Hydroxyl radical electron transfer

Rapid e / h recombination, the reverse of equation 3, necessitates that D andM be pre-adsorbed prior to light excitation of the Ti02 photocatalyst. In the case of a hydrated and hydroxylated Ti02 anatase surface, hole trapping by interfacial electron transfer occurs via equation 6 to give surface-bound OH radicals (43,44). The necessity for pre-adsorbed D andM for efficient charge carrier trapping calls attention to the importance of adsorption—desorption equihbria in... 

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

By combining these reactions, hydroxyl radicals, generated with the photocatalyst and the electron transfer reagent, should react with methane to produce m yl radicals. In our... 

Reduction of Ketones and Enones. Although the method has been supplanted for synthetic purposes by hydride donors, the reduction of ketones to alcohols in ammonia or alcohols provides mechanistic insight into dissolving-metal reductions. The outcome of the reaction of ketones with metal reductants is determined by the fate of the initial ketyl radical formed by a single-electron transfer. The radical intermediate, depending on its structure and the reaction medium, may be protonated, disproportionate, or dimerize.209 In hydroxylic solvents such as liquid ammonia or in the presence of an alcohol, the protonation process dominates over dimerization. Net reduction can also occur by a disproportionation process. As is discussed in Section 5.6.3, dimerization can become the dominant process under conditions in which protonation does not occur rapidly. 

For the lability of alkoxy-type radicals, see Ando, W. (ed.). (1992). Organic Peroxides. Wiley, New York. In the same way, olefin epoxidation with peracids can be simply viewed as an electron transfer, followed by mesolytic cleavage of the peracid anion radical to carboxylate and hydroxyl radical, followed by homolytic coupling and proton loss. See also Nugent, W.A., Bertini, F. and Kochi, J.K. (1974). J. Am. Chem. Soc. 96,4945... 

Thus, two routes of transformation are possible for the Fe2+(H202) complex one-electron transfer to form the hydroxyl radical and two-electron transfer to form the ferryl ion. It is difficult to prove experimentally the formation of the ferryl ions because they are very reactive, so that this route of interaction of H202 with Fe2+ remains hypothetical to a great extent. Another change in the mechanism of H202 decomposition with increasing pH is related to the acidic dissociation of H02 (pKa = 4.4)... 

However, the existence of an extremely reactive bound hydroxyl radical is questionable because it is difficult to understand why it does not immediately react with adjacent molecules (most of the reactions of hydroxyl radicals proceed with the rates close to a diffusion limit). Therefore, the mechanism proposed by Zhang et al. [7,8] seems to be much more convincing. They suggested that the genuine oxidizing free radical formed during SOD inactivation is the bicarbonate radical anion CO/, which is formed as a result of the oxidation of bicarbonate. It has also been suggested that DMPO OH is formed by the addition of water to an intermediate of the reaction of DMPO with CO/ via a nucleophilic or electron transfer mechanism. 

The above electron trapping process promotes the electron transfer effects, which improves the photocatalytic activity of Eu-TiOz. At the same time, the positively charged vacancies (h+) remaining on the dye molecule can extract electrons from hydroxyl species in solution to produce hydroxyl radicals... 

Spin trapping is an often-used technique in the study of possible radical production in biological systems (for reviews see Kalyanaraman, 1982 Mason, 1984 Mottley and Mason, 1989), particularly by the detection and monitoring of spin adducts of the hydroxyl and hydroperoxyl ( OOH) radicals in view of their relation to possible damage mechanisms. This is a large area of research which it is not possible to cover in a limited review, and the treatment will therefore be restricted to a discussion of the electron transfer properties of biochemical systems (for a review on the application of the Marcus theory to reactions between xenobiotics and redox proteins, see Eberson, 1985) and... 

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

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