Raman Spectroscopic Studies and Ab Initio Calculations on Conformational Isomerism of 1-Butyl-3-methylimidazolium Bis-(trifluoromethanesulfonyl)amide Solvated to a Lithium Ion in Ionic Liquids: Effects of the Second Solvation Sphere of the Lithium Ion

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Abstract
Raman spectra of the ionic liquid, 1-butyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)amide [C4mIm][TFSA] containing a LiTFSA salt were measured for the lithium salt mole fractions xLi = 0.000, 0.053, 0.106, and 0.171 in the temperature range of 273−350 K. The lithium ion solvation number of 2 at ambient temperature is kept constant in higher temperatures examined in this study. Thermodynamic quantities, such as Gibbs free energy, ΔisoG0; enthalpy, ΔisoH0; and entropy, ΔisoS0, for conformational isomerism of TFSA− from trans to cis isomers in the neat ionic liquid and also in the first solvation sphere of the lithium ion were successfully evaluated for the first time. In the neat ionic liquid, the thermodynamics quantities indicates that the trans isomer is slightly stabilized by enthalpy, though the enthalpic advantage is reduced by entropy to yield nearly equal Gibbs free energy. For the TFSA− in the first solvation sphere of the lithium ion, the ΔisoG0, ΔisoH0, and TΔisoS0 were obtained at 298 K to be −4, −9.4, and −5 kJ mol−1, respectively, and the cis isomer is clearly more favored due to the larger enthalpy relative to that for the neat ionic liquid. However, gas phase quantum calculations for the lithium ion solvated clusters of [Li(TFSA)2]− were reported to be opposite to the experimental isomerization enthalpy. In this study, additional MP2 level ab initio calculations were carried out for the lithium ion solvated clusters with a countercation of 1-ethyl-3-methylimidazolium [C2mIm] in gas phase to yield the energy difference of −8.8 kJ mol−1 from [C2mIm][Li(trans-TFSA)2] to [C2mIm][Li(cis-TFSA)2]. The ab initio calculations revealed the important roles of the surrounding imidazolium cation as the second solvation sphere of the lithium ion and agree with the Raman experimental fact that the cis-TFSA− solvated to the lithium ion is more stabilized relative to the trans with relatively large enthalpy.
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