Determination of the structure of the violet pigment C22H12Cl2N6O4 from a non-indexed X-ray powder diagram.

The violet pigment methylbenzimidazolonodioxazine, C22H12Cl2N6O4 (systematic name: 6,14-dichloro-3,11-dimethyl-1,3,9,11-tetrahydro-5,13-dioxa-7,15-diazadiimidazo-[4,5-b:4',5'-m]pentacene-2,10-dione), shows an X-ray powder diagram consisting of only ca 12 broad peaks. Indexing was not possible. The structure was solved by global lattice energy minimizations. The program CRYSCA [Schmidt & Kalkhof (1999), CRYSCA. Clariant GmbH, Pigments Research, Frankfurt am Main, Germany] was used to predict the possible crystal structures in different space groups. By comparing simulated and experimental powder diagrams, the correct structure was identified among the predicted structures. Owing to the low quality of the experimental powder diagram the Rietveld refinements gave no distinctive results and it was difficult to prove the correctness of the crystal structure. Finally, the structure was confirmed to be correct by refining the crystal structure of an isostructural mixed crystal having a better X-ray powder diagram. The compound crystallizes in P1, Z=1. The crystal structure consists of a very dense packing of molecules, which are connected by hydrogen bridges of the type N-H...O=C. This packing explains the observed insolubility. The work shows that crystal structures of molecular compounds may be solved by lattice energy minimization from diffraction data of limited quality, even when indexing is not possible.