Analyzing the interaction energies between cation and anion in ionic liquids: The subtle balance between Coulomb forces and hydrogen bonding

Potential applications of ionic liquids depend on the properties of this class of liquid material. To a large extent the structure and properties of these Coulomb systems are determined by the intermolecular interactions among anions and cations. In particular the subtle balance between Coulomb forces, hydrogen bonds and dispersion forces is of great importance for the understanding of ionic liquids. All these issues are addressed by using a suitable combination of experimental and theoretical methods including specially synthesized imidazolium-based ionic liquids, far infrared spectroscopy (FIR) and density functional theory (DFT) calculations. The key statement is that although ionic liquids consist solely of anions and cations and Coulomb forces are the dominating interaction, a local and directional interaction such as hydrogen bonding has significant influence on the structure and properties of ionic liquids. In this review we mainly summarize the results we achieved within our project of the priority programme “Ionic Liquids” (SPP 1191), which has been funded by the German Science Foundation (DFG) between 2008 and 2012.