Low-frequency modes of ionic liquids studied with terahertz-time domain spectroscopy and ab initio MO calculations

We have obtained the dielectric spectra of ionic liquids with the terahertz time-domain spectroscopy (THz-TDS) in a frequency range from 3.3 cm^-1 to 140 cm^-1. Ionic liquids composed of dialkylimidazolium or quaternary ammonium have been examined. The ionic liquids show broad but structured THz dielectric spectra, which could be assigned to the bands of inter-ion motions. The DFT calculations of respective cation-anion pairs give inter-ion vibrational modes in the THz region. The bending-like inter-ion motions, which originate from the rotational, translational, or torsional motions of the ions, are located below 100 cm^-1 whereas the inter-ion stretching motions are located above 100 cm^-1.