Combined quantum chemical/Raman spectroscopic analyses of Li+ cation solvation: Cyclic carbonate solvents—Ethylene carbonate and propylene carbonate

• DFT calculations used to examine EC and PC coordination to lithium cations.
• A methodology is provided to accurately determine EC solvation numbers.
• PC solvation numbers cannot be accurately determined from a Raman analysis.

Combined computational/Raman spectroscopic analyses of ethylene carbonate (EC) and propylene carbonate (PC) solvation interactions with lithium salts are reported. It is proposed that previously reported Raman analyses of (EC)n–LiX mixtures have utilized faulty assumptions. In the present studies, density functional theory (DFT) calculations have provided corrections in terms of both the scaling factors for the solvent's Raman band intensity variations and information about band overlap. By accounting for these factors, the solvation numbers obtained from two different EC solvent bands are in excellent agreement with one another. The same analysis for PC, however, was found to be quite challenging. Commercially available PC is a racemic mixture of (S)- and (R)-PC isomers. Based upon the quantum chemistry calculations, each of these solvent isomers may exist as multiple conformers due to a low energy barrier for ring inversion, making deconvolution of the Raman bands daunting and inherently prone to significant error. Thus, Raman spectroscopy is able to accurately determine the extent of the EC…Li+ cation solvation interactions using the provided methodology, but a similar analysis of PC…Li+ cation solvation results in a significant underestimation of the actual solvation numbers.

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