Halogen bonds enhanced by σ-hole and π-hole interactions: a comparative study on cooperativity and competition effects between X∙∙∙N and S∙∙∙N interactions in H3N∙∙∙XCN∙∙∙SF2 and H 3N∙∙∙XCN∙∙∙SO2 complexes (X   = F, Cl, Br and I).

Halogen bonding, a noncovalent interaction between a halogen atom X in one molecule and a negative site in another, plays an important role in fields as diverse as molecular biology, drug design, and crystal engineering. In this work, the H3N∙∙∙XCN∙∙∙SF2 and H3N∙∙∙XCN∙∙∙SO2 (X  =  F, Cl, Br, I) complexes are theoretically investigated to find ways to enhance the halogen bond interaction. Cooperative effects are found when X∙∙∙N and S∙∙∙N bonds coexist in the same complex. The ab initio calculations are carried out using at the MP2/aug-cc-pVTZ level, through analysis of surface electrostatic potentials VS(r), interaction energies and the topological analysis based on the quantum theory of atoms in molecules. Particular attention is paid to understand the origin of the X∙∙∙N and S∙∙∙N interactions in the ternary complexes. The cooperativity between both types of the interaction is mainly caused by the electrostatic effects.