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Ion Pairing in Protic Ionic Liquids Probed by Far-Infrared Spectroscopy: Effects of Solvent Polarity and Temperature

Authors

  • Dr. Koichi Fumino,

    1. Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr.-Lorenz-Weg 1, 18059, Rostock (Germany)
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  • Verlaine Fossog,

    1. Universität des Saarlandes, Physikalische Chemie, 66123 Saarbrücken (Germany)
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  • Peter Stange,

    1. Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr.-Lorenz-Weg 1, 18059, Rostock (Germany)
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  • Kai Wittler,

    1. Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr.-Lorenz-Weg 1, 18059, Rostock (Germany)
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  • Wigbert Polet,

    1. Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr.-Lorenz-Weg 1, 18059, Rostock (Germany)
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  • Prof. Dr. Rolf Hempelmann,

    1. Universität des Saarlandes, Physikalische Chemie, 66123 Saarbrücken (Germany)
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  • Prof. Dr. Ralf Ludwig

    Corresponding author
    1. Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr.-Lorenz-Weg 1, 18059, Rostock (Germany)
    2. Leibniz-Institut für Katalyse an der Universität Rostock e.V. Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
    • Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr.-Lorenz-Weg 1, 18059, Rostock (Germany)

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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.

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