Theoretical characterization of the trends in halogen bonding

Theoretical investigations of the systematic trends in the spectroscopically observable properties of small intermolecular complexes bound via halogen bonding are reviewed. As a representative example of the charge-transfer or electron donor-acceptor complexes XY...B of dihalogens XY with different electron acceptors B, the case where B is an amine, in particular NH3 or a methylated amine, has been selected. Additionally, a survey of the calculated properties of complexes of the type C - X...B, formed between molecules carrying the C-X group, such as halomethanes, and amines is provided. The borderline case of complexes with C - X... H - C interactions, in which halogen bonding and hydrogen bonding occur simultaneously, is included as well. The close analogy between the features characteristic of halogen bonding and those typical for hydrogen bonding is pointed out. The stabilization energies, structural parameters, theoretical vibrational spectra, and other properties useful for describing the charge transfer in these complexes are discussed in detail.