Crystal structure study of selected xanthine derivatives

The crystal structures of two compounds belonging to a group of theophylline derivatives have been solved by X-ray powder diffraction methods. Despite the fact that these compounds seemed to be rather simple (23 atoms in an asymmetric unit), the preferred orientation and dominant zone problems created very serious obstacles in the investigations, even for the most modern powder diffraction methods. The crystal structure of the first compound, 8-phenylazoxanthine, C₁₄H₁₄N₆O₂, was finally solved from laboratory diffractometer data, while in the case of the second compound, 8-(3-bromo­benzyl­idene)­xanthine, C₁₅H₁₄N₅O₂Br, it was not possible to reliably index its diffraction pattern until data had been collected at the high-resolution powder diffractometer ID31 at the ESRF Grenoble. The serious problems encountered during an ab initio crystal structure solution from powder data of these pharmaceutical samples are described and discussed.