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.