The nature and geometry of intermolecular interactions between halogens and oxygen or nitrogen

The nature of intermolecular interactions between carbon-bonded halogens (C-X, X = F, Cl, Pr, or I) and electronegative atoms (El N, O and S) has been analysed, focusing on the role of specific attractive forces and the anisotropic repulsive wall around halogen atoms. Searches of the Cambridge Structural Database show that electronegative atoms in various hybridization states clearly prefer to form contacts to Cl, Br, and I (but not F) in the direction of the extended C-X bond axis, at interatomic distances less than the sum of the van der Waals radii. Ab initio intermolecular perturbation theory calculations show that the attractive nature of the X ... EI interaction is mainly due to electrostatic effects, but polarization, charge-transfer, and dispersion contributions all play an important role. The magnitude of the interaction for the chloro-cyanoacetylene dimer is about 10 kJ/mol, demonstrating the potential importance of these kinds of nonbonded interactions. The directionality of the interaction is explained by the anisotropic electron distribution around the halogen atom, causing a decreased repulsive wall and increased electrostatic attraction for electronegative atoms in the observed preferred position. In contrast, carbon-bonded hydrogens show no directionality in their contacts to the halogen atoms, because the angular dependence of the electrostatic energy is reversed and acts to counter rather then to reinforce the effect of the anisotropic repulsive wall.