The topology of π•••H hydrogen bonds

Unsaturated hydrocarbons are routinely used as proton acceptors because their pi clouds possess a capacity to interact with Lewis acids and cationic species. Thus, in order to understand the formation of pi hydrogen-bonded complexes, which in many cases are considered as weakly bound systems, the topological quantification of the charge density ruled by quantum mechanic theorems is very useful. Although elaborated by Richard Bader 40 years ago, the Quantum Theory of Atoms in Molecules (QTAIM) has only recently become widely known, and it has been applied in several scientific fields, allowing great insights in chemical bonding studies of intermolecular systems, in particular those formed by pi center dot center dot center dot H hydrogen bonds.