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16 Ralf Steudel · Bodo Eckert Irradiation at low temperatures is another method to convert a-S8 into polymeric sulfur [60]. Initially, the irradiation of a-S8 with visible light (l<420 nm) or UV radiation at temperatures of 2–70 K produces free radi- cals by homolytic dissociation of S-S bonds as demonstrated by electron spin resonance (ESR) spectroscopy [61]. On warming to room temperature the ESR signals fade away since the radicals decay by recombination as well as by triggering a ring-opening polymerization resulting in the formation of polymeric sulfur [60]. 2.2 Molecular and Crystal Structures The following section provides a brief summary of the molecular and crystal structures of the solid allotropes of sulfur mentioned in the Introduction. More specific details about the structures of most of the allotropes can be found in the cited literature. A conclusion concerning the characteristics of the molecular as well as of the crystalline structures of sulfur will be drawn at the end of this section. There are no crystalline forms known for the low atomic molecules S2 to S5 although these molecules are present in the gaseous and liquid phase [49, 59]. Since the cyclic molecules S16, S17, S19, and Sn (n21) have not yet been prepared, no molecular and crystal structure data are available. However, a mixture of large sulfur rings Sx(PDF Image | Topics in Current Chemistry
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