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

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.