Spectroscopic evidence for hydrogen-bonded cationic clusters in the solid, liquid and the gaseous phases of hydroxy- and carboxy-functionalized ionic liquids

The unique properties of ionic liquids (ILs) result from the tunable mélange of Coulomb interactions, hydrogen bonding, and dispersion interactions among the constituent ions. In hydroxy-functionalized ILs, local and directional hydrogen bonds (H-bonds) lead to the anticipated formation of ion pairs but also to the elusive formation of cationic clusters. Here, we show how hydrogen-bonding motifs in the bulk liquid and gas phase of hydroxy-functionalized ILs shed light on the general nature of hydrogen bonding. Infrared spectroscopy, nuclear magnetic resonance, neutron diffraction, and molecular dynamics simulations provide information about the structure, strength, and dynamics of cationic clusters in the bulk liquid ILs. Cryogenic ion vibrational predissociation (CIVP) spectroscopy along with density functional theory calculations has established a clear picture about the specific contacts within isolated H-bonded cationic clusters formed in the gas phase. In carboxy-functionalized ILs doubly H-bonded cationic dimers are formed, resembling the archetype H-bond motif known for carboxylic acids. Surprisingly, this H-bond motif was the only one present in the crystal structures, in the liquid state at low temperatures and even in the gaseous phase in ternary cationic complexes wherein one counterion is removed. This information from experiment, simulation, and theory provides a fundamental understanding of hydrogen bonding, in particular between ions of like charge.