Sulfur radical species, complex

The kinetics of the reactions of various substituted thio phenols with liquid sulfur have been investigated, and intermediates of the general form RS H and HS H have been identified. The studies suggest that the reaction is usually free radical involving an initiation period in which a steady state concentration of sulfur radical species is established, The reaction appears to be second order in mercaptan and third order in sulfur. These findings have been interpreted using a complex sulfur radical species involving several Sg molecules. 

A review by Brandt and van Eldik provides insight into the basic kinetic features and mechanistic details of transition metal-catalyzed autoxidation reactions of sulfur(IV) species on the basis of literature data reported up to the early 1990s (78). Earlier results confirmed that these reactions may occur via non-radical, radical and combinations of non-radical and radical mechanisms. More recent studies have shown evidence mainly for the radical mechanisms, although a non-radical, two-electron decomposition was reported for the HgSC>3 complex recently (79). The possiblity of various redox paths combined with protolytic and complex-formation reactions are the sources of manifest complexity in the kinetic characteristics of these systems. Nevertheless, the predominant sulfur containing product is always the sulfate ion. In spite of extensive studies on this topic for well over a century, important aspects of the mechanisms remain to be clarified and the interpretation of some of the reactions is still controversial. Recent studies were... 

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... 

That the species formed by reaction between H2S and alkali metal atoms could be more complex than originally envisaged (14) is supported by the observation of secondary polymeric sulfur radicals on annealing. 

Generally, for oxidations, as demonstrated in the case of the complexes, [Fe(SPh)(CO)L(Ti5-C5R5)], it is assumed that electron lo occurs from the HOMO of the compound. However, it has been suggested that the HOMO in [Fe(SPh)(CO)2(Ti5-Cp )] is the sulfur lone pair and not a metal-centered orbital. The loss of one electron from this orbital leads to the formation of a radical species with the unpaired electron primarily localized on the ligand. The dimeric product is formed from couplingof two such species. 

Little is known concerning the chemistry of nickel in the atmosphere. The probable species present in the atmosphere include soil minerals, nickel oxide, and nickel sulfate (Schmidt and Andren 1980). In aerobic waters at environmental pHs, the predominant form of nickel is the hexahydrate Ni(H20)g ion (Richter and Theis 1980). Complexes with naturally occurring anions, such as OH, SO/, and Cf, are formed to a small degree. Complexes with hydroxyl radicals are more stable than those with sulfate, which in turn are more stable than those with chloride. Ni(OH)2° becomes the dominant species above pH 9.5. In anaerobic systems, nickel sulfide forms if sulfur is present, and this limits the solubility of nickel. In soil, the most important sinks for nickel, other than soil minerals, are amorphous oxides of iron and manganese. The mobility of nickel in soil is site specific pH is the primary factor affecting leachability. Mobility increases at low pH. At one well-studied site, the sulfate concentration and the... 

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