THE NATURE OF -CL-CENTER-DOT-CENTER-DOT-CENTER-DOT-CL- INTERMOLECULARINTERACTIONS

The controversy as to whether there is a specific attractive intermole cular force between chlorine atoms, of the charge-transfer or donor-ac ceptor type, is resolved using various analyses of experimental crysta l structure data and theoretical calculations. The occurrence of Cl... Cl intermolecular contacts which are shorter than would be expected fr om the conventional isotropic van der Waals radius is shown to be most common in the crystal structures of fully or highly chlorinated hydro carbons, 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 th e chloromethane dimer show that the charge-transfer contribution to th e intermolecular energy is negligible, the electrostatic forces are we ak, and the repulsive wall is anisotropic. Calculations on the electro static interactions between other chlorinated hydrocarbons show that t hese results will also apply to other Cl...Cl interactions. Thus a rea listic anisotropic model for the repulsion, dispersion, and electrosta tic forces between chlorinated hydrocarbons should be capable of predi cting the observed crystal structures with ''short'' Cl...Cl intermole cular separations.