Energetic and geometrical evidence of nonbonding character of some intramolecular halogen···oxygen and other Y···Y interactions.

We present high-level ab initio calculations for representative group of molecules possessing the intramolecular interaction of the X···O type (X = F, Cl, Br, and I). Few examples of molecules with intramolecular O···O and F···F interactions are also investigated. We show that, although most often investigated as bonding, all these interactions are nonbonding or repulsive, as indicated by positive values of interaction energies obtained by means of a few estimating methods, by thorough studies of geometrical changes that take place during the X···O contact formation and by the lateral contact of electron density isosurfaces of X and O atoms, which both are characterized by negative values of electrostatic potential. The presented conclusion about the repulsive nature of the intramolecular X···O and similar interactions is in opposition to the proclamation of the quantum theory of atoms in molecules of Bader that the presence of a bond path and a corresponding bond critical point is always to indicate the bonding, thus, stabilizing interaction.