Tools for studying ion solvation and ion pair formation in ionic liquids: isotopic substitution Raman spectroscopy

Isotopic H/D or ^6/7Li substitution Raman spectroscopy was applied to new kinds of ionic liquids; N-methylimidazole (C₁Im) and acetic acid (CH₃COOH) as the pseudo-protic ionic liquid (pPIL), and both of the neat and the 2,2,3,3-tetrafluoropropyl ether (HFE) diluted Li-glyme solvate ionic liquids (SIL) [Li(Gn)][TFSA] (Gn, glyme n = 3 or 4); TFSA, bis(trifluoromethanesulfonyl)amide) to clarify the proton transfer or the Li⁺ solvation/ion pair formation. The isotopic substitution Raman (ISR) spectra were obtained as the difference between the samples containing the same composition except the substituted isotope. The calculated and theoretical ISR spectra were also evaluated for comparison. With the C₁Im-CH₃COOH(D) pPIL, the Raman bands attributable to the C₁Im/C₁HIm⁺ gave signals of differential shape, and they were well reproduced with the curve fitting by taking the small amount of C₁HIm⁺ and CH₃COO^- generation into consideration. The ISR spectra for the SIL were well explained by the formation of the Li-TFSA contact ion pair (CIP) and the solvent shared ion pair (SSIP) in the [Li(G3)][TFSA] SIL. In addition, the ISR spectra for the HFE-diluted [Li(G4)][TFSA] SIL clearly proved that the HFE hardly coordinates to the Li⁺ in the HFE-diluted SIL. Here, the ISR spectroscopy is proposed as a new tool for studying the ion solvation and the ion pair formation in ionic liquids.