Anion Conformation of Low-Viscosity Room-Temperature Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(fluorosulfonyl) Imide

Anion conformation of a low-viscosity room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl) imide (EMI⁺FSI^-) has been studied by Raman spectra and theoretical DFT calculations. Three strong Raman bands were found at 293, 328, and 360 cm^-1, which are ascribed to the FSI^- ion. These Raman bands show significant temperature dependence, implying that two FSI^- conformers coexist in equilibrium. This is supported by theoretical calculations that the FSI^- ion is present as either C₂ (trans) or C₁ (cis) conformer; the former gives the global minimum, and the latter has a higher SCF energy of about 4 kJ mol^-1. Full geometry optimizations followed by normal frequency analyses show that the observed bands at 293, 328, and 360 cm^-1 are ascribed to the C₂ conformer. The corresponding vibrations at 305, 320, and 353 cm^-1 were extracted according to deconvolution of the observed Raman bands in the range280−400 cm^-1 and are ascribed to the C₁ conformer. The enthalpy ΔH° of conformational change from C₂ to C₁ was experimentally evaluated to be ca. 4.5 kJ mol^-1, which is in good agreement with the predicted value by theoretical calculations. The bis(trifluoromethanesulfonyl) imide anion (TFSI^-) shows a conformational equilibrium between C₁ and C₂ analogues (ΔH° = 3.5 kJ mol^-1). However, the profile of the potential energy surface of the conformational change for FSI^- (the F−S−N−S dihedral angle) is significantly different from that for TFSI^- (the C−S−N−S dihedral angle).