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

Abstract

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.

Structural investigation ofN,N′-methylenebisacrylamideviaX-ray diffraction assisted by crystal structure prediction

“…On the other hand, the rough information provided by the energy function, generally evaluated by means of simple potential models, can be successfully combined with that contained in (not indexed) PXRD data, and it was shown that combining the PXRD profile with the CSP energy function can noticeably facilitate the structure solution (Bond & Jones, 2002;Schmidt et al, 2005;Day et al, 2006).…”

Pigment Orange 5: crystal structure determination from a non-indexed X-ray powder diagram

“…In most cases the growth of single crystals fails. Hence many crystal structures of organic pigments have been determined from powder diffraction data, either with indexing, [5][6][7] or even without a previous indexing [8][9][10].…”

Crystal Structures of Pigment Red 170 and Derivatives, as Determined by X‐ray Powder Diffraction

“…Most methods2 for the determination of crystal structures from X‐ray powder diagrams require the diagram to be indexed, that is, all lattice parameters and the possible space groups must be known. There are two alternative approaches: 1) search for an isotypical compound with known structure, or 2) predict the crystal structures by global minimization of the lattice energy, followed by simulation of powder diagrams and comparison with the experimental diffractogram 3. To solve the structure of α‐ 1 a we had to combine the two approaches.…”

“…Pigments with desirable properties are generally characterized by very dense packings (e.g., 1.74 g cm −1 for the triphenedioxazine C 22 H 12 Cl 2 N 6 O 4 3 and 2.03 g cm −1 for the anthanthrone pigment C 22 H 8 Br 2 O 2 6c). Dense packings impede photochemical reactions that require a change in molecular geometry; in the case of photodissociation the fragments remain trapped at their locations and have a good chance for recombination.…”

“…22 The structure was solved by crystal‐structure prediction using the program CRYSCA. CRYSCA performs global lattice energy mimimizations for flexible molecules;6, 11, 23, 24 with this approach we could recently solve the structure of the violet pigment C 22 H 12 Cl 2 N 6 O 4 from a non‐indexed powder diagram containing only 12 peaks 3. For γ‐ 1 a , several thousand crystal structures with given lattice parameters were generated with the symmetries P 2 1 / c , P 2 1 / n , and P 2 1 / a ( Z =4).…”