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Solid Sulfur Allotropes 33 Table 11 Molecular structure data of orthorhombic S11 at 163 Ka [38] Point symmetry Av. bond lengths (pm) Range Av. bond angles () Range Av. torsion angles ()b Range 2–C2 (approximately) Molecule 1 205.7 203.7–211.0 106.0 103.8–108.4 97.1 70.5–140.5 Molecule 2 205.5 203.2–207.7 106.3 103.3–108.6 96.9 69.3–137.1 a Errors of bond lenghts 0.3–0.4 pm, of angles 0.1 b Absolute values (molecule 1) and 208 pm (molecule 2) with two adjacent short bonds (203– 204 pm). The related torsion angles of 141 (1) and 137 (2), respectively, are the largest ever observed in sulfur rings. However, crystalline S11 is of medi- um stability similar to S6, S9, S10, and S13. Both independent molecules of S11 in the crystal have the same conforma- tion, but the parameters of equivalent bonds show significant differences which are exceeded only in the case of solid S13 (Table 11). 2.2.1.8 Orthorhombic S12 Solid cyclo-dodecasulfur was first synthesized by Schmidt and Wilhelm in 1966 [98]. The authors obtained pale-yellowish, plate-like rectangular needles of solid S12 (from benzene solution). In the same year an X-ray structural analysis was reported by Kutoglu and Hellner [99]. A refined anal- ysis of crystalline S12 was performed by Steidel et al. in 1981, together with an examination of the molecular addition compound CS2·S12 [100]. Accord- ingly, S12 crystallizes in the orthorhombic space group Pnnm–D2h12 (no. 58) with a unit cell containing two molecules on sites of C2h symmetry. As was discussed in the case of cyclo-decasulfur S10, for the even-mem- bered S12 ring one might have expected a highly symmetrical molecular structure of D6d symmetry in analogy to S6 (D3d) and S8 (D4d). However, the X-ray single crystal analyses revealed an approximate D3d conformation of S12 (see Fig. 12) [99, 100]. By ab-initio quantum chemical calculations it has been shown that the D6d structure is about 226 kJ mol1 less stable than the D3d structure and, moreover, that it is not even a minimum on the potential energy surface [68]. In the S12 molecule the atoms are located in three parallel planes: six atoms in the median plane and three atoms each in an upper and in a lower plane. An arrangement in which the atoms would be located in two planes (D6d structure) would yield torsion angles of 129 being far from the opti- mum value of about 90. As a result, the bond lengths would be larger as a consequence of bond weakening due to the large torsion angles. In the D3dPDF Image | Topics in Current Chemistry
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