Sulfur radicals

The reaction with meicaptans is beheved to generate initiating sulfur radicals ... 

Nowadays, ultramarine-type pigments are produced synthetically. Inside the zeolite structure the highly reactive sulfur radical anions are well protected which explains the stability of the blue color over thousands of years in air. However, the species responsible for the blue color should not be confused with the sulfur radical cations responsible for the blue color of sulfur solutions in fuming sulfuric acid (oleum) and similar oxidizing mixtures... 

By mass spectrometry sulfur radical anions with up to 25 atoms have been detected and there is photoelectron spectroscopic evidence for chainlike as well as cychc isomers of 8g and 87 [141]. 

Very approximately the S2O content of a gas mixture can be estimated from the color of the condensate at -196 °C in a glass trap (provided that all other components are colorless at this temperature). Due to the formation of highly colored decomposition products the condensate is yellow at <2 mol% S2O, orange-yellow at 5-10%, orange at 20-30%, cherry-red at 40-70%, and dark-red at >85% [10]. These colors [14] are caused by small sulfur molecules like S3 and 84 [15, 16] as well as by sulfur radicals formed in the radical-chain polymerization of S2O to polysulfuroxides (S 0)x and SO2 [10, 17] ... 

The species S3 (absorbing at 420 nm) and S4 (absorbing at 530 nm) have been detected by reflection spectra in the condensate but the formation of S4 is unexplained [16]. S3 and SO2 have also been observed by Raman spectroscopy in such samples [15] (the expected S4 Raman line at 678 cm was probably obscured by the SS stretching mode of S2O at 673 cm but a shoulder at the high-frequency side of the S2O line indicates that some S4 may have been present). While the reddish colors turn yellow on warming at about -120 °C, the sulfur radicals could be observed by ESR spectroscopy up to 0 °C [10]. If the condensation of S2O gas is performed very slowly at -196 °C the condensate is almost colorless and turns red only if the temperature is allowed to increase slowly. Hence, it has been suspected that S2O is actually colorless like SO2. 

On condensation at low temperatures, on dissolution in inert solvents or on raising its partial pressure substantially above 1 mbar (100 Pa) S2O polymerizes with partial disproportionation. Since sulfur radicals have been detected in such condensates by ESR spectroscopy [10] it has been proposed that a radical-chain reaction takes place according to Scheme 5. 

Glasbeek M (2001) Excited State Spectroscopy and Excited State Dynamics of Rh(III) and Pd(II) Chelates as Studied by Optically Detected Magnetic Resonance Techniques. 213 95-142 Glass RS (1999) Sulfur Radical Cations. 205 1-87 Gobbi L, see Diederich F (1999) 201 43-129... 

Z. B. Alfassi (Ed.), The Chemistry of Sulfur Radicals, Wiley, Chichester, 1999. 

Stephen Hilton obtained his B.Sc. at King s College London in 1996 followed by a Ph.D. under the supervision of Professor Keith Jones and Dr Sheetal Handa. In 2002 he carried out a postdoctoral fellowship under the supervision of Professor William Motherwell at University College London. In 2006 he moved to his current position at The Institute of Cancer Research as a postdoctoral fellow in Medicinal Chemistry. His research interests lie in the area of natural products containing sulfur, radical chemistry and anticancer drug targets. 

In addition to silicon, sulfur groups have been used as auxiliaries for amide oxidation reactions (Scheme 31) [63], However, in these cases the mechanism of the reaction is different. The sulfur is oxidized to form a sulfur radical cation that is then eliminated from the molecule in order to... 

The thermochemistry of sulfur radicals in the gas phase has been reviewed. Methylsulfonyl radicals and cations have been produced by femtosecond collisional electron transfer in the gas phase. When formed by vertical collisional electron transfer from cation CH3SO2+, radical CH3S02 dissociates to CH3 and SO2. Radical CH30S0 exists as a mixture of syn (19a) and anti (19b) isomers which are stable when formed by collisional electron transfer to the corresponding cation. Dissociation of both isomers of CH30S0 formed CH3 and SO2 via isomerization to methylsulfonyl radical. An ab initio study on the formation of the thiyl peroxyl radical has also been reported. Julolidylthiyl radicals (20) were formed by femtosecond photo-dissociation of the corresponding disulfide and have been observed... 

S4 + [77-80]. The observation of EPR signal for these solutions led the authors to believe that the species are Sg" and S4+ radicals presumably in equilibrium with their dimers Si6 + and Sg +, respectively. By using the FPR spectrum of 92% enriched sulfur. Low and Beaudet [81] concluded that the sulfur radical in 65% oleum is S5 +. ... 

A remarkable example of a persistent radical anion is the semiprecious stone, lapis lazuli, known and appreciated as a pigment since ancient times. The species imparting the blue hue is trisulfur radical anion, S , accompanied by variable fractions of S, which introduces a green tint the sulfur radical anions are incarcerated... 

A Stern-Volmer plot obtained in the presence of donors for the stilbene isomerization has both curved and linear components. Two minimal mechanistic schemes were proposed to explain this unforeseen complexity they differ as to whether the adsorption of the quencher on the surface competes with that of the reactant or whether each species has a preferred site and is adsorbed independently. In either mechanism, quenching of a surface adsorbed radical cation by a quencher in solution is required In an analogous study on ZnS with simple alkenes, high turnover numbers were observed at active sites where trapped holes derived from surface states (sulfur radicals from zinc vacancies or interstitial sulfur) play a decisive role... 

Irradiation of proteins, peptides, and amino acids at — 196°C. produces radicals different from those at room temperature. No sulfur radicals are observed at this temperature. 

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