A new class of ion-ion interaction: Z-bond

The hydrogen-bond interactions in ionic liquids have been simply described by the conventional hydrogen-bond model of A-Ha <-B. Coupling with the strong electrostatic force, however, hydrogen bond between the cation and anion shows particular features in the geometric, energetic, electronic, and dynamic aspects, which is inherently different from that of the conventional hydrogen bond. A general model could be expressed as (+)[A-Ha <-B](-), in which A and B represent heavy atoms and "+" and "-aEuroe represent the charges of the cation containing A atom and anion containing B atom, respectively. Because the structure shows a "zig-zag" motif, this coupling interaction is defined here as the Z-bond. The new model could be generally used to describe the interactions in ionic liquids, as well as bio-systems involved in ions, ionic reaction, and ionic materials.