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Solid Sulfur Allotropes 37 temperature. The motif in S14 is similar to that of S12. Therefore, the struc- ture of S14 can be derived by opening the S12 ring and adding an S2 frag- ment. 2.2.1.11 Solid S15 Up to now solid forms of cyclo-pentadecasulfur were obtained only as a lem- on-yellow powder or as light-yellow flakes, both containing traces of other sulfur rings (S14, S16) and slowly transforming to a sticky mass at room tem- perature. Attempts to grow single crystals of S15 were not successful [47]. However, Raman and UV spectra of S15 in solution have been measured which imply a ring structure [47]. The few Raman scattering lines in the stretching region (400–480 cm1) indicate degenerate vibrations which may exclude a very low molecular symmetry. Since in sulfur rings the wavenum- ber of a stretching vibration is directly correlated with the bond distance, an average value of the bond lengths of S15 of 207 pm (range 203–210 pm) was estimated [4, 47]. In addition, the bond length is correlated with the dihedral angle giving for the latter one a range of 30–140 in S15 [47, 101]. According to a density functional calculation S15 molecules are of C2 sym- metry with internuclear distances ranging from 205.3 to 206.5 pm, bond an- gles from 104.1 to 109.4, and absolute torsion angles from 77.1 to 112.3 [102]. 2.2.1.12 Allotropes of S18 cyclo-Octadecasulfur forms two crystalline allotropes each consisting of an- other molecular conformer usually called a-S18 and b-S18. However, since Greek letters should generally be used to designate different crystal struc- tures of one and the same molecule (e.g., a-, b-, and g-S8), we suggest here to term the allotropes with respect to the molecular conformation (see Fig. 14) in order to avoid misunderstanding. The S18 molecules in both al- lotropes are similar with the exception that four torsion angles are opposite in sign, two of them referring to the endo- and exo-conformation. In both conformers the atoms are placed in two almost parallel planes. While the endo-conformer is of C2h symmetry (containing a twofold rotation axis and a mirror plane perpendicular to that axis) the exo-conformer is of the lower Ci symmetry (inversion center as the only symmetry element). The variation of bond parameters of endo-S18 is much smaller than that of exo-S18. Crystals of endo-S18 (a-S18) are intense lemon-yellow, rhombic plates and belong to the orthorhombic crystal system. The molecules are arranged to form a pseudohexagonal close packing which accounts for the higher densi- ty of the endo-S18 crystal. Crystals of exo-S18 (b-S18) are monoclinic. The crystal and molecular structure data are summarized in Table 15. A detailed analysis and modeling of the intermolecular forces in 12 sulfur allotropesPDF Image | Topics in Current Chemistry
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