Ion−Ion Interactions and Conduction Mechanism of Highly Conductive Fluorohydrogenate Ionic Liquids

Ion−ion interactions in highly conductive fluorohydrogenate ionic liquids (ILs) are discussed in this study. Low-temperature crystal structures of DMIm(FH)2F and DMIm(FH)3F (DMIm = 1,3-dimethylimidazolium) are determined by single-crystal X-ray diffraction to obtain the location of each ion in the crystal lattice. Interaction energies between the imidazolium cation and fluorohydrogenate anions are evaluated with the aid of quantum mechanical calculations where the configuration of the ions is taken from the crystal structures of DMIm(FH)2F and DMIm(FH)3F as well as the previously determined EMImFHF (EMIm = 1-ethyl-3-methylimidazolium). The calculation suggests that the interaction energies are mainly dominated by electrostatic interactions as in the cases of other imidazolium salts, and the low viscosity and high conductivity of fluorohydrogenate ILs are derived from their dynamic properties. The HF unit exchanging between fluorohydrogenate anions weakens the cation−anion interactions and produces smaller anionic diffusion species.