Analytical Sciences

Raman Spectroscopic Study on Alkaline Metal Ion Solvation in 1-Butyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquid

Yasuhiro UMEBAYASHI,* Taishi YAMAGUCHI,* Shuhei FUKUDA,* Takushi MITSUGI,* Munetaka TAKEUCHI,* Kenta FUJII,** and Shin-ichi ISHIGURO*

*Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi, Fukuoka 812-8581, Japan
**Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan

The Raman spectra for 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [BMI][TFSA] containing alkaline metal salts of TFSA^-, MTFSA (M = Li, Na, K and Cs), were recorded in the frequency range of 200 - 1800 cm^-1, with varying salt concentrations at 298 K. With Li⁺ and Na⁺ ions, at the frequency range of 730 - 760 cm^-1, new Raman bands ascribable to the anion bound to the ions appeared at higher frequency relative to that found in the neat ionic liquid. On the other hand, with K⁺ and Cs⁺ ions, single Raman bands were solely observed. According to the difference Raman spectra for the ionic liquids containing K⁺ and Cs⁺, evaluated by subtracting Raman spectra for the neat ionic liquid, it turned out that two-state approximation, i.e., bulk TFSA^- and TFSA^- bound to K⁺ and Cs⁺ ions, could hold, as Li⁺ and Na⁺ ions. By careful analyses of Raman band intensity arising from bulk TFSA^- as a function of the salt concentration, the solvation numbers for the respective ions were successfully evaluated to be 1.95 for Li⁺, 2.88 for Na⁺, 3.2 for K⁺ and 3.9 for Cs⁺, respectively. By taking into account that TFSA^- acts as a bidentate ligand, the atomic coordination numbers are proposed to be 4, 6, 6 and 8 for Li⁺, Na⁺, K⁺ and Cs⁺, respectively. Raman shifts for the TFSA^- bound to the metal ions relative to that of the bulk TFSA^- were plotted against the ionic radii for the solvated alkaline metal ions estimated via Shannon's ionic radii, to yield a straight line with a slope of almost unity, suggesting that the electrostatic interaction predominantly operates in the ion-ion interaction between the alkaline metal ions and TFSA^-, as expected. Moreover, the Raman spectra in the frequency range of 370 - 450 cm^-1 strongly depend on the alkaline metal ions, indicating that cis TFSA^- is favored in the first solvation sphere of the Li⁺ ion of a relatively small ionic radius, and that such a preferred conformational isomerism of TFSA^- diminishes with an increase of the ionic radii of the central metal ions.

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