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

Abstract

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 C2 and C1 symmetry, respectively, constitute global and local minima, with an energy difference 2.2-3.3 kJ mol(-1). The wagging omega-SO2 vibration appears at 396 and 430 cm(-1) for the C1 conformer and at 387 and 402 cm(-1) for the C2 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 C2 to C1 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 C1 and C2 conformers of TFSI- in the liquid EMI+TFSI-, and the C2 conformer is more favorable than the C1 one.

Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids

“…A change in the preferred conformer of the [Tf 2 N] À anion at the surface depending on the alkylchain length of the cation was recently found by K. Nakajima et al 36 This fact was considered by the choice of an ion pair with a syn-conformer of the anion for [EMIm]Tf 2 N and one with the respective anti-conformer is chosen for [OMIm]Tf 2 N (see the displayed structure of the two utilized ion pairs in Fig. 1 52 have shown that the syn-conformer of the anion exhibits a slightly higher energy than the anti-conformer. This energetic order is also present in our calculations.…”

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

“…It was concluded that a conformational equilibrium indeed must exist between the C 1 and C 2 conformers of the [Tf 2 N] À ion in the liquid À ion, known also from crystal structures [166]. Figure adapted from Fujii et al [165] [C 2 mim][Tf 2 N]. The C 2 conformer is more favorable than the C 1 one [165].…”

“…In addition to these studies, the structure of the bis(trifluoromethylsulfonyl)imide ([Tf 2 N] À ) anion in the liquid state has been investigated by means of IR and Raman spectroscopy and ab-initio self-consistent Hartree-Fock and DFT calculations on the free ion, aiming at a determination of the equilibrium geometry and understanding of the vibrational spectrum [137,[163][164][165]. A pronounced delocalization of the negative charge on the nitrogen and oxygen atoms was found, and a marked double-bond character of the S-N-S moiety for the anion [163,164].…”

“…22. According to the DFT calculations (followed by normal frequency analyses), two conformers of C 2 and C 1 point group symmetry (a two-fold rotational axis and no symmetry), respectively, constitute global and local minima, and have an energy difference of 2.2-3.3 kJ mol À1 [165]. The wagging omega-SO 2 vibration appeared at 396 and 430 cm À1 for the C 1 conformer and at 387 and 402 cm À1 for the C 2 one.…”

“…Figure adapted from Fujii et al [165] [C 2 mim][Tf 2 N]. The C 2 conformer is more favorable than the C 1 one [165]. Three different geometries (named cis and trans by the authors) have recently been determined in the X-ray crystal structure of the salt Li 2 [C 2 mim][Tf 2 N] 3 [166].…”