Temperature and Pressure Dependence of the Infrared Spectrum of 1-Ethyl-3-Methylimidazolium Trifluoromethanesulfonate Ionic Liquid

Abstract

The infrared absorption spectrum of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI–TfO) was investigated at ambient pressure and variable temperatures between 120 and 330 K, or at room temperature and variable pressures up to 10 GPa. Upon cooling, the ionic liquid crystallizes; on the contrary, upon compression no evidence of crystallization can be obtained from the infrared spectra. Moreover, Density Functional Theory (DFT) calculations were applied to gain a better description of the ionic couple. The ωB97X-D functional, including not only the empirical dispersion corrections but also the presence of a polar solvent, gives a good agreement with the infrared spectrum and suggests that TfO resides above the plane of the imidazolium, with the shorter distance between the O atom of the anion and the C2 atom of the imidazolium ring equal to 2.23 Å.

Evidence of the CH···O HydrogenBonding in Imidazolium-Based Ionic Liquids from Far-Infrared Spectroscopy Measurements and DFT Calculations

“…At 110K, two pairs of narrow peaks are well detectable, one with peak frequencies at 205 and 215 cm −1 , and the other one with peak frequencies between 515 and 520 cm −1 , which can be safely attributed to the anion vibrations, in agreement with previous computational results obtained for the isolated anion [ 8 , 44 ]. On heating above 250 K, both doublets merge into two broad bands, respectively at 210 and 515 cm −1 , signaling the occurrence of a solid–liquid transition, which was already evidenced for this IL by differential scanning calorimetry and mid-infrared spectroscopy measurements [ 44 , 45 ].…”