Orthorhombic Sn

The preparation of ryc/o-undecasulfur was first reported by Sandow et al. in 1982 [36]. Results of an X-ray structural analysis of a single crystal of Sn were published by Steidel et al. in the same year [37] and by Steudel et al. in 1986 [38]. Crystals of Sn are yellow, rod-like and of rhombic-bipyramidal shape. 

The unit cell contains eight molecules, although there are only two independent molecules in the asymmetric unit which occupy sites of Ci symmetry. The crystal system is orthorhombic (space group Pcali-Civ, no. 29) with lattice parameters a=1493.3(10), fc=832.1(5), and c=1808.6(12) pm at about 163 K. The molecular symmetry, however, is approximately C2 with a twofold rotation axis as the only element of symmetry (see Fig. 11). As a typical odd-membered ring Sn shows an unusually large bond of about 211 pm 

Both independent molecules of Su in the crystal have the same conformation, but the parameters of equivalent bonds show significant differences which are exceeded only in the case of soHd S13 (Table 11). 

As was discussed in the case of Qrdo-decasulfur Sio, for the even-mem-bered S12 ring one might have expected a highly symmetrical molecular structure of Dga symmetry in analogy to Sg (193d) and Sg (D4d). However, the X-ray single crystal analyses revealed an approximate conformation of S12 (see Fig. 12) [99, 100]. By ab-initio quantum chemical calculations it has been shown that the Dga structure is about 226 kj mol less stable than the Dsa structure and, moreover, that it is not even a minimum on the potential energy surface [68]. 

Structure the much more favorable value of the torsion angles of about 88 is allowed. In contrast to the structures of Se and Ss in which a cis-cis arrangement is repeated three-times and four-times [motifs (H—)3, (H—)4], respectively, the bonding in S12 shows a regular cis-trans repetition [motif  

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