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Solid Sulfur Allotropes 17 The packing of the molecules in crystalline structures has only a slight influ- ence on the molecular geometry which is found typically in the range of a few percent variation of bond lengths and bond angles. A characteristic parameter to describe the conformation of sulfur mole- cules is the sign of the dihedral angle. For a sulfur chain or ring the order of the signs of the torsion angles is the so-called “motif”. Each of the molecular species shows a typical motif reflecting the molecular symmetry and shape. 2.2.1 Allotropes Consisting of Cyclic Molecules 2.2.1.1 Rhombohedral S6 S6 was first synthesized by Engel in 1891 [9]. Since the molecular structure was not known at that time, this sulfur allotrope was called Engels sulfur, r- sulfur, as well as e-sulfur (see [3]). Later, in 1914, Aten determined the mo- lecular mass to S6 molecular units [63], which has given cyclo-hexasulfur an additional name, Atens sulfur. Crystals of S6 form orange-yellow hexagonal prisms. Several attempts have been made to characterize the molecular and crystal structure by X-ray diffraction studies (see [8]). In consequence, a chair conformation for the S6 ring molecule has been suggested; the crystal structure was attributed to a rhombohedral lattice with one molecule per primitive cell occupying a site of C3i (S6) symmetry. The first complete description of crystalline S6 was reported by Donohue et al. in 1961 on the basis of an X-ray diffraction study [64]. The rhombohe- dral structure was verified, and the molecular symmetry was ascertained to be D3d. Since cyclo-hexasulfur decomposes rapidly under X-ray irradiation at standard temperature-pressure (STP) conditions Steidel et al. reinvestigat- ed the molecular and crystal structure at 183 K giving results with a higher accuracy [65]. The molecular and crystal lattice parameters are summarized in Tables 1 and 2. As listed in Table 2 and shown in Fig. 1, each of the S6 molecules has 18 short intermolecular contacts in the range 350–353 pm (at ~300 K). This fact, in combination with the compact molecular structure, accounts for the high density of rhombohedral S6 (for comparison see the structure data of orthorhombic sulfur, Table 6). On the other hand, the compact molecular structure is responsible for a certain strain in the bond geometry which is expressed by a relatively large deviation of the torsion angle from an un- strained value of about 90, therefore, making the molecule unstable [66]. Since the lattice parameters depend significantly on the temperature (Ta- ble 2), it is possible to estimate the coefficient of isobaric thermal expansion roughly to about 2.8104 K1. Up to now no six-membered sulfur allotrope other than rhombohedral S6 has been found. In addition, from theoretical structure analysis it is reasonable to assume that the chair conformation is energetically more favorable thanPDF Image | Topics in Current Chemistry
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