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The stacking disorder in the hexagonal polymorph of tris(bicyclo[2.1.1]hexeno)benzene, C 18 H 18 , is explained by lattice-energy minimisations. The compound crystallises in layers with  P (-6)2 m  layer-group symmetry. Each layer can be placed in one of three possible positions. Possible stacking sequences were derived from order-disorder (OD) theory and by a combinatorial approach. The resulting periodic model structures were optimised by lattice-energy minimisations. The calculations show that  eclipsed  arrangements of layers (sequence  e.g . ABA) are energetically less favourable than  non-eclipsed  ones (sequence  e.g . ABC). The reason was found in the deviation from  P (-6)2 m  symmetry. Molecules in  eclipsed  layers are almost parallel to the layer plane, whereas molecules in  non-eclipsed  layers are inclined to it by about 3° leading to a more efficient packing. The influence of the next-nearest layers was found to be not a direct one, but mediated by the distortion of the layers between them. Using Boltzmann statistics, the stacking probabilities for all four-layer sequences were calculated. The results match well with the probabilities derived from the diffuse scattering by Bürgi  et al.  (2005) (Z. Kristallogr. 220, 1066–1075). The lattice-energy minimisations allowed to determine the actual local structures in all individual layers including packing effects like rotation of molecules, lateral shifts, and to calculate the stacking layer thickness, depending on the actual layer sequences.

Explanation for the stacking disorder in tris(bicyclo[2.1.1]hexeno)benzene using lattice-energy minimisations