A-H···π Hydrogen Bonds-Where Are Proton Acceptor Sites Located?

Different types of A-H···π hydrogen bonds are analyzed. Two extreme categories of this interaction may be specified: the first category where the A-H bond is directed to a center of the π-electron system and the second category where this bond is directed far from such a center to a carbon atom, for example, if a π-electron donor is a system of carbon centers. However, the categorization is more complicated, since there are numerous intermediate cases. The Cambridge Structural Database searches were performed here to find the first category of interactions; these are π-electron systems with the triple C≡C bond, or with the 6-member benzene ring, or with the 5-member cyclopentadienyl anion ring. The MP2/6-311++G(d,p) results of calculations on simple complexes, where acetylene, ethylene, benzene, or cyclopentadienyl anions act as a proton acceptor, are also described. These results are supported by the Natural Bond Orbital method and the Quantum Theory of Atoms in Molecule approach calculations.