Molecular coordination numbers in crystal ­structures of organic compounds

The calculation of molecular coordination numbers (MCNs), and topological and geometrical analysis of the environment of molecules in the crystal structures of 23 067 organic compounds, shows that Kitaigorodskii's close-packing model, assuming the predominance of MCN = 12, works correctly in only a few cases, whereas MCN = 14 is the most frequent. To explain this fact the close-packing model is extended with the model of the thinnest covering of space by deformable molecules. It is shown that the packing of molecules of arbitrary shape and composition can be better described with geometrical, but not topological, parameters of their short-range environment, which is conveniently characterized by molecular Voronoi-Dirichlet polyhedra.