Halogen/π interaction and cooperativity effect between dihalogen molecules and unsaturated hydrocarbons: An ab initio study

Ab initio calculations on the complexes formed between unsaturated hydrocarbon(R) and dihalogen molecules(XY), R⋯(XY)n (n = 1-2), have been performed at MP2/aug-cc-PVDZ level of theory. Geometrical structures, interaction energies and topological parameters derived from the theory of atoms in molecules (AIM) developed by Bader have been studied systematically to characterize the halogen⋯π interactions. The present theoretical investigation indicates that such interactions in all trimolecular complexes R⋯(XY)2 are significantly stronger than those in the corresponding bimolecular series R⋯(XY), demonstrating the existence of cooperativity effect. This may provide a theoretical basis for our understanding the reaction mechanism of the electrophilic addition of halogen to the unsaturated hydrocarbons.