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

Hikari Watanabe; Nana Arai; Jihae Han; Yui Kawana; Seiji Tsuzuki; Yasuhiro Umebayashi

doi:10.1007/s44211-022-00121-7

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

Anal Sci. 2022 Aug;38(8):1025-1031. doi: 10.1007/s44211-022-00121-7. Epub 2022 Jun 8.

Authors

Hikari Watanabe¹, Nana Arai², Jihae Han², Yui Kawana², Seiji Tsuzuki³, Yasuhiro Umebayashi⁴

Affiliations

¹ Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan.
² Graduate School of Science and Technology, Niigata University, 8050 Ikarashi, 2-no-cho, Nishi-ku, Niigata City, 950-2181, Japan.
³ Japan Advanced Chemical Energy Research Center (ACERC), Institute of Advanced Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan.
⁴ Graduate School of Science and Technology, Niigata University, 8050 Ikarashi, 2-no-cho, Nishi-ku, Niigata City, 950-2181, Japan. yumescc@chem.sc.niigata-u.ac.jp.

PMID: 35672500
DOI: 10.1007/s44211-022-00121-7

Abstract

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.

Keywords: Inter-ionic interaction; Ionic liquid; Isotopic substitution Raman spectroscopy.

Abstract

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