Cation-anion hydrogen bonds: a new class of hydrogen bonds that extends their strength beyond the covalent limit. A theoretical characterization.

The existence of O-H···O hydrogen bonds having a strength within the -80 to -210 kcal/mol range, that is, in the range of strength of covalent bonds and well beyond the so-called covalent limit (-50 kcal/mol), is reported on complexes where the O-H proton donor and O acceptor groups are located in ions of opposite sign. A complete analysis of short distance O-H···O hydrogen bonds between charged fragments was performed for cases where the OH and O groups are both located on charged molecules. It shows that these interactions (a) are nonsymmetrical for the O-H and H···O distances, (b) have a noncovalent H···O bond critical point, and (c) have a strong and energetically stable electrostatic component when the OH and O groups are located in oppositely charged molecules. These cation-anion O-H···O interactions are energetically stable, satisfy the usual topology for hydrogen bonds, HBs, and also have the same directionality found in other HBs. Therefore, they should be considered as a new class of HBs, the cation-anion hydrogen bonds.