotropic spectra are presented in what follows.…”