Infrared Spectroscopy of Ionic Liquids Consisting of Imidazolium Cations with Different Alkyl Chain Lengths and Various Halogen or Molecular Anions with and without a Small Amount of Water

Infrared spectroscopy was performed on ionic liquids (ILs) that had imidazolium cations with different alkyl chain lengths and various halogen or molecular anions with and without a small amount of water. The molar concentration normalized absorbance due to +C-H vibrational modes in the range of 3000 to 3200 cm-1 was nearly identical for ILs that had imidazolium cations with different alkyl chain lengths and the same anions. A close correlation was found between the red-shifted +C-H vibrational modes, the chemical shift of +C(2)-H proton, and the energy stabilization of the hydrogen-bonding interaction. The vibrational modes of the water molecules interacting with anions in the range between 3300 and 3800 cm-1 was examined. The correlation between the vibrational frequencies of water, the frequencies of +C-H vibrational modes, and the center frequency of intermolecular vibrational modes due to ion pairs was discussed.