High-Z' structures of organic molecules: their diversity and organizing principles.

A list has been compiled of 284 well determined organic structures having more than four crystallographically independent molecules or formula units (i.e. Z' > 4). Another 22 structures were rejected because the space group or unit cell was probably misassigned; the rate for that type of error is then only 7%. The space-group frequencies are unusual; half the structures are in Sohncke groups, partly because the fraction of enantiopure structures of resolvable enantiomers is higher than for lower Z' structures. Careful investigation of the 284 structures has shown that they are very diverse; no simple classification can describe them all. Organizing principles have, however, been recognized for almost all of them. The most common features are simple modulations and hydrogen-bonded aggregates; only 14% of the structures have neither. In 50% of the structures n molecules are related by a pseudotranslation that would be a crystallographic translation but for small molecular displacements and rotations. In 70% of the structures there are aggregates (e.g. n-mers, columns or layers) held together by strong intermolecular interactions; those aggregates usually have approximate local symmetry. Because the n-fold modulations and the n-mers often have n < Z', 85% of the structures with Z' > 5 have several features that combine to give the high Z' value. The number of different molecular conformations is usually small, i.e. one or two in 84% of the structures. More exotic packing features, such as ordered faults and alternating layers of different types, are found in ca 30% of the structures. A very few structures are so complex that it is difficult to understand how the crystals could have formed.