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 par ticular attention to factors affecting crystal density. Packing coefficient s 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 f actors associated with high crystal density are difficult to identify. Howe ver, crystal density is higher for compact polycyclic molecules, since they have smaller molecular volumes. Also, hat, rigid molecules pack better tha n flexible, twisted ones. On the other hand, substituents such as alkyl or nitrile groups tend to lower the packing efficiency. High crystal density d oes not necessarily lead to high lattice energy, and, in particular, hydrog en 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. (C) 2000 Elsevier Science Ltd. All rights reserved.