H2O2 hydrated electron reactions

Ionizing radiations (a, ft and y) react unselectively with all molecules and hence in the case of solutions they react mainly with the solvent. The changes induced in the solute due to radiolysis are consequences of the reactions of the solute with the intermediates formed by the radiolysis of the solvent. Radiolysis of water leads to formation of stable molecules H2 and H2O2, which mostly do not take part in further reactions, and to very reactive radicals the hydrated electron eaq, hydrogen atom H" and the hydroxyl radical OH" (equation 2). The first two radicals are reductants while the third one is an oxidant. However there are some reactions in which H atom reacts similarly to OH radical rather than to eaq, as e.g. abstraction of an hydrogen atom from alcohols, addition to a benzene ring or to an olefinic double bond, etc. 

Mezyk SP (1995) Rate constant determination for the reaction of sulfhydryl species with the hydrated electron in aqueous solution. J Phys Chem 99 13970-13975 Mezyk SP, Bartels DM (1995) Direct EPR measurement of Arrhenius parameters for the reactions of H atoms with H2O2 and D atoms with D2O2 in aqueous solution. J Chem Soc Faraday Trans 91 3127-3132... 

From a fundamental point of view the origins of H2O2 and H2 take place at the earliest step after ionization basically the recombination of OH forms H202, and the recombination of hydrated electron forms H2 and a non-scavengeable part of H2 directly originates from the ionization of water molecule and then enters the reaction mechanism of water radiolysis very early. 

The discovery in 1962 of the intense absorption band of eaq (Amax 720 nm, Cmax 1900 m mor ) [56] in pulse radiolysis experiments on aqueous solutions was made almost simultaneously at Mount Vernon Hospital [57] and Manchester [58], and provided an extremely useful method for measuring the rate constants for the reaction of this species with a variety of compounds. As mentioned in the Introduction, reactions of the hydrated electron are electron-transfer reactions. The first paper dealing with this type of measurement appeared in 1963 [59] and contained the rate constants for the reactions of Caq with H, H2O2, and O2. Many other rate constants for the reactions of Caq were determined in the following years. The NDRL/ NIST Solution Kinetics Database, Version 3, which covers the literature up to 1994, contains nearly two thousand entries for this type of reaction, almost all of them obtained by means of pulse radiolysis [7a]. In many cases, the rate constant for a given reaction has been determined more than once for example, the rate constants for the reaction of Caq with H+, NOs , C6H5NO2, Ag+, Cu +, and MV + (l,l -dimethyl-4,4 -bipyridinium) have been determined 19, 16, 14, 11, 10, and 8 times, respectively [7a]. 

The radiolysis of water produces hydrated electrons (e, ", G = 2.9), hydrogen atoms (G = 0.55) and hydroxyl radicals (G = 2.8) which react with the solute molecules. In addition, the radiolysis of aqueous solutions leads to formation of molecular products H2O2 (G = 0.75) and gaseous hydrogen (G = 0.45). Also produced are hydronium ions (HjO, G = 2.9). In most cases the molecular products do not interfere with the reactions of the radicals. To study the reaction of one radical with the solute without interference from other radicals, scavengers for the other radicals should be added. ... 

However, within the climate system there is no liquid water where radiation smaller than 195 nm exists. Liquid water photolysis under the formation of H2O2 was first proven by Tian (1911) for wavelengths < 190 nm. Other early studies on the influence of sunlight on aqueous solutions will be cited in Chapter 5.3.5. In the following, we will term the hydrated electron with the symbol or H2O- unless it exists only in H2O clusters where n = 6-50 (reaction 5.75). In the climate system, there are only two possibilities to produce either chemically or under solar light conditions via so-called photosensitizers. The only known direct chemical production (Hughes and Lobb 1976) is ... 

Studies on the catalatic activity of additional Cu" chelates confirm the previous conclusions that the complex must possess a suitable redox potential and two adjacent free sites to promote the efficient decomposition of H2O2. Malmstrom has discussed the asymmetric nature of oxidases which contain several copper atoms per molecule, including laccase and ceruloplasmin. The rate constant for the reaction of the hydrated electron... 

In contrast, a different reaction mechanism has been proposed by Comotti et al. for the oxidation of glucose over Au/AC (Figure 3.24) [175]. In their mechanism, the first step is the adsorption of hydrated alcoholate, which is formed in advance by alkali, onto Au catalytic sites. The electron-rich Au species might be formed by Au-alcoholate intermediates and then oxygen could be adsorbed onto Au by nucleophilic attack to form Au -02 or Au -02 Then, carboxylic adds are formed by two-electron reduction of oxygen. In this reaction, H2O2 must be formed as a by-product and was in fact detected experimentally. 

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