A critical evaluation of isotropic potential functions for chlorine |
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Authors: | Michael G Munowitz George L Wheeler Steven D Colson |
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Institution: | Sterling Chemistry Laboratory , Yale University , New Haven, Connecticut, 06520, U.S.A. |
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Abstract: | A survey of many crystal structures shows that short intermolecular chlorine-chlorine contacts are anisotropic. This observation challenges the currently used potential function models which allow only for isotropic exp-6 and coulomb interactions. In this study the polymorphic p-dichlorobenzene DCB system is used to illustrate the difficulties encountered by relying on the isotropic atom-atom approximation. It is found that exp-6 potential functions cannot be extrapolated beyond the range of contacts used in their derivation. Addition of coulomb terms provides needed additional flexibility to the potential function but the derived C-Cl bond polarization is not found to be a constant of the molecule, i.e. the optimum value is not the same for each phase of DCB. Furthermore, no single exp-6 potential function could satisfy the structural constraints provided by the three phases of crystalline DCB and by hexachlorobenzene. These findings are typical of curve-fitting methods which employ an incorrect mathematical form for the curve and indicate the necessity of including anisotropic terms in the interaction potential. |
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