Attraktive Wechselwirkungen zwischen Ionen gleicher Ladung in Ionischen Flüssigkeiten: Der Einfluss von Druck, Polarität und molekularen Mimetika

• Antragsteller: Professor Dr. Ralf Ludwig
• Fachliche Zuordnung: Physikalische Chemie von Molekülen, Flüssigkeiten und Grenzflächen, Biophysikalische Chemie
• Förderung: Förderung von 2016 bis 2023
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 286149019

• Erstellungsjahr: 2023

Zusammenfassung der Projektergebnisse

In the first funding period, we showed that the interaction strength of the anion, the polarizability of the cation, and the length of hydroxyl alkyl groups of the cation control the delicate balance between hydrogenbonded cation-anion and cation-cation clusters in bulk ionic liquids. We investigated the kinetic and thermodynamic stabilities of clusters of like-charged ions. The plethora of exciting results about the attractive interaction between ions of like charge provoked further questions, which we addressed in the renewal proposal: Can we control the delicate balance between cation-anion ion-pairs and the cation-cation clusters by changing the dielectric medium using solvents of different polarity? How important are specific interactions with solvent molecules? Can we enhance hydrogen bonding and cluster formation by replacing the cations by their molecular mimics? Are the structural motifs of cationic and molecular gas phase clusters also present in the bulk liquid phase? We tackled these challenges with a suitable combination of synthesis of well-selected ionic liquids, bulk and gas phase infrared spectroscopy, and quantum chemical methods. Overall, we provided a comprehensive understanding of ‘‘like–likes–like’’ charge attraction (L3Chat) in hydroxyfunctionalized ionic liquids (ILs) by a plethora of methods including infrared spectroscopy (IR), solid state NMR spectroscopy, neutron diffraction, differential scanning calorimetry (DSC), molecular dynamics (MD) simulations and quantum chemistry. We demonstrated that hydrogen bonds (HBs) between cation and cation (c–c) are stronger than that between cation and anion (c-a), despite the repulsive Coulomb interaction in the first and attractive Coulomb interaction in the latter case. We showed for a well-suited set of hydroxyl-functionalized ILs that the formation of cationic clusters is tunable by polarizable cations, weakly interacting anions and increasing hydroxylalkyl chain lengths. Combined FT-IR and solid-state NMR spectroscopy provided a suitable characterizing of the cationic clusters in terms of size, shape and H-bond motifs. That the H-bonds in the cationic clusters are stronger as those in cationanion pairs is supported by redshifted OH IR frequencies, downfield NMR proton chemical shifts and smaller deuteron quadrupole coupling constants. Thus, the spectroscopic features also shed light onto the discussion about the stability of so-called “Anti electrostatic hydrogen bonds”. A combination of FT-IR, solid-state NMR spectroscopy and neutron diffraction (ND) measurements further allowed to determining the populations of the (c–a) and (c–c) cluster species as a function of temperature. The by far largest temperature range is accessible by simple infrared (IR) spectroscopy. We presented a new method for deriving the number of cations involved in (c–a) ion pairs from IR spectra in the OH stretch region. This procedure provides access to the equilibria of (c–a) and (c–c) hydrogen bonds as a function of temperature allowing derivation of the transition enthalpy. Our approach should be also applicable to spectra of molecular liquids as a function of temperature or to spectra of their mixtures in solvents with varying concentration, if the high frequency OH-band is assignable to distinct molecular species such as monomers. With molecular dynamics (MD) simulations, we have studied the HB kinetics of each of these HBs via HB population and reactive flux correlation functions. Respective HB lifetimes are available, either as integrals over HB population correlation functions or as inverse rates of HB breaking. For the case of hydroxyl-functionalized ILs, we have to deal with four different HB lifetimes, each valid in its own right, and each associated with different aspects of HB breaking and HB reformation. Surprisingly, the H-bond life times of the (c-c) clusters are longer than those of the (c-a) clusters despite the repulsive Coulomb interaction in the first case and the attractive Coulomb interaction in the latter case. We also continued the fruitful collaboration with the Johnson group at Yale University. Cryogenic Ionic Vibrational Predissociation (CIVP) spectroscopy of mass-selected cluster ions in the gas phase revealed the molecular-level contacts among the ionic constituents. Double resonance experiments allowed to distinguishing isomers. These methods supported the analysis of the broad FT-IR spectra of the bulk ILs, which only provide a qualitative picture of the local interactions at play.

Projektbezogene Publikationen (Auswahl)

• Cooperatively enhanced hydrogen bonds in ionic liquids: closing the loop with molecular mimics of hydroxy-functionalized cations. Physical Chemistry Chemical Physics, 21(33), 18092-18098.
  Niemann, Thomas; Strate, Anne; Ludwig, Ralf; Zeng, Helen J.; Menges, Fabian S. & Johnson, Mark A.
  
• Influence of Hydrogen Bonding between Ions of Like Charge on the Ionic Liquid Interfacial Structure at a Mica Surface. The Journal of Physical Chemistry Letters, 10(23), 7368-7373.
  Niemann, Thomas; Li, Hua; Warr, Gregory G.; Ludwig, Ralf & Atkin, Rob
  
• Chain Length Dependence of Hydrogen Bond Linkages between Cationic Constituents in Hydroxy-Functionalized Ionic Liquids: Tracking Bulk Behavior to the Molecular Level with Cold Cluster Ion Spectroscopy. The Journal of Physical Chemistry Letters, 11(3), 683-688.
  Zeng, Helen J.; Menges, Fabian S.; Niemann, Thomas; Strate, Anne; Ludwig, Ralf & Johnson, Mark A.
  
• Clusters of Hydroxyl-Functionalized Cations Stabilized by Cooperative Hydrogen Bonds: The Role of Polarizability and Alkyl Chain Length. Molecules, 25(21), 4972.
  Philipp, Jule & Ludwig, Ralf
  
• Controlling “like–likes–like” charge attraction in hydroxy-functionalized ionic liquids by polarizability of the cations, interaction strength of the anions and varying alkyl chain length. Physical Chemistry Chemical Physics, 22(5), 2763-2774.
  Niemann, Thomas; Zaitsau, Dzmitry H.; Strate, Anne; Stange, Peter & Ludwig, Ralf
  
• Counting cations involved in cationic clusters of hydroxy-functionalized ionic liquids by means of infrared and solid-state NMR spectroscopy. Physical Chemistry Chemical Physics, 22(13), 6861-6867.
  Strate, Anne; Neumann, Jan; Niemann, Thomas; Stange, Peter; Khudozhitkov, Alexander E.; Stepanov, Alexander G.; Paschek, Dietmar; Kolokolov, Daniil I. & Ludwig, Ralf
  
• Cyclic Octamer of Hydroxyl‐functionalized Cations with Net Charge Q=+8e Kinetically Stabilized by a ‘Molecular Island’ of Cooperative Hydrogen Bonds. ChemPhysChem, 21(21), 2367-2367.
  Philipp, J. K.; Fritsch, S. & Ludwig, R.
  
• Kinetics of Hydrogen Bonding between Ions with Opposite and Like Charges in Hydroxyl-Functionalized Ionic Liquids. The Journal of Physical Chemistry B, 125(1), 281-286.
  Neumann, Jan; Paschek, Dietmar; Strate, Anne & Ludwig, Ralf
  
• Hydrogen Bonds between Ions of Opposite and Like Charge in Hydroxyl-Functionalized Ionic Liquids: an Exhaustive Examination of the Interplay between Global and Local Motions and Intermolecular Hydrogen Bond Lifetimes and Kinetics. The Journal of Physical Chemistry B, 125(19), 5132-5144.
  Neumann, Jan; Ludwig, Ralf & Paschek, Dietmar
  
• Three in One: The Versatility of Hydrogen Bonding Interaction in Halide Salts with Hydroxy‐Functionalized Pyridinium Cations. ChemPhysChem, 22(18), 1850-1856.
  Al Sheakh, Loai; Niemann, Thomas; Villinger, Alexander; Stange, Peter; Zaitsau, Dzmitry H.; Strate, Anne & Ludwig, Ralf