Two fluoradene derivatives: pseudosymmetry, eccentric ellipsoids and a phase transition

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) hav e a PT cell that contains two independent molecules (Z = 2) and that is alm ost centered. Crystals of 7b-(2,4-dinitrophenyl)fluoradene (C25H14N2O4) hav e both a P2(1)/c cell with Z = 1 and a P2(1)/c cell with Z = 2. The molecul ar volumes in these two P2(1)/c structures differ by 0.7%, but the structur es are otherwise virtually the same; the two independent molecules in the l arger cell are related by a pseudotranslation. Some of the atomic ellipsoid s in the P2(1)/c, Z = 1 structure are very large and eccentric, and there a re some hints in the diffraction pattern of an incipient phase transition, but the Z = I and Z = 2 phases are clearly different. The P2(1)/c, Z = 2 cr ystal at 295 K probably contains some volume fraction of the Z = I phase; w hen the temperature is lowered to 273 K this fraction is decreased markedly . The pronounced pseudosymmetry in the P (1) over bar and P2(1)/c structure s that have Z = 2 has been investigated by analysing the atomic coordinates , by performing refinements in the smaller pseudocells and by making separa te Wilson plots for the classes of reflections which are systematically str ong and systematically weak. All three approaches are informative, but they reveal different information. Least-squares fits of coordinates of corresp onding atoms measure the similarity of the molecular conformations. The Wil son plots allow a quantitative comparison of the intensities of the strong and weak reflections and thus an assessment of the deviations of the true s tructure from the smaller pseudocell structure. Comparison of the atomic di splacements obtained in the full and pseudocell refinements shows where the structural distortions are largest and provides an indication of their dir ections.