Cation-cation clusters in ionic liquids: Cooperative hydrogen bonding overcomes like-charge repulsion

Abstract

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.

Theoretical Aspects of Ionic Liquids for Soft‐Matter Sciences

“…Hydrogen bonding between the ions, which often exists in ionic liquids, further complicates this question because hydrogen bonding may cause ionic liquids to form network structures that spatially connect the ions . In this case, the electrostatic screening effect would also be disparate from the prediction by the Debye−Hückel theory for dilute electrolyte solutions, not only because the ion concentration is high but also because the correlation between the ions could be substantially suppressed by hydrogen bonding . This suggests a fundamental difference between the electrostatic interactions of ionic liquids and the electrostatic interactions of solvents containing large amounts of inorganic salts.…”

Spectroscopic Evidence for Clusters of Like‐Charged Ions in Ionic Liquids Stabilized by Cooperative Hydrogen Bonding

“…We also calculated the B3LYP/6‐31+G*‐D3 energies and free energies of the c–a and c–c clusters . For example, the tetramers c–a are slightly lower in energy than the c–c and c–c‐cyc structures by about 5.83 and 1.0 kJ mol −1 per ion pair.…”

Polyion Covalency: Exotic Species from the Unexplored World of Electrostatically Shielded Molecular Ion Chemistry

“…[3] Subsequent studies also provided experimental evidence for AEHB clustering in ionic liquids and other fluid environments. [4] Such cation-cation or anion-anion AEHB species are profoundly counterintuitive,s erving to fundamentally shift debate between classical-type electrostatic versus quantum-type covalencyc onceptions of Hbonding. [5,6] Further aspects of electrostatic opposition to Hbonding are illustrated in recent theoretical prediction of the ion product (K W )o fl iquid water.…”