Chair: Urszula Rychlewska (Poland), Co-chair: Boris Kamenar (Croatia)

Microsymposium C2 on Structure Systematics and Structure-Function Relationship in Organic Chemistry covered a wide range of topics. The first part of the MS was devoted to phenomena which solely take place in solid state such as polymorphism, hydration and inclusion. The second part concentrated on weak interactions both intra- and intermolecular in condensed media. The closing presentation concerned use of CCD detectors in small molecule crystallography.

Exactly 200 years after Klaprothís discovery of two polymorphic forms of calcium carbonate, calcite and aragonite, Frank H. Herbstein (Technion, Haifa, Israel) in his invited review lecture presented examples of different forms of polymorphism. He introduced a physics-chemistry scale for polymorphism and suggested a new scheme for nomenclature of polymorphs, which he hopes will eventually replace present chaotic terminology. A question from the audience whether to consider hydrates and solvates of a compound as polymorphs, to which Herbsteinís answer was not favorable, has lead us to the next talk by Fokke J.J. Dijksma (University of Edinburgh, Scotland) about the anhydrous brucine and its two hydrates. The crystal structures have been compared with brucine cocrystals and, on the basis of DSC and TGA as well as X-ray powder diffraction, it has been suggested that dehydration of the tetrahydrate proceeds via dihydrate, which has very different structure, and therefore mediates the reversal of layers between the tetrahydrate and anhydrous brucine. While brucine is used to separate racemic mixtures by co-crystallization, some hexapedal host compounds presented by Susan A. Bourne (University of Cape Town, South Africa) show molecular recognition properties. In particular hexakis (3-hydroxy-3,3-diphenyl-2-propynyl) benzene forms complexes of different stability with such similar guests as carbonyl-type oxygens and etheric oxygens, an observation which may lead to practical applications.

A provocative opinion about the forces responsible for holding the organic world together, was presented by Jack D. Dunitz (ETH-Zentrum, Zurich, Switzerland) in his review lecture which opened the second part of the MS. While discussing the stabilizing energy for solids and liquids he stressed that a crystal structure corresponds to an equilibrium at which attractive and repulsive forces must balance. At a normal contact distance the energy contribution may be stabilizing or destabilizing, but the force is invariably repulsive. Thus the crystal is held together mainly by repulsions that oppose the disentanglement of interlocking molecules rather than by attractive forces. He pointed out again that close packing of the molecules is the dominating feature of organic crystal structures. Teresa Borowiak from Adam Mickiewicz University in Poznaò, Poland, used her own X-ray results and the data extracted from the Cambridge Structural Database to demonstrate the importance of weak 1,4 - non-bonded interactions of the S...O and S...S type in stabilizing the geometry and planarity of 3,4í-diquinolinyl sulfides.

Finally, Martin Nieger from the University of Bonn, Germany exchanged with the audience his experience in using CCD detectors for collecting the X-ray data for small molecules. While technical details still raise some questions, e.g. the accuracy of the determination of unit cell dimensions, it is generally agreed that nowadays CCD-detector systems are becoming widely used in an increasing number of laboratories.

U. Rychlewska, Chair (ECM-18 report session C2)