Report on the Organic Crystal Chemistry Symposium
Rydzyna, Poznan, August, 1997

The International Symposium on Organic Crystal Chemistry (I.S.O.C.C.), held at Rydzyna's exquisitely restored chateau in western Poland 17-21 August, 1997, is the l0th in a series begun in 1977 and continued every two or three years. While earlier meetings concentrated on techniques and developments in the determination and discussion of crystal structures, albeit with certain themes highlighted, recent symposia have been more concerned with the nature of inter-molecular interactions, the prediction and design of crystals with specific properties, and theoretical investigations of molecular structure. Thus for the l0th I.S.O.C.C.,co-sponsored by the I.U.Cr., guest-host interactions in inclusion compounds, crystal engineering applications of the Chemical Structure Database (CSD), and models for structure prediction figured largely in the programme. With Joel Bernstein (Beer-Sheva) as Programme Chair, it was inevitable that polymorphism and hydrogen bonding would be prominent in the discussions.

The meeting format is somewhat like that of a rather intensive Gordon or Biirgenstock Conference, with no concurrent sessions, and with enhanced opportunities for scientific interaction by the residence in-house (or in-chateau) of most of the 100 or so participants, about half from Poland but the remainder spread across a dozen countries. Each day there were about five 50 min. talks and several shorter ones; discussion in the poster sessions was stimulated by beer on tap. The strong field of British contributions straddled the themes of the symposium: Susan Price (London) on accurate electrostatic models in prediction of molecular structures; Sam Motherwell (Cambridge) on non-bonding interactions (Isostar) and packing visualization with the CSD and the Protein Data Bank (PDB); John Osborn (Bradford) with a commended poster comparing lattice-energy calculations with sublimation energies as a basis for polymorph prediction; and two from Chemical Engineering departments, recognising the industrial relevance of organic crystallography: Kevin Roberts (Edinburgh) on processing speciality chemicals and Roger Davey (Manchester) on kinetic processes in polymorph isolation.

In his opening lecture on quantum crystallography, the Honorary Chairman, Nobel Laureate, Jerome Karle (Washington D.C.), showed how high-quality crystallographic data can be utilized in a projector-matrix approach (with fancy manipulation, as he put it) to enhance the information from quantum mechanics. It was possible to calculate electron density, energy, etc. for free small molecules such as maleic anhydride or small peptides, and for fragments of macromolecules. As an illustration of the power of theoretical methods, Karle also strongly commended the careful electrostatic interaction calculations by A. Szarecka (Poznan) for 2-hydroxacetamide in the gas phase and as dimers in solution. Taking what she termed a lowtech. approach to structure predictions, involving the most (and least) common molecular and space-group symmetries, Carolyn Brock (Lexington, KY) noted the strong tendency of ions to occupy m and T elements and the robustness of I4 symmetry. Enthalpy differences were likely to determine ultimately the formation of some cis and trans isomers. Peter Zorky's (Moscow) extensive surveys with the CSD showed that the spatial arrangement of molecules in an organic crystal structure is influenced not to much by close packing as by specific directional intermolecular interactions, especially hydrogen bonds. Local symmetry is important for orthogonal and sandwich benzene-ring contacts but not Cl.... Cl.

With a superb array of slides which, allegedly for the benefit of organic chemists, contrived to incorporate sanitary engineering in crystal engineering, Christer Aakeröy (Manhattan, KS) illustrated the diverse ways in which molecules can orientate and pack to yield novel materials. The selectivity and directionality of hydrogen bonding, and the sensitivity of specific molecular interaction to substituent effects, can be exploited to link chains into a microscopic scaffolding of 2-d motifs and layers. Chains, tapes and layers may be assembled by hydrogen bonds linking synthons into supramolecular solids. John MacDonald (Flagstaff, AZ) was able to engineer organic structures by hydrogen-bonded layer motifs in a series of imidazolinium carboxylate salts with relatively short C-H ^� 0 hydrogen bonds as well as N-H ^�0 bonds. Elsewhere, discussing 14-atom hydrogen-bonded rings in specific b-foldamer helices of peptides synthesised from b-aminoacids, Isabella Karle (Washington, DC) observed that not all the NH groups engage in hydrogen bonds. Tayhas Palmore (Davis, CA) reported the use of cyclic dipeptides as robust 1-d synthon tapes, exploiting the sensitivity of hydrogen bonding to the different functional groups present. Atom force microscopy can illustrate the morphological consequences of direction-dependent growth rates of the supramolecular structure of tapes of 2,5-diketopiperazine packed as decks. Veneta Videnova-Adrabinska (Wroclaw) stressed the continuity between organic molecular and supramolecular structure and illustrated a design strategy of symmetry-constrained molecular recognition with urea cocrystals. For Mike Zaworotko (Halifax, Nova Scotia), crystal engineering meant designing new classes of hydrogen-bonded solids: organic cocrystals of complementary components (at least one organic), including chicken-wire grids, diamondoid networks and ionic honeycomb grids.

Several of the most highly commended (by an international group under William Duax (Buffalo, NY)) of the 50 or so posters were of inclusion compounds and host-guest interactions: Anita Coetzee (Cape Town) on kinetics of desolvation of substituted fluorenylbiphenyl hosts; Katherine Gifford Nash (Cape Town) on Toda multipedal hosts with host: guest ratios of 1:2 to 1:6; and Maria Gdaniec (Poznan) on cholic acid, a versatile host, for which, with thiobenzphenone as guest, weak extra reflections hinted at long-range interchannel communication between guests. In a vigorous talk, Mark Hollingsworth (Bloomington, IN) reported on the synthesis of ferroelastic urea inclusion compounds that undergo domain switching under anisotropic compression.

Overall, Chair Urszula Rychlewska and Co-chair Maria Gdaniec (Adam Mickiewicz University, Poznan) are to be congratulated in maintaining the spirit of the Rydzyna Symposium as organic solid state chemistry continues to evolve. In inclusion - compound terminology, communication channels were very evident between hosts and guests and between guests, stimulated particularly by the comments on most topics of Joel Bernstein and Bill Duax. As with the 9th ISOCC, it is planned to publish selected papers in a special issue of J.Molec. Structure.

Derry W.Jones
University of Bradford

Click here to return to BCA homepage