Pigment Red 168 (4,10-Dibromoanthanthrone, see picture) is an industrially important pigment with two known polymorphic phases. The crystal structure of the red α-phase was solved by single crystal structure analysis.^[1] Since the crystallinity of the metastable orange β-phase is always poor, powder patterns are not indexable. Therefore we tried to solve the structure by means of lattice energy minimizations.
4,10-Dibromoanthanthrone has the molecular symmetry 2/m. The most common space groups for molecules of 2/m symmetry are P-1, P2₁/c, C2/c and Pbca with the molecule located on the inversion centre.^[2,3] The structure of the β-phase was predicted using the program CRYSCA^[4]. It was assumed that the molecule is located on an inversion centre. The calculations were performed with a whole molecule in the corresponding subgroups of the above-mentioned space groups. Subsequently, the predicted structures were optimized with a force field having anisotropic atom-atom potentials, which was developed for halogenated aromatic molecules.^[5] In order to find out which of the proposed structures correspond to the experimental polymorphs of α and β, powder patterns were calculated and compared with the experimental patterns.
The α-phase could be identified without problems. Due to the low crystallinity of the β-phase it was difficult to select which of the proposed structures is the correct one.


[1] E. F. Paulus, G. M. Day, M. U. Schmidt, in preparation.
[2] E. Pidcock, W. D. S. Motherwell, J. C. Cole, Acta Cryst. 2003, B59, 634-640.
[3] A. I. Kitaigorodskii: “Organic Chemical Crystallography” 1961, New York: Consultants Bureau.
[4] M. U. Schmidt, H. Kalkhof, Frankfurt am Main, 1997–2002.
[5] G. M. Day, unpublished.

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