Conformational equilibrium of bis(trifluoromethanesulfonyl) imide anion of a room-temperature ionic liquid: Raman spectroscopic study and DFT calculations

The structure of bis(trifluorornethanesulfonyl) 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 U mol(-1). The wagging omega-SO2 vibration appears at 396 and 430 cm(-1) for the C, conformer and at 387 and 402 cm(-1) for the C, 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.