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.

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