Statistical analysis of some structural properties of solid hydrocarbons

The crystal structures of 391 hydrocarbon molecules, comprising 18,249 atoms, retrieved from the Cambridge Structural Database, have been examined. Centrosymmetric space groups occur more frequently than in general organic compounds. Tables of atomic volume and surface increments are given. Averages of various packing indices are calculated. Molecular size is correlated to the packing energy, as computed by empirical potentials; methods to estimate accurately sublimation energies are developed, and trends in groups of compounds are discussed in terms of molecular structural properties. These trends survive changes in the potential energy parameters. It is found that flat, rigid, unsaturated molecules have the best chances to form a compact crystal, while alkyl substituents are strong perturbing factors. Bulk moduli and thermal conductivities are estimated, and it is found that the modulus is a convenient index of packing efficiency and correlates to molecular shape and symmetry effects. A principal-component analysis reveals three main factors, which can be interpreted as size, packing score, packing failure coordinates; the shape effect cannot yet be deconvoluted from these factors. The analysis of the distribution of atomic contributions to the cohesion energy allows a verification of the homomeric principle and reveals the intramolecular screening factors that diminish the cohesionmore » at certain atoms, as opposed to the effects of irregular molecular shape, which diminish the cohesion more uniformly at all atoms. Cylindric molecules pack in such a way that their elongation axes are very nearly parallel in the solid state. Future uses of the packing analysis hydrocarbon files are sketched.« less