Structural analysis of organic powders and microcrystals with synchrotron radiation.
J.Maixner, M.Huák
Institute of Chemical Technology Prague, Technická 5, 166 28 Prague, Czech Republic
maixnerj@vscht.cz, husakm@vscht.cz
Molecular and crystal structure determination used to be connected with 3D analysis of reciprocal space of suitably perfect and large enough single crystals (average size of 0.500 mm and volume of 0.1mm3). The difficulties to grow such single crystals mainly due to the time consuming partly trial crystallization procedures are cause that directs the structure analysis of organic compounds to use of either very small single crystals (microcrystals with avarage size of 0.050 mm and volume of 0.0001 mm3 up to submicrocrystals with size of a powder like crystallite with size of 1 mm and volume of 1mm3) or to use polycrystalline materials - powders when suitable single micro- or submicrocrystals are not available. Both approaches need to use high flux of primary beam and ab initio structure solution from powders furthermore high resolution to separate the number of severely overlapping reflections due to a compression of a reciprocal space into one dimension. These requirements can be achieved by exploitation of synchrotron radiation.
Ab initio structure determination from microcrystals and powder diffraction data is currently of great interest in crystallography and materials science. A number of crystal structures have been solved using either solution in reciprocal space (single crystal direct methods modified to use powder diffraction data for phasing - Altomare et al., 2004) or so-called direct-space methods as new approaches for structure solution using the Monte Carlo method, the simulated annealing method or the genetic algorithm. The later methods directly fit a calculated diffraction pattern to the observed pattern, and thus they do not require the pattern decomposition process for extracting integrated intensities. To solve structures having a large number of unknown parameters is, however, practically impossible without utilizing data other than those of diffraction intensities and a rigid-body or flexible rigid-body approximation is introduced to reduce the number of variables.
The lecture will give some information about handling micro crystals, filling capillaries to avoid texture formation, indexing powder data as the bottleneck of ab initio structure solution (CRYSFIRE-Shirley, 1999), application of the simulated annealing (FOX Favre-Nicolin,V. and Černý R., 2002) and Rietveld refinement (GSAS - Von Dreele 2000, EXPGUI B.H. Toby ).
We will present the structure solution of metergoline form II (solution form powder data with preferred orientation and from micro crystal) and the results of the temperature behavior of simvastatin in the range from 150K up to 295K giving 2 new polymorphic forms solved from synchrotron powder data.