Structural understanding of a polymorphic system by structure solution andrefinement from powder X-ray diffraction data: the alpha and beta phases of the latent pigment DPP-Boc

The latent pigment 1,4-dioxo-2,5-di-tert-butoxycarbonyl-3,6-diphenyl-1,2,4, 5-tetrahydropyrrolo[3,4-c]pyrrole (abbreviated DPP-Boc) has been found to e xist in the solid state in two polymorphic forms, denoted the alpha and bet a phases. Above ambient temperature, a thermal decomposition reaction occur s in both polymorphs to produce the commercially important pigment DPP. Sig nificant differences have been observed in kinetic aspects of this reaction in the alpha and beta phases, and such differences in reactivity must ulti mately originate from structural differences between the two polymorphs. In this paper we focus on establishing a structural understanding of these ma terials. The structure of the alpha phase of DPP-Boc has been determined pr eviously from single crystal X-ray diffraction data, although certain featu res of the reported structure are unsatisfactory. Here we report a revised structure determination of the alpha phase by Rietveld refinement from powd er X-ray diffraction data. The structure of the beta phase of DPP-Boc, whic h was previously unknown, has been solved directly from powder X-ray diffra ction data using our Monte Carlo technique and Rietveld refinement. A detai led description and comparison of the structural properties of the alpha an d beta phases of DPP-Boc is presented.