Liquid structure of room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis-(trifluoromethanesulfonyl) imide

The liquid structure of 1-ethyl-3-methylimidazolium bis-(trifluoromethanesulfonyl) imide (EMI+TFSI-) has been studied by means of large-angle X-ray scattering (LAXS), H-1, C-13, and F-19 NMR, and molecular dynamics (MD) simulations. LAXS measurements show that the ionic liquid is highly structured with intermolecular interactions at around 6, 9, and 15 angstrom. The intermolecular interactions at around 6, 9, and 15 angstrom are ascribed, on the basis of the MD simulation, to the nearest neighbor EMI+...TFSI- interaction, the EMI+...EMI+ and TFSI-...TFSI- interactions, and the second neighbor EMI+...TFSI- interaction, respectively. The ionic liquid involves two conformers, C-1 (cis) and C-2 (trans), for TFSI-, and two conformers, planar cis and nonplanar staggered, for EMI+, and thus the system involves four types of the EMI+...TFSI- interactions in the liquid state by taking into account the conformers. However, the EMI+...TFSI- interaction is not largely different for all combinations of the conformers. The same applies also to the EMI+...EMI+ and TFSI-...TFSI- interactions. It is suggested from the C-13 NMR that the imidazolium C-2 proton of EMI+ strongly interacts with the O atom of the -SO2(CF3) group of TFSI-. The interaction is not ascribed to hydrogen-bonding, according to the MD simulation. It is shown that the liquid structure is significantly different from the layered crystal structure that involves only the nonplanar staggered EMI+ and C-1 TFSI- conformers.