Probing Electron Density of H-Bonding between Cation-Anion of Imidazolium-Based Ionic Liquids with Different Anions by Vibrational Spectroscopy

Attenuated total reflection infrared spectroscopy and density functional theory calculation have been employed to Study the spectral properties of imidazolium-based ionic liquids (ILs) with different anions. ILs based on 1-butyl-3-methylimidazolium cation with different anions, OH-, CF3CO2-, HSO4-, H2PO4-, Cl-, PF6-, and BF4-, are investigated in the present work. It has been shown that the C-2-H stretching vibration of the imidazolium ring is closely related to the electron density of H-bonding between the two closest cations and anions for pure ILs. The electron density of H-bonding between cation and anion with different anions decreases in the order [OH](-) > [H2PO4](-) > [HSO4](-) > [CF3CO2](-) > [CI](-) > [BF4](-) > [PF6](-). For aqueous ILs, with increasing water content, the aromatic C-H stretching vibration of the imidazolium cation showed systematic blue-shifts. Especially for BmimOH, the nu(C2-H) undergoes a drastic blue-shift by 58 cm(-1), suggesting that the formation of the strong hydrogen bonds O-H center dot center dot center dot O may greatly weaken the electron density of H-bonding between the cation and anion of ILs.