Hydrogen Atom and Hydrated Electron

Current interest in the hydrated electron has sparked increased activity in the entire field of solvated electrons. If Conant and Hall had the hydrated electron at their disposal, they might have refrained from writing that 44Much important chemistry has been obscured by our slavish devotion to water. Now, because of the interconversion of hydrogen atoms and hydrated electrons in alkaline solutions and because the hydrated electron is a primary product in many photochemical processes, intensified studies on aqueous systems at the experimental and theoretical levels may be predicted. 

The conversion of hydrogen atoms to hydrated electrons is an exception to the rule of rapid reactivity in this case, the reaction is relatively slow, first order in [OH-], and has a second-order rate constant of 3 x 107 M-1 s-1.45 In less alkaline solutions, the direct reaction of the hydrogen atom with water is quite slow and yields H2 + OH. As a result of these low rate constants, the interconversion between hydrogen atoms and hydrated electrons can be catalyzed by weak bases such as F-and NH3. 

Therefore, in anoxic medium and (for example biogenic) in situ hydrogen production, this is an important pathway to initiate reduction chains by electron transfer processes. The hydrogen atom and hydrated electron are interconvertible. Reaction... 

In aqueous solution, the indirect action of radiation plays an important role, since the primary radiolysis products (Scheme 5.26) - that is, hydroxyl radicals, hydrogen atoms and hydrated electrons - are highly reactive. They are formed with G (OH ) = 2.7, G(H ) = 0.55, and G(e ) = 2.65. 

To explain the conversion of the Fe to Fe, investigators have postulated that the two hydrated ions are linked by a structured chain of water molecules. The assumption is that one of the electrons of the Fe valence shell enters the valence shell of an oxygen atom of a neighboring water molecule. The water molecule releases a hydrogen-free radical (one hydrogen atom and one electron), which is transferred from water molecule to water molecule until it reaches a water molecule close to Fe, at which point the electron is transferred to Fe, yielding Fe. ... 

Proteins OH radicals, H atoms and hydrated electrons react very rapidly with proteins, with rate constants on the order of 10 to 10 L mol s . Due to the given geometric structure of most proteins, the probability of a certain site to become attacked depends on its location. In other words, amino acid moieties situated in the inner part of a protein are less likely to be attacked by reactive species than are those in outer parts. OH radicals may abstract hydrogen from the backbone or side groups and add to aromatic rings or sulfur atoms, as shown in Scheme 5.31. 

Anbar, M. and Neta, P. (1967). A compilation of specific biomolecular rate constants for the reaction of hydrated electrons, hydrogen atoms and hydroxyl radicals with inorganic and organic compounds in aqueous solutions. Int. J. Appl. Radiat. Isot. 18, 493-497. 

Buxton, G. V., Greenstock, G. L., Helman, W. P., Ross, A. B. (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solutions. J. Phys. Chem. Ref. Data 17, 513-886. 

The second-order rate constant for the reaction of a hydrogen atom with a hydroxide ion to give an electron and water (hydrated electron) is 2.0 x 10 M s . The rate constant for the decay of a hydrated electron to give a hydrogen atom and hydroxide ion is 16M s. Both rate constants can be determined by pulse radiolytic methods. Estimate, using these values, the pA of the hydrogen atom. Assume the concentration of water is 55.5M and that the ionization constant of water is 10 M. 

The yields of these so-called primary species, present at the time when radical combination in, and diffusive escape from, the spurs is complete, were obtained by adding solutes to the water to capture the radicals and by measuring the stable identifying products. It was from a number of these studies that it became clear that the reducing radical must exist in two forms, which turned out to be the hydrogen atom and the hydrated electron (e q). For example, Hayon and Weiss [6] found that the yields of H2 and Cl produced by irradiating solutions of chloroacetic acid varied with pH in a manner that was consistent with the following reactions ... 

The hydrated proton, so far in this text, has been written as H+(aq), but a bare proton might be expected to interact very strongly with a water molecule to give the ion, H30+, that would also be hydrated and written as H30+(aq). The interaction takes the form of an extra covalent bond between a hydrogen atom and the formally positively charged O1 ion, itself electronically identical with the nitrogen atom. The H30+ ion is isoelectronic with the NH3 molecule. The ion H30+ is called variously hydronium, hydroxonium or nximium. 

Buxton GV, Greenstock C L, Helman WP, Ross A B (1988) Critical Review of Rate Constants for Oxidation of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (0H°/0° ) in Aqueous Solutions, Journal of Physical and Chemical Reference Data 17 513-886. 

Michael BD, Hart EJ. The rate constants of hydrated electron, hydrogen atom, and hydroxyl radical reactions with benzene, 1,3-cyclohexadiene, 1,4-cyclo-hexadiene, and cyclohexene. J Phys Chem 1970 74 2878-2884. 

Hydrogen atoms and hydroxyl radicals react with aliphatic compounds mainly by H-abstraction from the chain, although reactions with certain substituents are also important. With hydrated electrons the functioned group is the only site of reaction and its nature determines the reactivity. The reactions of hydrated electrons are by definition electron transfer reactions. The rate of reaction of a certain substrate will depend on its ability to accommodate an additional electron. For example, in an unsaturated compound the rate may depend on the presence of a site with a partial positive charge. Thus acrylonitrile and benzonitrile are three orders of magnitude more reactive toward e q than are ethylene and benzene. On the other hand, this large difference does not exist in the case of addition of H and OH. 

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