THE NATURE AND GEOMETRY OF INTERMOLECULAR INTERACTIONS BETWEEN HALOGENS AND OXYGEN OR NITROGEN

The nature of intermolecular interactions between carbon-bonded haloge ns (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 force s 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. A b initio intermolecular perturbation theory calculations show that the attractive nature of the X ... EI interaction is mainly due to electr ostatic effects, but polarization, charge-transfer, and dispersion con tributions all play an important role. The magnitude of the interactio n for the chloro-cyanoacetylene dimer is about 10 kJ/mol, demonstratin g the potential importance of these kinds of nonbonded interactions. T he directionality of the interaction is explained by the anisotropic e lectron distribution around the halogen atom, causing a decreased repu lsive wall and increased electrostatic attraction for electronegative atoms in the observed preferred position. In contrast, carbon-bonded h ydrogens show no directionality in their contacts to the halogen atoms , because the angular dependence of the electrostatic energy is revers ed and acts to counter rather then to reinforce the effect of the anis otropic repulsive wall.