9,10 Phenanthrenequinone, not your average structure

Abstract

Abstract 9,10-phenanthrenequinone, C14H8O2, is both polymorphic and polytypic. The β form can have different symmetries arising from different stackings of ordered layers perpendicular to c p*. These structure options are best understood using occupancy wave modulations of a 1 : 1 disordered parent structure in space group P21/c. The disorder is resolved by having ordered layers of C1̅ symmetry in a cell with a = 2a p, b = 2b p. Ordering the stacking sequence allows two orientations of a Z = 8, Cc, c = c p structure and two orientations of a Z = 16, F1̅, c = 2c p structure. It was found that for some crystals it is necessary to model the reflection data assuming uncorrelated mosaic blocks of different space group symmetry as well as orientation. Reliable geometry for both the Cc and F1̅ components were then obtained. Likewise, the α form can be described as arising from a 1 : 1 disordered parent structure in space group Pnmn where the disorder is resolved by having ordered layers of P2/c symmetry perpendicular to a p* in a cell with b = b p, c = 2c p. Ordering the stacking sequence allows two orientations of a Z = 4, P21 cn structure with a = a p and two orientations of a Z = 8, B2/c structure with a = 2a p (a non standard setting of P2/c).