Photoreduction of hydrogen peroxide by hydrogen

Abstract

The photoreduction of hydrogen peroxide in water under hydrogen of up to 100 atm pressure has been investigated. The reaction involves a chain mechanism with the quantum efficiency for the disappearance of hydrogen peroxide being strongly dependent upon the concentration of hydrogen peroxide. A maximum quantum efficiency of about 35 mol/einstein occurs at a hydrogen peroxide concentration of about 5 X 10-4 M. At higher hydrogen peroxide concentrations the reaction (HO• + H202 -*• H20 + H02•), which leads to chain termination by subsequent H02* reactions, is competitive with the chain-propagating reaction (HO• + H2 -* H20 + H•) so that the quantum efficiency decreases with increasing [H202], At lower hydrogen peroxide concentrations the chain-propagating step ( -+ H202 -*• H20 + HO•) cannot compete with chain termination by ( • + 02 -*• H02•) so the yield again decreases. Scavenging of hydrogen atoms at lower hydrogen peroxide concentrations is not well understood. The participation of H02• radical is inferred from the observation of a strong pH effect which shows an inflection point at a pH near the pKa of H02• (4.88). A complex mechanism for the reaction is proposed and analyzed by numerical integration of the resulting rate expressions. This part of the work is an example of the usefulness of recently developed numerical integration techniques in the analysis of complex nonlinear reaction mechanisms.

Computational Modeling for the Oxidation Reactions of the Cysteine Residues with the Superoxide and the Organic Radical Species

“…• + H • → HO 2 − ), and so its rate constant is practically assumed to equal that of the hydrogen-abstraction reaction of the perhydroxyl radical. 32,33 Hence, our calculation results represent the first insight into the kinetics of this reaction, where we found that the Gibbs energy barrier of the TS 1 0 range is 14.7−18.2 kcal/mol. On the basis of Eyring's formula, the Gibbs energy barrier range of TS 1 0 yields an estimated rate constant of ∼10 2 to 1.0 M −1 s −1 , where the upper limit is in good agreement with Winterbourn and Metodiewa results.…”

“…Reaction represents the most thermodynamically favorable reaction channel, as previously reported by Cardey et al Hence, reactions and represent competing reaction channels for cysteine oxidation even at high pH where the perhydroxyl radical has a small abundance. Contrary to reaction , reaction is not kinetically accessible with a Gibbs energy barrier of 22.0 kcal/mol and a corresponding rate constant of 10 –3 M –1 s –1 as previously reported . So, the rate constant for reaction is about 3–5 orders of magnitude lower than that of reaction .…”

“…At physiological pH ∼ 7, the O 2 •– is the dominant species, and hence reaction represents the best candidate for the oxidation of the cysteine residues. To our knowledge, there is no experimental kinetic data for the hydrogen-abstraction reaction of superoxide anion (O 2 • + H • → HO 2 – ), and so its rate constant is practically assumed to equal that of the hydrogen-abstraction reaction of the perhydroxyl radical. , Hence, our calculation results represent the first insight into the kinetics of this reaction, where we found that the Gibbs energy barrier of the TS 1 0 range is 14.7–18.2 kcal/mol. On the basis of Eyring’s formula, the Gibbs energy barrier range of TS 1 0 yields an estimated rate constant of ∼10 2 to 1.0 M –1 s –1 , where the upper limit is in good agreement with Winterbourn and Metodiewa results.…”

“…Contrary to reaction 1, reaction 3 is not kinetically accessible with a Gibbs energy barrier of 22.0 kcal/mol and a corresponding rate constant of 10 −3 M −1 s −1 as previously reported. 33 So, the rate constant for reaction 3 is about 3−5 orders of magnitude lower than that of reaction 1. Finally, reaction 2 is both thermodynamically and kinetically favorable.…”

Kinetics of the interaction of perhydroxyl and superoxide radicals with hydrogen peroxide. The Haber-Weiss reaction

“…A number of such studies included determinations of kj which are listed in Table I. [3][4][5][6][7][8][9][10][11] Early qualitative evidence against the occurrence of the Haber-Weiss step was found by George in mixtures of solid K02 and 95% hydrogen peroxide.12 However, we now know that information about the Haber-Weiss step cannot easily be extracted from the kinetics of such heterogeneous mixtures. Some other studies published lately suggested that the reaction either does not occur or is very slow.13•14 Similar conclusions were also drawn from thermodynamic and molecular orbital considerations.…”