Sulfur allotropes analysis

Since the oxides do not have to be isolated, the sulfur solution after addition of the peroxyacid solution is simply kept in the refrigerator until S,(, has formed which is then isolated by cooling and recrystallization When both Sg and S g are dissolved in CS and the solution is cooled, then, under special concentration conditions, a new sulfur allotrope crystallizes out as orange-yellow opaque crystals of m.p. 92 °C. This compound has been shown by vibrational spectroscopy and X-ray structural analysis to consist of equal amounts of Sg and molecules in their usual conformations. In solution the mean molecular weight of 258 corresponding to 8 atoms per molecule indicates complete dissociation This is the first example of an allotrope of a chemical element consisting of molecules of different sizes. 

Up to now no six-membered sulfur allotrope other than rhombohedral 85 has been found. In addition, from theoretical structure analysis it is reasonable to assume that the chair conformation is energetically more favorable than... 

The crystalline structure parameters of y-Sy and 5-87 are summarized in Table 3. While the asymmetric unit of y-87 is built by one molecule, in -87 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 87 which have been studied so far. A detailed analysis and modeling of the intermolecular forces in 12 sulfur allotropes using a non-spherical sulfur atom potential yielded a slightly higher lattice energy for y-Sy than for S-Sy [73]. 

Crystals of endo-Su (a-Sig) 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 density of the endo-Sis crystal. Crystals of exo-Sig (PSu) 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 allotropes... 

Qualitatively, solid sulfur allotropes can best be characterized by Raman spectroscopy which is an extremely sensitive method. Depending on the number of atoms in the molecule and of the molecular symmetry quite different spectra are obtained [151]. A quantitative analysis of even complex mixtures of sulfur rings can be achieved by reversed-phase HPLC analysis after dissolution in carbon disulfide and using a UV absorbance detector... 

Most informative in this context is vibrational spectroscopy since the number of signals observed depends on the molecular size as well as on the symmetry of the molecule and, if it is part of a condensed phase, of its environment. In particular, Raman spectroscopy has contributed much to the elucidation of the various allotropes of elemental sulfur and to the analysis of complex mixtures such as hquid and gaseous sulfur. 

Irradiation of sulfur by various means yields discoloration of elemental sulfur, but it is doubtful that these forms will ever lead to pure allotropes. Above 92°C, a-sulfur converts into monoclinic sulfur. The kinetics of the transformation depend on various factors. Thermal analysis of single crystals of a-sulfur shows that the conversion is so slow that a-sulfur can be heated to 112°C, where it melts, before j8-sulfur is formed (19). 

Elemental selenium and elemental sulfur (any allotrope of either one) react on heating (melting) to give a large number of cyclic molecules Se Sm. While the eight-membered rings are much preferred, HPLC analysis has shown that both six- and seven-membered species are also formed. For a review of these cyclic selenium sufides, see ref. (55). 

Up to now, the various solid allotropes of polymeric sulfur can only be identified by X-ray structural analysis since the molecular vibrations and therefore the Raman spectra are very similar [151]. A chance to distinguish between the different allotropes by vibrational spectroscopy may only exist if the lattice vibrations are recorded which are expected to be different enough, especially for the fibrous and the laminar structures. However, this experiment requires samples of a high degree of crystallinity such measurements have to be performed at low temperatures to avoid broadening of the Raman lines. 

To analyze sulfur ring mixtures quantitatively by HPLC requires the determination of the calibration functions A=/(c) by analysis of solutions of differing concentrations c for all the allotropes available as pure materials (A peak area). In most cases this function has the form A=a-c with the slope depending systematically on the ring size or the number of sulfur atoms, respectively [89]. 

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