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.