DESIGNER CRYSTALS: INTERMOLECULAR INTERACTIONS, NETWORK STRUCTURES AND SUPRAMOLECULAR SYNTHONS. Gautam R.Desiraju, School of Chemistry, University of Hyderabad, Hyderabad 500 046, India

Crystal engineering attempts an understanding of the packing of molecular solids towards the design of new materials with desired aesthetic or functional properties. The Cambridge Structural Database can be used to identify patterns of intermolecular interactions that control crystal packing. Strong O-H...O and N-H...O hydrogen bonds are generally used for crystal design but weaker, polarisation-induced interactions, such as say, C-H...O, O-H...[[pi]], Br...Br and O...I may often be reliable. Still, a major concern in crystal engineering is that the interactions which control crystal packing are weak and numerous. The same molecular structure could be associated with several crystal structures and this problem of polymorphism is not very helpful.

The growth and development of supramolecular chemistry has led to the consideration of a crystal as the ultimate supermolecule. Going further, crystal engineering becomes the supramolecular equivalent of organic synthesis and like any other form of synthesis, it is characterised by target definition, methodology and strategy. The crystal is a supermolecule and so, a target in crystal engineering should be defined supramolecularly, that is as a network. Synthetic methodology corresponds to interaction properties while strategy invokes the concept of supramolecular synthons. These are robust combinations of interactions that incorporate geometrical and chemical recognition features of molecules. As in traditional synthesis, retrosynthetic analysis is of value, because it permits general design strategies leading to an identification of several molecular structures which could crystallise in the same target network. Such an intertwining of crystallography with organic chemistry has the promise of leading to new vistas in property-driven and goal-oriented crystal engineering.