Vibrational spectroscopic study of the protic ionic liquid 1-H-3-methylimidazolium bis(trifluoromethanesulfonyl)imide

The solid and liquid phase vibrational spectra (Raman and FTIR/ATR) of 1-H-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (HMImTFSI) ionic liquid were recorded at temperatures up to 150 °C. The interpretation of the spectra indicates the existence of ion pairs in the liquid state in which, the anion and the cation, are bridged mainly through hydrogen bonds HBs between the nitrogen of the imidazole ring and the oxygen atoms of TFSI− anion. At room temperature, HMImTFSI is crystalline with TFSI− adopting the trans conformation. Upon melting, the spectra indicate the existence of both conformations (cis and trans) while at elevated temperatures cis conformation becomes dominant. The enthalpy of equilibrium between the two rotamers is, also, calculated from the reduced isotropic Raman spectra using the wagging SO2 vibrations in 380–450 cm−1 region, and it was found ∼8.5 kJ/mol.