This work has been supported by the DFG Priority Programme 1191 "Ionic Liquids" and by the DFG Sonderforschungsbereich SFB 652. We thank Verlaine Fossog (Physikalische Chemie, Universität des Saarlandes) for providing us with the protic ionic liquid TEAMS.Supporting information for this article is available on the WWW under http://dx.

Direct spectroscopic evidence for hydrogen‐bonded clusters of like‐charged ions is reported for ionic liquids. The measured infrared O−H vibrational bands of the hydroxyethyl groups in the cations can be assigned to the dispersion‐corrected DFT calculated frequencies of linear and cyclic clusters. Compensating the like‐charge Coulomb repulsion, these cationic clusters can range up to cyclic tetramers resembling molecular clusters of water and alcohols. These ionic clusters are mainly present at low temperature and show strong cooperative effects in hydrogen bonding. DFT‐D3 calculations of the pure multiply charged clusters suggest that the attractive hydrogen bonds can compete with repulsive Coulomb forces.

The properties of ionic liquids are determined by the energy-balance between Coulomb-interaction, hydrogen-bonding, and dispersion forces. Out of a set of protic ionic liquids (PILs), including trialkylammonium cations and methylsulfonate and triflate anions we could detect the transfer from hydrogen-bonding to dispersion-dominated interaction between cation and anion in the PIL [(C6 H13 )3 NH][CF3 SO3 ]. The characteristic vibrational features for both ion-pair species can be detected and assigned in the far-infrared spectra. Our approach gives direct access to the relative strength of hydrogen-bonding and dispersion forces in a Coulomb-dominated system. Dispersion-corrected density functional theory (DFT) calculations support the experimental findings. The dispersion forces could be quantified to contribute about 2.3 kJ mol(-1) per additional methylene group in the alkyl chains of the ammonium cation.

Polarity controls the equilibrium constants and free energies of contact ion pairs (CIPs) and solvent-separated ion pairs (SIPs) in mixtures of protic ionic liquids and molecular solvents. The subtle balance between the ionic species was studied by far-infrared difference spectra and related DFT-calculated properties for solvents of low and high polarity and for different solvent concentrations.

We characterize the double-faced nature of hydrogen bonding in hydroxy-functionalized ionic liquids by means of neutron diffraction with isotopic substitution (NDIS), molecular dynamics (MD) simulations,and quantum chemical calculations.N DIS data are fit using the empirical potential structure refinement technique (EPSR) to elucidate the nearest neighbor H···O and O···O pair distribution functions for hydrogen bonds between ions of opposite charge and the same charge.D espite the presence of repulsive Coulomb forces,t he cation-cation interaction is stronger than the cation-anion interaction. We compare the hydrogen-bond geometries of both "doubly charged hydrogen bonds" with those reported for molecular liquids,such as water and alcohols.Incombination, the NDIS measurements and MD simulations reveal the subtle balance between the two types of hydrogen bonds:T he small transition enthalpysuggests that the elusive like-charge attraction is almost competitive with conventional ion-pair formation.