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Liquid Sulfur 91 bonds” at the chain-ends (i.e., free radicals). In fact, Gardner and Fraenkel [37] obtained ESR spectra of very pure sulfur melts in the temperature range of 189–414 C and observed an increasing spin concentration with increas- ing temperature. These spectra consist of a single broad line at g=2.024. The radicals are formed by homolytic cleavage of sulfur-sulfur bonds: SS Ð2S ð3Þ From the temperature dependence of the signal intensity (amplitude) the activation energy for this reaction was derived as 140€20 kJ mol1 (SS bonds) or 70 kJ mol1(spins). From the line-width and its temperature dependence the lifetime of the free radicals was estimated which is obviously determined by the following rapid radical displacement reaction: SiþSS Ð SiSþS ð4Þ The activation energy for reaction (4) was derived as 12 kJ mol1 and the lifetime was estimated from the line-width as 1.6109 s at 216 C and 6.51010 s at 414 C. Evidently, the radicals move rapidly around in the melt and novel chain-like and cyclic molecules are constantly formed and de- stroyed. If the melt is cooled to temperatures below 150 C irreversible radi- cal recombination will occur. Koningsberger and DeNeef used a spectrometer of high sensitivity and applied time-averaging techniques to improve the signal-to-noise ratio. They observed isotropic ESR signals of Lorentzian line-shape (g=2.017€0.005) al- ready at 153 C (but not at lower temperatures) and up to 800 C [38]. The spin concentration was estimated as two electrons per 5107 atoms at 172 C and per 5103 atoms at 445 C (boiling point) [69]. These numbers would represent the average chain-lengths of the diradicals at the given tempera- tures if only chains and no rings were present which is of course not the case. Therefore, these numbers are upper limits for the average chain- lengths of the diradicals in liquid sulfur. The chemistry in liquid sulfur was modeled by the following four reac- tions (C=diradical chain, R=ring) and the enthalpy values given were de- rived using a theoretical analysis presented in [69] for the temperature re- gion of 250–550 C: Ring opening : SR8 Ð SC8 DH0 1⁄4 153kJmol1 ð5Þ Chain dissociation : SCn Ð SCi þSCj DH01⁄4 149 kJ mol1 ð6Þ Ringaddition : SCi þSR8 Ð SCj DH01⁄4 10kJmol1 ð7Þ Radical displacement : Si þ S S Ð SiS þ S ð8Þ The enthalpy of the last reaction is of course zero. Poulis et al. [39] measured the magnetic susceptibility of liquid sulfur in the temperature range of 120–580 C. The susceptibility is strongly influ-PDF Image | Topics in Current Chemistry
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