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

摘要

We have obtained the dielectric spectra of ionic liquids with the terahertz time-domain spectroscopy (THz-TDS) in a frequency range from 3.3 cm1 to 140 cm1. 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 cm1 whereas the inter-ion stretching motions are located above 100 cm1.