The role of molecular electrostatic potentials in the formation of a halogen bond in furan⋅⋅⋅XY and thiophene⋅⋅⋅XY complexes.

The halogen bonding of furan⋅⋅⋅XY and thiophene⋅⋅⋅XY (X=Cl, Br; Y=F, Cl, Br), involving σ- and π-type interactions, was studied by using MP2 calculations and quantum theory of "atoms in molecules" (QTAIM) studies. The negative electrostatic potentials of furan and thiophene, as well as the most positive electrostatic potential (V(S,max) ) on the surface of the interacting X atom determined the geometries of the complexes. Linear relationships were found between interaction energy and V(S,max) of the X atom, indicating that electrostatic interactions play an important role in these halogen-bonding interactions. The halogen-bonding interactions in furan⋅⋅⋅XY and thiophene⋅⋅⋅XY are weak, "closed-shell" noncovalent interactions. The linear relationship of topological properties, energy properties, and the integration of interatomic surfaces versus V(S,max) of atom X demonstrate the importance of the positive σ hole, as reflected by the computed V(S,max) of atom X, in determining the topological properties of the halogen bonds.