A Statistical Study of Density and Packing Variations among Crystalline Isomers

Crystal structures of groups of isomeric hydrocarbons, oxahydrocarbons and azahydrocarbons have been retrieved from the Cambridge Structural Database. Correlations among crystal and molecular descriptors were sought, with particular attention to factors affecting crystal density. Packing coefficients do not differ much from 0.74, so organic molecules have roughly the same packing efficiency as a close packed assembly of spheres. Molecular shape factors associated with high crystal density are difficult to identify. However, crystal density is higher for compact polycyclic molecules, since they have smaller molecular volumes. Also, flat, rigid molecules pack better than flexible, twisted ones. On the other hand, substituents such as alkyl or nitrile groups tend to lower the packing efficiency. High crystal density does not necessarily lead to high lattice energy, and, in particular, hydrogen bonding seems to have no immediate effect on crystal density. Results of bivariate statistics were confirmed by principal component analysis. These results may be of interest for practical applications in crystal chemistry and crystal physics.