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Solid Sulfur Allotropes 19 any other isomeric forms like boat or twisted conformations [67, 68]. The chair-boat interconversion reaction has a barrier of about 126 kJ mol1 [69]. 2.2.1.2 Allotropes of S7 The first chemical preparation of cyclo-heptasulfur S7 was reported by Schmidt et al. in 1968 who obtained solid S7 as intense yellow needle-like crystals [16]. The molecular structure of homocyclic S7 is of special interest insofar as it is not possible to construct a puckered ring in which the bond lengths, bond angles, and especially the dihedral angles typical for even membered rings such as S6, S8, and S12 are preserved [70]. A first X-ray structural anal- ysis of solid S7 was attempted by Kawada and Hellner in 1970 who only de- rived a two-dimensional projection of the molecule. However, it was evident that the molecule must have a chair conformation and that the various dihe- dral angles of the molecule must differ significantly. Infrared and Raman spectroscopic studies have shown that S7 crystallizes in at least four different allotropic forms (a-, b-, g-, and d-S7) [21]. Most likely all of the crystalline allotropes consist of the same type of heptamer. Detailed X-ray structural studies have proved this assumption for g-S7 and d-S7 [20, 71]. In solution S7 undergoes rapid pseudorotation, i.e., the ring atoms become equivalent on average (observed by Raman spectroscopy and by 77Se NMR spectroscopy in the case of the related molecule 1,2-Se2S5) [72]. a-S7 crystallizes as intense yellow needle-like, lancet shaped crystals, which are disordered [20, 21]. b-S7 was always obtained as a powder by decomposition of d-S7 crystals [20, 21]. g-S7 crystallizes similar to a-S7, though under special conditions g-S7 can be obtained as single crystals in the monoclinic space group P21/c–C2h5 [20]. d-S7 forms block-shaped, tetragonal-bipyramidal and sarcophagus-like single crystals of the monoclinic space group P21/n–C2h2 [20, 21, 71]. The crystalline structure parameters of g-S7 and d-S7 are summarized in Table 3. While the asymmetric unit of g-S7 is built by one molecule, in d-S7 we have to deal with two independent molecules due to intermolecular forces. Figure 2 represents a view of the unit cells of the two allotropes of S7 which have been studied so far. A detailed analysis and modeling of the in- termolecular forces in 12 sulfur allotropes using a non-spherical sulfur atom potential yielded a slightly higher lattice energy for g-S7 than for d-S7 [73]. The molecular structures of the heptamers of both allotropic forms are nearly the same. The molecules have a chair conformation and their symme- try is close to Cs, but the site symmetry is actually C1 due to intermolecular interactions. Four neighboring atoms [S4 to S7 in Fig. 3] are located in a plane; in consequence, the torsion angle is close to the unfavorable value of 0 (motif: ++0+). The large internuclear distance of the bond S6-S7 is the result of the repulsion of the 3pp lone-pairs at these two atoms [21, 74].PDF Image | Topics in Current Chemistry
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