Analytical Sciences

Abstract − Analytical Sciences, 24(10), 1297 (2008).

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.