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00 Some of the intermolecular C · C contacts are somewhat short. b. 13, 6 = 1 0 . , the structure contains two independent molecules in general positions, and the molecules totally lose their symmetry. 4 Â, a common figure for aromatic hydrocarbons with a crystalline structure of this type. 45 8. Molecules of Symmetry mm and mmm (xi) c. 2Ja9 Ζ = 4. The molecular symmetry is mmm, but in the crystal the molecule occupies a general position. In the structure, the role of one "crystallographic" molecule is played by a pair of molecules related by a center of inversion (Fig.

24 1 Structure of Crystals translations and with a twofold screw axis parallel to a translation axis. The next operation was to select space groups in which these layers are possible and are stacked so as to yield high coordination numbers. It is obvious, for example, that the stacking of layers with the aid of a mirror symmetry plane is excluded here. Detailed consideration will be given to only the planar case. This consideration will bring to light the main principles underlying the approach to the selection of space groups convenient for a molecular crystal.

Pyrene. Projection ab. 46 1 Structure of Crystals forming an "island" are stacked with a displacement (Fig. 40). 54 Â. 96 Â. This is a rare (if not the only) case of a packing in which a molecule of symmetry mmm in the free state takes up a general position in the crystal. d. 15 A, 0 = 1 1 0 . 8 ° , space group: C | h = P 2 J a , Ζ = 2. 40 A. However, these molecules are stacked with a displacement (Fig. 44 A. 5 A. For molecules 8. Molecules of Symmetry mm and 47 Fig. 41. Coronene. Stacking o f molecules related through translation b.