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Liquid Sulfur 95 Fig. 1 Schematic representation of the Raman spectra of some sulfur allotropes consist- ing of homocycles (fundamental modes given only; after [80]) narrower Raman lines are obtained. Green and blue laser lines usually initi- ate photoreactions even at low temperatures. 3.1.1 Raman Spectra of Liquid Sulfur Several authors have reported Raman spectra of sulfur melts without obtain- ing any novel information besides the fact that cyclo-S8 is the main con- stituent and that the polymer content increases with temperature, in particu- lar above 159 C [49, 81–85]. These spectra show the strong lines of S8 at 151, 218 and 474 cm1 and—at higher temperatures—the broad line of S1 at ca. 460 cm1 which prevents the detection of any weaker lines of the other components of the melt. With rising temperature the S8 lines also get broad- er and broader [83]. Only the use of modern spectrometers revealed the weak lines of additional components [80]: High-purity sulfur sealed in a glass ampoule was measured in the range 110–300 C using a krypton ion laser (647.1 nm) after the thermal and chemical equilibria had been estab- lished. In addition to the lines of S8 two weak lines at 264 and 235 cm1 (€1 cm1) were observed at temperatures above 130 C. These lines can defi- nitely be assigned to the symmetrical ring bending vibrations of S6 and S7, respectively [86]. The additional Raman signals of these two molecules are obscured by the dominating and very broad S8 lines. Compared to the solid allotropes a- and b-S8 the lines of liquid S8 of a1 and e2 symmetry are shifted to lower wavenumbers as higher the temperature of the melt [80]. At tem- peratures where the melt contains a substantial concentration of polymeric sulfur a broad Raman “band” in the region 350–460 cm1 was observed which partly must be caused by the polymer [80].PDF Image | Topics in Current Chemistry
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