Molecular van der Waals symmetry affecting bulk properties of condensed phases: melting and boiling points

Qualitative analysis of the trends in melting temperatures T (m) and boiling temperatures T (b) of organic compounds and related substances supports a concept of van der Waals symmetry suggested earlier by the authors. Data for substituted methanes, ethanes, ethylenes, benzenes and cyclohexanes, reveal linear T (b) versus M-alpha patterns but usually non-uniform T (m) versus M-alpha patterns (where M is the molecular mass, alpha varies from 0.5 to 1). In the same set of isomers, T (m) and T (b) often change in different succession. To explain the observed trends, a local van der Waals field U that acts on a molecule in a solid or liquid phase, was represented by a sum of an averaged long-range field U-0 from all molecules, and a short-range "contact" term W determined by a molecular shape. Variations of T (m) and T (b) among closely related substances correspond to different strength of perturbation of U-0((C)) (in a crystal) and U-0((L)) (in a liquid) by W. Higher T (b) of isomers with medium-symmetry molecules (like 1,2-disubstituted benzenes and cyclohexanes) reflect the better fitting of their molecules to a local molecular environment in a liquid (that contains vacancies). Highly symmetrical quasispherical molecules (e.g., C2Cl6 or C6F12) produce a stabilised solid and destabilised liquid state, hence their molecular crystals easily sublime rather than melt at ambient pressure.