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Author: Knorr, Anne × Author: Ludwig, Ralf × End date: 2019 × Start date: 2010 × Type: Text × WGL Institute: LIKAT ×

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Now showing 1 - 3 of 3
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    Cation-cation clusters in ionic liquids: Cooperative hydrogen bonding overcomes like-charge repulsion
    ([London] : Macmillan Publishers Limited, part of Springer Nature, 2015) Knorr, Anne; Ludwig, Ralf
    Direct spectroscopic evidence for H-bonding between like-charged ions is reported for the ionic liquid, 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate. New infrared bands in the OH frequency range appear at low temperatures indicating the formation of H-bonded cation-cation clusters similar to those known for water and alcohols. Supported by DFT calculations, these vibrational bands can be assigned to attractive interaction between the hydroxyl groups of the cations. The repulsive Coulomb interaction is overcome by cooperative hydrogen bonding between ions of like charge. The transition energy from purely cation-anion interacting configurations to those including cation-cation H-bonds is determined to be 3–4 kJmol−1. The experimental findings and DFT calculations strongly support the concept of anti-electrostatic hydrogen bonds (AEHBs) as recently suggested by Weinhold and Klein. The like-charge configurations are kinetically stabilized with decreasing temperatures.
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    Spectroscopic evidence of 'jumping and pecking' of cholinium and H-bond enhanced cation-cation interaction in ionic liquids
    (Cambridge : RSC Publ., 2015) Knorr, Anne; Fumino, Koichi; Bonsa, Anne-Marie; Ludwig, Ralf
    The subtle energy-balance between Coulomb-interaction, hydrogen bonding and dispersion forces governs the unique properties of ionic liquids. To measure weak interactions is still a challenge. This is in particular true in the condensed phase wherein a melange of different strong and directional types of interactions is present and cannot be detected separately. For the ionic liquids (2-hydroxyethyl)-trimethylammonium (cholinium) bis(trifluoro-methylsulfonyl)amide and N,N,N-trimethyl-N-propylammonium bis(trifluoromethylsulfonyl)amide which differ only in the 2-hydroxyethyl and the propyl groups of the cations, we could directly observe distinct vibrational signatures of hydrogen bonding between the cation and the anion indicated by ‘jumping and pecking’ motions of cholinium. The assignment could be confirmed by isotopic substitution H/D at the hydroxyl group of cholinium. For the first time we could also find direct spectroscopic evidence for H-bonding between like-charged ions. The repulsive Coulomb interaction between the cations is overcome by cooperative hydrogen bonding between the 2-hydroxyethyl functional groups of cholinium. This H-bond network is reflected in the properties of protic ionic liquids (PILs) such as viscosities and conductivities.
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    Spectroscopic Evidence for Clusters of Like-Charged Ions in Ionic Liquids Stabilized by Cooperative Hydrogen Bonding
    (Weinheim : Wiley-VCH, 2016) Knorr, Anne; Stange, Peter; Fumino, Koichi; Weinhold, Frank; Ludwig, Ralf
    Infrared spectroscopy and density functional theory calculations provide strong evidence for the formation of clusters of like-charged ions in ionic liquids. With decreasing temperature, cooperative hydrogen bonding overcomes repulsive electrostatic interaction. The resulting cyclic tetramers nicely resemble well-known molecular clusters of alcohols.