Geometries, vibrational frequencies and interaction energies of the complexes oxirane…XY and thiirane…XY (XY＝Cl2, Br2, ClF, BrF, BrCl) were examined by the counterpoise-corrected (CP-corrected) gradient optimization that explicitly corrects the basis set superposition error (BSSE) at the B3LYP/6-311++G(d,p) and B3LYP/aug-cc-pVDZ levels of theory, respectively. The topological properties of halogen-bonded complexes were investigated by the topological analysis of electron density. The nature of halogen bond was investigated, indicating that this kind of halogen bond is between covalent and ionic bonds, with the electrostatic interaction being dominant. The formation of halogen bonds results in an elongation of X—Y bonds, red-shift of vibrational frequencies, which is accompanied by the changes of the integration over atomic basin. The change of bond length, the energy of a halogen bond and the electron density of a critical bond are related to the electronegativity of dihalogen molecules.

Key words: intermolecular interaction, halogen bond, B3LYP, topological analysis of electron density