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The nature of intermolecular interactions between carbon-bonded halogens (C-X, X ) F, Cl, Br, 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‚‚‚El 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.

The distributed multipole analysis procedure, for describing a molecular charge distribution in terms of multipole moments on the individual atoms (or other sites) of the molecule, is not stable with respect to a change of basis set, and indeed, the calculated moments change substantially and unpredictably when the basis set is improved, even though the resulting electrostatic potential changes very little. A revised procedure is proposed, which uses grid-based quadrature for partitioning the contributions to the charge density from diffuse basis functions. The resulting procedure is very stable, and the calculated multipole moments converge rapidly to stable values as the size of the basis is increased.

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The controversy as to whether there is a specific attractive intermolecular force between chlorine atoms, of the charge-transfer or donor-acceptor type, is resolved using various analyses of experimental crystal structure data and theoretical calculations. The occurrence of Cl-Cl intermolecular contacts which are shorter than would be expected from the conventional isotropic van der Waals radius is shown to be most common in the crystal structures of fully or highly chlorinated hydrocarbons, and thus a consequence of close packing of anisotropic atoms, rather than evidence for a specific attractive force. Intermolecular perturbation theory calculations on the Cl-Cl interactions within the chloromethane dimer show that the charge-transfer contribution to the intermolecular energy is negligible, the electrostatic forces are weak, and the repulsive wall is anisotropic. Calculations on the electrostatic interactions between other chlorinated hydrocarbons show that these results will also apply to other Cl-Cl interactions. Thus a realistic anisotropic model for the repulsion, dispersion, and electrostatic forces between chlorinated hydrocarbons should be capable of predicting the observed crystal structures with "short" Cl-Cl intermolecular separations.