Significance of high Z' structures in crystallization mechanisms

School of Chemistry, University of Hyderabad
○Gautam R. Desiraju

Crystal structures with multiple molecules in the asymmetric unit (high Z') have been noted from the earliest days of crystallography. In former times, these structures might have posed problems with structure solution and refinement and in this way, be a nuisance. Today, there are a few of these structures which cast some light on the mechanism of crystallization. In this talk, some polymorphic systems will be described wherein one of the forms has a higher value of Z' and could be a kinetic polymorph. The other has a smaller value of Z' and could be the thermodynamic polymorph. The symmetry independent molecules in the high Z' structure could evolve into symmetry related molecules in the low Z' structure. Both synthons and symmetry could evolve in this process, and a study of the two structures may offer insights into the mechanism of crystallization of small organic molecules.