Prediction and characterization of halogen bonds involving formamidine and its derivatives

Ab initio calculations have been carried out for the complexes of formamidine (FA) and some representative halogenated molecules XY (X=Cl, Br, and I; Y=F, CCH, CF3, CN, and NC). The FA-(Z) complex combines with the halogenated molecule through a halogen bond, while the FA-(E) complex is stabilized jointly by both a halogen bond and a X⋯H interaction. The FA-(E) complex is more stable than the FA-(Z) counterpart, with the interaction energy of -3.4 to -23.4kcal/mol, indicating that FA is a good electron donor in halogen bonding. The methyl substituent particularly one at the imino nitrogen atom of FA has an enhancing effect on the strength of halogen bond. The similar effect is found for the phenyl and pyridyl substituents, depending on the FA conformation and substitution position of pyridyl. The stability of stronger halogen bonding is mainly attributed to electrostatic and polarization energies, which is different from the weak one with an electrostatic nature.