Ab initio and atoms in molecules analyses of halogen bonding with a continuum of strength

Ab initio calculations at the MP2/aug-cc-PVDZ level of theory have been carried out to investigate a large number of complexes formed between halogen-containing molecules and several electron donors. It is shown that the strength of certain halogen bonds turns out to be greater than that of well-studied hydrogen bonds. Bader's atoms in molecules (AIM) theory has also been applied, and it is found that the electron densities at the halogen bond critical points correlate with the interaction energies, which indicates that the electron density can serve as a convenient measure of the strength of the different types of halogen bonding. Similar to hydrogen bonds, halogen bonds can be classified in a range from weak bonds, which are regarded as electrostatic interactions, to strong bonds, which have some degree of covalent character.