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Solid Sulfur Allotropes 29 2.2.1.4 Allotropes of S9 Although crystalline S9 was prepared in 1970 by Schmidt et al. [32], the structure was determined only in 1996 by Steudel et al. [33]. cyclo-Nonasul- fur crystallizes as deep-yellow needles in at least in two allotropic forms (a- and b-S9) as shown by Raman spectroscopy [33, 93]. The space group of a- S9 is P21/n(=C2h2) with two independent molecules in the unit cell occupying sites of C1 symmetry. The molecular symmetry is approximately C2. The av- erage bond length is 205.25 pm ranging from 203.2 to 206.9 pm. One single short bond of about 203–204 pm in each of the two molecules is neighbored by two longer bonds of about 206–207 pm. The bond angles vary from 103.7 to 109.7 with an average value of 107.2 for both molecules in the cell. The torsion angles show a much wider range, the absolute values varying from 59.7 to 115.6. The molecular structure was found to be in accordance with results of density functional and ab-initio molecular orbital calcula- tions, especially the pattern of the torsion angles (motif) [67, 68, 94]. The lattice parameters of monoclinic a-S9 are a=790.2 pm, b=1390.8 pm, c=1694.8 pm, and b=103.2 at 173 K [33]. If for each atom of the two inde- pendent molecules the shortest intermolecular distance is considered, one finds 18 of such contacts ranging from 338.9 to 364.3 pm [33]. The structure of b-S9 has not yet been determined. However, the Raman spectrum of b-S9 suggests that the molecules of this allotrope have the same molecular conformation as those of a-S9 [33]. 2.2.1.5 Monoclinic S10 Crystalline S10 was first prepared by Schmidt et al. in 1965 [16, 95]. The mo- lecular structure of cyclo-decasulfur is interesting insofar as a D5d symmetry was expected for this even-membered ring, in analogy to S6 (D3d) and S8 (D4d) [96]. However, X-ray studies on single crystals have shown that the S10 molecule is of D2 symmetry [35, 97]. The only symmetry elements are three orthogonal twofold axes of rotation (C2). The conformation of the molecule is similar to the one of S12 since the atoms are located in three planes (rather than two as in S6 and S8). In fact, S10 can formally be composed of two iden- tical S5 units cut from the S12 molecule. The mean bond length in S10 is 205.6 pm (S12: 205.2 pm), but the variation of the bond lengths around the molecule shows a clear alternation (Fig. 9). Two sets of torsion angles, about 77 and +123, are correlated with “nor- mal” bond distances of about 203 to 205 pm, and large distances of about 207 to 208 pm, respectively. By ab-initio quantum chemical calculations it was found that the D5d con- former is about 29 kJ mol1 less stable than the D2 structure [68]. The lower stability of the D5d structure can be explained considering the torsion angles of both structures. While the D5d structure forces the torsion angles to bePDF Image | Topics in Current Chemistry
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