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 asymmetrya_s 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 C_nH_2n+2, reduced melting temperatures of monosubstituted derivatives with highly-symmetric molecular cores (e.g. toluene), and non-trivial mp orders among the isomers of disubstituted ethylenes C₂H₂X₂ (“gem- < cis- < trans-”), substituted benzenes C₆H₄X₂ (“meta- < ortho- < para-”), C₆H₃X₃ (“1,2,4- < 1,2,3- < 1,3,5-”), C₆H₂X₄ (“1,2,3,5- < 1,2,3,4- < 1,2,4,5-”), and disubstituted naphthalenes. The observed mp trends do not display any notable correlation with molecular moments of mass and charge distribution, thus being dictated mostly by van der Waals forces.