Crystal structures elucidated from X-ray powder diffraction data without prior indexing

OCEANA, a new genetic algorithm-based program for structure elucidation from non-indexed X-ray powder diffraction data, uses Lamarckian evolution to search for the best molecular position and orientation over a range of cell parameters for a given space group. As OCEANA does not require an indexed powder pattern, it is particularly suited for analysis of organic molecular crystals, which often crystallize in relatively large, low-symmetry unit cells. The high degree of peak overlap in the powder patterns of such compounds makes cell determination and pattern indexing very difficult and presents a major obstacle to success when using any routine that requires this information. OCEANA searches within a user-defined range of cell parameters for a specified space group and uses the experimental density to eliminate unit cells that are not viable. Several examples based on compounds with known crystal structures are presented to demonstrate the effectiveness of this method, and the crystal structure of a new compound, bis(5-quinoline)diacetylene (5QDA), is also reported.