Centrosymmetric orthorhombic space group

The structure of 203 has been redetermined312 by refinements of the XD data in the centrosymmetric orthorhombic space group Pnca. The molecule has C2 symmetry in the crystal. The side bonds in the bicyclobutane unit are longer, Cl—C2 1.540 (6) A, the bridge bond is shorter, Cl —C6 1.442 (5) A, and the fold angle of 110.7 ° (calculated from the coordinates) is narrower than in free 186. 

With the dilemma not resolved, Pandarese et al. carefully reexamined many individual crystals from the benzene-grown batch, and discovered that approximately 20 per cent of them belonged to the non-centrosymmetric orthorhombic space group P2i2i2i, thus explaining the source of the nonlinear effects. A method for purifying m-nitrophenol and growing crystals was developed by Wojcik and Marqueton in 1989 and the structure of the non-centrosymmetric orthorhombic form was published by Hamzaoui et al. in 1996. 

Well-crystallized specimens were obtained for PrMnO, and TbMnOj. All crystals had black color and were obtained as a single phase. The measurement data of PrMnO, and TbMnOj are shown in Table 1. The conditions of systematic absences of X-ray peaks for each sample were k + I 2n for Okl, h 2n for hkG, h 2n forhOO, k 2n for OkO and / 2n for 00/. The possible space groups compatible with the above reflection conditions were centrosymmetric Pnma and acentrosymmetric Fn2,a. The orthorhombic space group Pnma was chosen for the initial refinement. 

Crystals of the NN-dimethylethanolaminogallane dimer (77) are monoclinic, space group P2Jc, whereas the compound with R = Me is orthorhombic, space group Pccn. However, both contain well-separated centrosymmetric dimers, having Ga202 rings." ... 

Point symmetry mmm (shorthand for 2lm, 2Jm, 21m) comprises three perpendicular mirror planes, which automatically generates three perpendicular twofold axes at their lines of intersection, and a center of symmetry at the origin. This point group applies to all orthorhombic centrosymmetric crystals, such as those of space group Pbca. 

The series CeTSn together with CeRhSb and the related pseudo-ternary compounds crystallize in a rather complex orthorhombic structure with the non-centrosymmetric space group Pn2ia (Higashi et al. 1993). This structure, which is shown in fig. 106 (left), is closely related to the e-TiNiSi structure. It forms puckered hexagonal Ni—Sn-Ni rings... 

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