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Author Umebayashi, Yasuhiro ♦ Yamaguchi, Taishi ♦ Fukuda, Shuhei ♦ Mitsugi, Takushi ♦ Takeuchi, Munetaka ♦ Fujii, Kenta ♦ Ishiguro, Shin-ichi
Source J-STAGE
Content type Text
Publisher The Japan Society for Analytical Chemistry
Language English
Abstract 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.
ISSN 09106340
Learning Resource Type Article
Publisher Date 2008-10-10
e-ISSN 13482246
Journal Analytical Sciences(analsci)
Volume Number 24
Issue Number 10
Page Count 8
Starting Page 1297
Ending Page 1304


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