Packing Studies of Organic Molecular Crystals

Graph-set analysis is employed to study the similarities and differences in the hydrogen-bond patterns in crystal structures of the enantiomeric and racemic amino acids and of some glutaric acid derivatives. This analysis is based on describing hydrogen-bond patterns topologically as if they were intertwined nets with molecules as the nodes and hydrogen bonds as the lines. The hydrogen-bond patterns of these structures are clearly described by these efficient shorthand notations, and the similarities and differences in the patterns are clearly shown by this analysis. Some graph sets are found to recur in all amino acid structures; similar results were found in glutaric acid derivatives. These results suggest that there are preferred hydrogen-bond modes for certain classes of compounds, which can be very valuable information for molecular recognition. Efforts also have been going into developing the graph-set methodology, thus making this method more systematic and complete. The rational design of crystalline material based on the size and shape of the molecules that lead to predictable features is studied. Two approaches, quasiracemate formation and shape mimicry, are utilized to encourage non-centrosymmetric packing arrangements. Sets of compounds were employed with slight differences in substituent groups. The differing groups were designed to have similar size and shape. Crystals of these compounds, both pure and in mixtures, were studied by methods of crystal morphology, X-ray powder diffraction, and X-ray single crystal structure determination. Several of the resulting crystals contained predicted pseudosymmetric arrangements of molecules, showing that molecular shape is a major and controllable factor affecting crystal packing.