Symmetry of van der Waals molecular shape and melting points of organic compounds
Abstract
Long-known empirical regularities in melting points (mp) among molecular crystals of organic compounds with a closely related stoichiometry are dictated by a short-range molecular van der Waals term in their crystal field. This term, which acts as a symmetry-breaking perturbation on the averaged intermolecular interactions, is determined by the “contact” potential in the vicinity of molecules, i.e. by the shape of their van der Waals surface. To reflect the topology of the crystal field, an approximate van der Waals group Ğ of the average local field acting on a molecule and asymmetryas of the molecular surface composed of atomic van der Waals spheres, as its total misfit to Ğ, were introduced. The scheme was applied to organic compounds whose molecules have a mirror symmetry plane; the shape of two-dimensional sections of their van der Waals surfaces by this plane was analysed. Calculated values of molecular asymmetry parallel as yet uninterpreted trends in melting point data such as oscillations between “even” and “odd” n-alkanes CnH2n+2, reduced melting temperatures of monosubstituted derivatives with highly-symmetric molecular cores (e.g.