"DIRECT-SPACE" TECHNIQUES FOR SOLVING THE STRUCTURES OF MOLECULAR CRYSTALS FROM POWDER DIFFRACTION DATA

Benson M Kariuki, Katerina Psallidas, Roy L Johnston, Kenneth D M Harris

School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

The determination of crystal structures from single-crystal diffraction data can generally be carried out routinely and straightforwardly. However, many crystalline solids can be obtained only as microcrystalline powders and are not suitable for investigation by conventional single- crystal diffraction methods. In the past, this problem has limited the ability to derive a detailed structural understanding of such materials. In order to expand our structural understanding of such materials, there is clearly a pressing need to exploit techniques that allow crystal structures to be solved from powder diffraction data. However, although traditional techniques for structure solution from powder diffraction have been applied successfully in several cases, these techniques suffer from important intrinsic limitations, and for the case of organic molecular crystals the challenges that must be overcome are particularly severe. For these reasons, we have focused in recent years on the development and implementation of new methodologies for structure solution from powder diffraction data - this research has led to new "direct-space" techniques for structure solution in which a hypersurface based on the profile R-factor is searched using Monte Carlo or Genetic Algorithm techniques. The lecture will describe the foundations of these techniques and will present recent results obtained from their application, with examples taken mainly from the field of organic solid state chemistry.