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Solid Sulfur Allotropes 35 Table 13 Molecular structure parameters of monoclinic S13 at 173 Ka [38] Point group of molecule Molecule Av. bond length (pm) Range Av. bond angle () Range Av. torsion angle ()b Range C2 (approximately) I 204.6 197.8–207.4 106.4 103.3–111.1 85.3 30.9–116.3 II 205.2 199.5–211.3 106.1 102.8–107.8 85.4 29.5–114.1 a Errors of bond lengths: 0.4–0.5 pm, of bond angles and torsion angles: 0.1–0.2 b Absolute values tances in crystalline S12 are 335 pm (four contacts), the shortest distances in CS2·S12 are 372 pm (CS2S12) and 347 pm (S12S12), respectively. Thus, the packing effects on S12 due to the intermolecular forces are smaller for S12 in CS2·S12 and, therefore, the structure of the S12 ring is closer to a perfect D3d geometry in CS2·S12 [100]. 2.2.1.9 Monoclinic S13 Crystals of S13 were first obtained by Sandow et al. [36] and the crystal as well as the molecular structure were studied by Steudel et al. [38] using X- ray diffraction on single crystals. S13 crystallizes in the monoclinic space group P21/c–C2h5 (no. 14) as hexagonal yellow plates (from CS2 or CHCl3). The unit cell is built by eight molecules on sites of C1 symmetry. The lattice parameters are a=1295(2) pm, b=1236(1) pm, c=1761(2) pm, b=110.41(9) at T=163 K. A certain disorder of the S13 crystal was deduced from the crystal- lographic structure data and from the Raman spectrum. The molecule cyclo-tridecasulfur is similar to S11. The S13 ring can be ob- tained formally from S12 by replacing one atom by an S2 unit. This proce- dure introduces a twofold rotation axis as the only symmetry element of the molecule and leaves the motif of the formerly S12 unit unaltered (see Fig. 11). The point group of the molecules is approximately C2, and the two- fold axis intersects the longest bond introduced by the S2 unit. This relative- ly long bond of about 209 pm is connected to a small torsion angle of about 30 and neighbored by two short bonds of about 199 pm. The two indepen- dent molecules of the asymmetric unit have different molecular parameters. As in the case of S11 one of them (molecule I) is closer to C2 symmetry than the other one (see Table 13).PDF Image | Topics in Current Chemistry
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