Solvent influence on imidazolium based ionic liquid contact pairs

Abstract

The interaction and solvent influence on two different imidazolium based ion pairs have been investigated by molecular dynamics simulations and Nuclear Magnetic Resonance. The cations 1,2,3,4,5-pentamethyl imidazolium and 1,3,4,5-tetramethyl imidazolium were considered with the imidazolate anion to evaluate the influence of the acidic hydrogen at the position 2 of the imidazolium ring on the ion pair formation in different solvents. The selected solvents are chloroform, dichloromethane, acetone, dimethylsulfoxide, and water, covering a broad range of polarity and permittivity. The binding free energy of the ion pair was computed by umbrella sampling. We observed that, with the increase of the dielectric constant, the ion pairs become more transient being separated in water. The free energies of binding corroborate ion pair stabilization by the hydrogen bond at carbon 2 of the imidazolium cations. In dichloromethane, we obtained weaker bound ion pairs than in acetone due to intercalation of dichloromethane into the ion pair. Thus, the ion pair stability is not only a consequence of the solvent's dielectric constant, but also due to local structural details.