Structures of two derivatives of the curved fluoradene ring system (C19H12) have been determined. Both have phases that are highly pseudosymmetric. At room temperature crystals of 7b-triisopropylsilylfluoradene (C28H32Si) have a P\bar 1 cell that contains two independent molecules (Z′ = 2) and that is almost centered. Crystals of 7b-(2,4-dinitrophenyl)fluoradene (C25H14N2O4) have both a P21/c cell with Z′ = 1 and a P21/c cell with Z′ = 2. The molecular volumes in these two P21/c structures differ by 0.7%, but the structures are otherwise virtually the same; the two independent molecules in the larger cell are related by a pseudotranslation. Some of the atomic ellipsoids in the P21/c, Z′ = 1 structure are very large and eccentric, and there are some hints in the diffraction pattern of an incipient phase transition, but the Z′ = 1 and Z′ = 2 phases are clearly different. The P21/c, Z′ = 2 crystal at 295 K probably contains some volume fraction of the Z′ = 1 phase; when the temperature is lowered to 273 K this fraction is decreased markedly. The pronounced pseudosymmetry in the P\bar 1 and P21/c structures that have Z′ = 2 has been investigated by analysing the atomic coordinates, by performing refinements in the smaller pseudocells and by making separate Wilson plots for the classes of reflections which are systematically strong and systematically weak. All three approaches are informative, but they reveal different information. Least-squares fits of coordinates of corresponding atoms measure the similarity of the molecular conformations. The Wilson plots allow a quantitative comparison of the intensities of the strong and weak reflections and thus an assessment of the deviations of the true structure from the smaller pseudocell structure. Comparison of the atomic displacements obtained in the full and pseudocell refinements shows where the structural distortions are largest and provides an indication of their directions."
Structure and nature of ice XIX
