The intermolecular interaction in the charge-transfer complexes between amines and halogens: A theoretical characterization of the trends in halogen bonding

The trends in the properties of prereactive or charge-transfer complexes formed between the simple amines NH3, CH3NH2, (CH3)(2)NH, and (CH3)(3)N and the halogens F-2, ClF, Cl-2, BrF, BrCl, and Br-2 were investigated by the ab initio restricted Hartree-Fock approach, the Moller-Plesset second-order method, and with several density functional theory variants using extended polarized basis sets. The most important structural parameters, the stabilization energies, the dipole moments, and other quantities characterizing the intermolecular halogen bond in these complexes are monitored, discussed, and compared. A wide range of interaction strengths is spanned in these series. Successive methyl substitution of the amine as well as increasing polarities and polarizabilities of the halogen molecules both systematically enhance the signature of charge-transfer interaction. These trends in halogen bonds of varying strength, in many aspects, parallel the features of hydrogen bonding.