MOLECULAR STRUCTURES BY POWDER DIFFRACTION
M. Tremayne1, W. Dong2 and C. J. Gilmore2
1School of
Chemistry, The Purdie Building, University of St. Andrews, St.
Andrews, Fife UK. KY16 9ST
2 Department of Chemistry, University of Glasgow, Glasgow UK. G12
8QQ
Keywords crystal structure
determination, molecular materials, powder diffraction
The crystal structure determination of molecular materials from X-ray powder diffraction data is a rapidly expanding field, and many new approaches are currently being developed to study these systems. Two such methods are the maximum entropy-likelihood method [1] and the Monte Carlo method [2].
In the maximum entropy-likelihood method (MICE), a phasing tree is built in which the nodes represent possible phase choices, and in which likelihood is used to assess the correct phases. At less than atomic resolution, the resulting maps are investigated visually, and molecular models are fitted to the electron density. In the Monte Carlo method the positions and conformations of molecules in the unit cell are modified using a Monte Carlo sampling approach, and the powder diffraction pattern used as a measure of correctness in deriving initial atomic co-ordinates.
We present here the results of an approach in which these two methods are combined: the maximum entropy-likelihood method is used to generate maps from which initial models are derived that are then modified using the Monte Carlo method before Rietveld refinement. Examples will be given of this method solving organic molecules.