title: Ion pairing in protic ionic liquids probed by far-infrared spectroscopy: effects of solvent polarity and temperature.
abstract: The cation-anion and cation-solvent interactions in solutions of the protic ionic liquid (PIL) [Et3NH][I] dissolved in solvents of different polarities are studied by means of far infrared vibrational (FIR) spectroscopy and density functional theory (DFT) calculations. The dissociation of contact ion pairs (CIPs) and the resulting formation of solvent-separated ion pairs (SIPs) can be observed and analyzed as a function of solvent concentration, solvent polarity, and temperature. In apolar environments, the CIPs dominate for all solvent concentrations and temperatures. At high concentrations of polar solvents, SIPs are favored over CIPs. For these PIL/solvent mixtures, CIPs are reformed by increasing the temperature due to the reduced polarity of the solvent. Overall, this approach provides equilibrium constants, free energies, enthalpies, and entropies for ion-pair formation in trialkylammonium-containing PILs. These results have important implications for the understanding of solvation chemistry and the reactivity of ionic liquids.
authors.affiliation: Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr.-Lorenz-Weg 1, 18059, Rostock (Germany).
authors.forename: Koichi
authors.initials: K
authors.lastname: Fumino
authors.forename: Verlaine
authors.initials: V
authors.lastname: Fossog
authors.forename: Peter
authors.initials: P
authors.lastname: Stange
authors.forename: Kai
authors.initials: K
authors.lastname: Wittler
authors.forename: Wigbert
authors.initials: W
authors.lastname: Polet
authors.forename: Rolf
authors.initials: R
authors.lastname: Hempelmann
authors.forename: Ralf
authors.initials: R
authors.lastname: Ludwig
keywords: FTIR spectroscopy; density functional calculations; ion pairs; ionic liquids; polarity