Triclinic and monoclinic space groups

1 Triclinic and monoclinic space groups (1) Space group Pi (no. 2), multiplicity = 2 

Hexamethylbenzene, CeMe6, exists as a plastic phase I above 383 K, a room-temperature phase II, and a low-temperature phase III below 117.5 K. Both phase II and phase III crystallize in space group PI with Z = L The molecule is therefore located at an inversion center, and the site symmetry I is much lower than the idealized molecular symmetry of 6h- The asymmetric unit consists of one-half of the molecule. 

Determination of the crystal structure of phase II by Lonsdale in 1929 unequivocally settled over 70 years of debate concerning the geometry and bonding of aromatic molecular systems. The measured bond lengths and crystal structure of hexamethylbenzene are shown in Fig. 9.6.1. The hexamethylbenzene molecules lie within planes approximately perpendicular to (111). Phase III is structurally very similar to phase II, but differs from it mainly by a shearing process between molecular layers that results in a pseudo-rhombohedral, more densely packed arrangement. 

Cisplatin is the commercial name of ds-diamminedichloroplatinum(II), which is widely used as an approved drug since 1978 for treating a variety of tumors. In the crystal structure, Z = 2, and the cw-PtCl2(NH3)2 molecule was found to occupy a general position. The measured dimensions and crystal packing are shown in Fig. 9.6.2. 

Naphthalene crystallizes in this space group with Z = 2 the measured molecular dimensions and molecular packing are displayed in Fig. 9.6.4. 

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