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Organic crystal chemistry

Among the most exciting, fastest growing, and potentially useful areas of X-ray crystallography are studies of supramolecule structures, molecular recognition, solid state reactions and crystal design. These topics have been the primary focus of a series of ten international symposia on Organic Crystal Chemistry(OCC '97) held in Poland since 1983. The most recent in the series, held in a 17th century castle in Rydzyna, Poland in Aug. 1997, organized by U. Rychlewska and chaired by J. Bernstein was especially rich in the area of crystal dynamics.

S. Motherwell gave a splendid survey of the new tools introduced into the Cambridge software package that allow users to view data in new ways, answering old questions and suggesting new ones. Specific intermolecular interactions can now be displayed on 6000 scatter plots. His illustration of the analysis of the location of the molecular centers in different space groups was particularly attractive. C. Brock also identified patterns characteristic of molecular interaction in the CSD and offered persuasive explanation for the stereo chemical and electronic properties that lead to the patterns.

S. Price described a program to predict low energy crystal forms and showed typical applications in which the calculated minimum energy crystal form agreed with the x-ray structure. The program predicts several alternative crystal forms of only a little higher energy.

T. Palmor eloquently described the use of molecular recognition in the manipulation of molecules into "tapes, layers and stacks" that produced crystals with tailored properties. She compared an x-ray structure and the atomic force microscope profile of the surface of that structure and correlated crystal growth and morphology with the underlying arrangement of molecules.

J. McDonald gave a lucid presentation of his rational for designing networks with a versatile pair of interactive molecules. He represented them by a symbol indicative of the interactive ability of the molecule and showed four basic types of layers his molecules could form. He envisioned the design of optics, ferroelectrics, magnets, and conductors, extending from microscopy to macroscopy based upon his interactive molecules.

W. Nieuwenhuygen described a series of isoforms of stacked ampiphylic layers. Typically one isoform was found to be composed to ampiphylic single layers and the other of stacks of double layers. Solvation appeared to play a role in determining whether single layer or double layer structures could form. J. Olf Hench described a way to resurrect a polymorph of a substance lost once a more stable polymorph had formed. To restore a lost polymorph you need to understand the phase diagram of, and solvent influences on, the substances and their solubility. R. Davey described preparing four different forms of sulfathiazol by controlling solvent and other conditions. He could control growth of different crystal faces and induce crystallization of specific polymorphs by use of conformational mimicry. Growth of a crystal face can be inhibited by contaminants that complement and bind to selective surfaces.

I. Karle discussed the interrelation of peptide conformation and solvent interaction, hydrogen bonding and crystal form and a structural role for some N-H phenyl ring interactions. S. Price described N-H phenyl interactions in the protein database in which N-H groups point at the center of rings as well as parallel to and over the planes of rings. The latter arrangement allows the N-H to have normal H bonds as well as association with the phenyl π cloud.

Kinga Sawinska and Hanna Maluszynska admire Jerome Karle's Buffalo tie during a vodka break in Rydzina. (Photo W. L. Duax)

K. Sawinska gave a fine talk on the structures of derivatives of Vitamin D that have a useful activity profile. Via careful analysis of several complex structures with multiple copies in the asymmetric unit and comparison with biochemical and pharmacological data Sawinska was able to identify the active form of this flexible molecule.

C. Aakeroy discussed crystal design and the proper placement of toilet bowls (sic, very sic) for maximum utility. M. Zaworotko presented a tour of his stable of structures. Mike promises to turn out even more structures every year thanks to area detectors. He will need multiplex projectors to show his structures in the year 2000. J. Lipkowski entertained us with tales of the hazards of swamp gas, a fascinating water ball (H2O)21 H+ (Zhang, Castlen, J. Chem. Phys. 1994, 1157) and another guided tour of clathrand land. A. Szarcka described ab initio calculations on potential building blocks for crystal engineering and A. Katrusiak showed the physical elongation of a crystal associated with a phase change.

Over fifty posters were presented on topics related to crystal chemistry. A jury of five (C. Aackeroy, C. Eckhardt, C. Brock, S. Price, and W. Duax) summarized the high lights of the poster session. S. Price praised J. Osborne's poster which compared crystal lattice energies with sublimation enthalpies calculated with three different force fields. The variability of the results demonstrated the need to choose potentials carefully. "Unless you are calculating the properties of argon, you will have to use an intermolecular potential that is only an approximation to the true potential." In a similar vein, M. Ciechewz Rutkowski compared minimum energy conformations derived from two programs with X-ray results on seven anticonvulsants. The calculated energies and molecular conformations were significantly and systematically different. Most disconcerting was her discovery that the same empirical energy program produced different results depending upon the software packages used. Z. Urbaycyk-Lipkowska presented a comprehensive analysis of the conformations of precursors and intermediates as well as the final macrocycles she is designing as recognition elements, to explain success in one attempted synthesis and failure in another. The vote of the jury resulted in a tie between the posters of U. Baurnerstu of Germany and A. Coetzee of S. Africa.

D. Jones gave a splendid parody wrap-up. He commented on the many honors that Jerry and Isabella Karle have received and noted there were a few in the audience who hadn't yet received many honors. Derry said he has been waiting for the call but phone service between Stockholm and his village was not good. He also noted that if you have the Nobel prize and close your eyes during a lecture people think your having another brilliant inspiration, but when he closes his eyes everyone knows he's sleeping. He ribbed Joel Bernstein about never being home and Ursula Rychlewska about needing a psychiatrist as a consequence of the exceptional effort she and her staff had exerted in making the meeting such a success.

W. L. Duax