Double Hole–Lump Interaction between Halogen Atoms

In this paper a theoretical study has been carried out to investigate the nature of the unusual halogen–halogen contacts in the complexes R–X···X–R (with R = −H, −Cl, −F and X = Cl, Br, I). AIM, NBO, and MEP analyses have been used to characterize X···X interactions. Formation of the unusual X···X interactions leads to a significant increase of electron charge density in the bonding region between the two halogen atoms. The geometry and stability of these complexes is mainly due to electrostatic interactions lump­(X1) → hole­(X2) and lump­(X2) → hole­(X1) [or equivalently [V_S,min(X1) → V_S,max(X2) and V_S,min(X2) → V_S,max(X1)] and the charge transfers LP­(X1) → σ*­(R–X2) and LP­(X2) → σ*­(R–X1). In other words, these findings suggest that the electrostatic interactions and the charge transfer play a substantial role in determining the optimal geometry of these complexes, as in conventional halogen bonds, even though the dispersion term is the most important attractive term for all the complexes studied here, save one.