Sulfur homocycles

The origin of the small Sy content of all commercial sulfur samples is the following. Elemental sulfur is produced either by the Frasch process (mining of sulfur deposits) or by the Claus process (partial oxidation of HyS) [62]. In each case liquid sulfur is produced (at ca. 140 °C) which at this temperature consists of 95% Ss and ca. 5% other sulfur homocycles of which Sy is the main component. On slow cooling and crystalhzation most of the non-Ss species convert to the more stable Ss and to polymeric sulfur but traces of Sy are built into the crystal lattice of Ss as sohd state defects. In some commercial samples traces of Ss or Sg were detected in addition. The Sy defects survive for years if not forever at 20 °C. The composition of the commercial samples depends mainly on the coohng rate and on other experimental conditions. Only recrystalhzation from organic solvents removes Sy and, of course, the insoluble polymeric sulfur and produces pure a-Ss [59]. 

Correspondingly, the bond alternation observed in many sulfur homocycles and molecules with cumulated sulfur bonds can be attributed to the relatively strong interaction of neighboring bonds. 

From CS2 solution S7O2 is obtained as intensely orange colored crystals which on heating spontaneously decompose at 60-62 °C with evolution of sulfur dioxide. S7O2 is far less soluble in CS2 (ca. 1 g at 0 °C) than S7O. The solution decomposes within 1 h to a mixture of sulfur homocycles and SO2. Solid S7O2 decomposes at 25 °C within minutes and quantitatively within 2 h, even in the dark. Heating in a high vacuum to 50-60 °C produces S2O and elemental sulfur. The El mass spectrum therefore exhibits peaks due to these decomposition products only [67]. 

Thus, the wavenumbers of the SS stretching modes may be used to estimate the bond lengths in sulfur homocycles assuming that the corresponding vibrations are not coupled with other modes. The results obtained in this way for S7O2 are shown in Fig. 3. 

In its reactions SsO shows properties typical for both sulfur homocycles and sulfoxides. With elemental chlorine SOCI2 and S2CI2 are formed, with bromine SOBr2 and S2Br2 are obtained. Water decomposes SsO to H2S and SO2 besides elemental sulfur while cyanide ions expectedly produce thiocyanate. The reaction with iodide in the presence of formic acid is used for the iodometric determination of the oxygen content [70] ... 

The effect shown in Fig. 9 is a result of the bond-bond interaction which is a characteristic feature for chains and rings of two-valent chalcogen atoms. It can also be recognized from the relatively large bond interaction force constants fir of such compounds. The stretching force constants /r(SS) of polysulfur compounds depend on the SS bond distances as shown in Fig. 10. The data used in this figure include several excited electronic states of the S2 molecule as well as the disulfide anion and a number of sulfur homocycles [77]. 

Fig. 9 Mutual dependence of neighboring internuclear distances in sulfur homocycles (after [77])...

The structure of S7 is clearly the most intriguing59 61 of all known structures of neutral sulfur homocycles. Several modifications are known (sometimes only by powder or Raman data) however, the structural features are always the same and show a large bond length alternation and a range of S-S distances from 199 pm (bond order > > 1.0) to 218 pm (bond order < < 1.0 Figure 13). Other isomers were assessed by quantum chemistry.61... 

Most sulfur homocycles have been synthesized by reacting (C5H5)2Ti8s with appropriate open-chain polysulfur dichlorides (a, [Pg.5997]

As other sulfur homocycles, S13 shows a very characteristic Raman spectrum. In the solid state the molecules are of approximate C2 synmietry [38]. 

Fig. 20 Dependence of the S-S bond distance in sulfur homocycles on the torsion angle Tss- Experimental values are shown by open triangles, values obtained by MD calculations are shown hjfull circles [67]...

In the crystalline state, the site symmetry is in general lower than the symmetry of the free molecule. The preferred lattice structures of sulfur homocycles are of the orthorhombic and monoclinic crystal systems. The molecular packing arrangements of the soKd allotropes are quite different In orthorhombic Si2, for example, the asymmetric unit is given by one quarter... 

The same argu holds for other odd-membered sulfur homocycles like S9 or Su, but the effect in these molecules is less dramatic. 

Since about 1970 it has been shown that almost all metastable sulfur homocycles between Se and S20 can be prepared as pure soHds by kinetically controlled syntheses at moderate temperatures [71]. The investigation of their physical properties and in particular of their spectra allowed the molecular composition of Hquid sulfur to be determined using modern spectroscopic and chromatographic techniques. As a result of these investigations the pure sulfur rings S7, S12, Sis, md S20 can now be prepared from quenched sulfur melts by fractional crystalHzation [71]. The analytical work will be described below while the preparation of solid sulfur allotropes is reported elsewhere... 

The retention times tR of sulfur homocycles systematically increase at identical conditions with the ring size allowing the estimation of tR for new S species by inter- or extrapolation. The capacity factors... 

Fig. 3 Mean bond energies of the sulfur homocycles S6...S23 in liquid sulfur relative to Sg. AE is the difference in mean bond energies between Sg and the particular ring (in kj mok ) [93]...

All known solid state structures shown in Fig. 1 contain sulfur homocycles. While the 84 cation is a regular square, the 8g cation adopts an approximately Cs symmetric exo-endo conformation with three transannular contacts and 819 consists of two seven membered rings linked by a penta-sulfur chain and incorporates two tricoordinate sulfur atoms. An analysis of the soHd state anion contacts of the 84 and 8g cations showed that the positive charge is delocalized almost equally over the entire cations [11, 23]. 

This process continues until long-chain sulfane monosulfonate ions HS S03 (n>5) have been formed from which both sulfur homocycles S as well as higher polythionate ions can be generated ... 

Fig.1 Schematic model for the composition of the particles in hydrophilic sulfur sols consisting of long-chain polythionates S (S03 )2 and sulfur homocycles S ...

The particles of Selmi sols prepared by the method of Janek [42] have been investigated by infrared and Raman spectroscopy as well as by ion-pair chromatography. They consist of polythionates S Os with m=4-16 as well as of sulfur homocycles S (n=6-10), but depending on the method of preparation and purification a certain amount of salts (Na2S04, NaCl) will be present in addition. The sulfur atoms in the zero oxidation state are distributed between elemental sulfur and polythionates in an approximate ratio of 2 1 [45]. 

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