Conformational Equilibrium of Bis(trifluoromethanesulfonyl) Imide Anion of a Room-Temperature Ionic Liquid:  Raman Spectroscopic Study and DFT Calculations

The structure of bis(trifluoromethanesulfonyl) imide (TFSI-) in the liquid state has been studied by means of Raman spectroscopy and DFT calculations. Raman spectra of 1-ethyl-3-methylimidazolium (EMI+) TFSI- show relatively strong bands arising from TFSI- at about 398 and 407 cm-1. Interestingly, the 407 cm-1 band, relative to the 398 cm-1 one, is appreciably intensified with raising temperature, suggesting that an equilibrium is established between TFSI- conformers in the liquid state. According to DFT calculations followed by normal frequency analyses, two conformers of C 2 and C 1 symmetry, respectively, constitute global and local minima, with an energy difference 2.2−3.3 kJ mol-1. The wagging ω-SO2 vibration appears at 396 and 430 cm-1 for the C 1 conformer and at 387 and 402 cm-1 for the C 2 one. Observed Raman spectra over the range 380−440 cm-1 were deconvoluted to extract intrinsic bands of TFSI- conformers, and the enthalpy of conformational change from C 2 to C 1 was evaluated. The enthalpy value is in good agreement with that obtained by theoretical calculations. We thus conclude that a conformational equilibrium is established between the C 1 and C 2 conformers of TFSI- in the liquid EMI+TFSI-, and the C 2 conformer is more favorable than the C 1 one.