Radicals from hydrated electron reactions

The authors suggested that di-n-propyl disulfide is formed by the reaction of both hydrated electrons and OH radicals, however it should be mentioned that the yield from hydrated electrons should be higher than from OH radicals as otherwise N20 will not reduce the yield of dipropyl disulfide as N20 merely converts eaq to OH radicals. The yield from each radical could be calculated from the yields in the presence of the various scavengers, but unfortunately these data were not given by the authors. 

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. 

A specific free radical can be produced from a precursor molecule either in an initiation step or a propagation step in which a reagent radical reacts with the precursor. Initiation requires either removal or addition of an electron or homolysis. Chemically this can be done in a number of ways, by using one-electron oxidants or reductants or by inducing homolysis in some way examples of these types of reactions include autoxidation [84-86], photochemical oxidation and reduction [87-90], and oxidation and reduction by metal ions and their complexes [91-93], In propagation reactions, the reagent radical might be the hydroxyl radical, the hydrated electron, or any other suitably reactive species that will interact with the precursor molecule in the desired manner. We will consider initiation reactions first. 

Reactions with the Primary Species. In aqueous solution the monomers are exposed to the action of the species formed from the water in the primary act. The rate constants published for the reactions of the hydroxyl radicals and hydrated electrons are included in Table I. Most of the hydroxyl rate constants were measured using thiocyanate and are therefore subject to the usual uncertainties of this method (5). No rate constants appear to have been published for the reactions of the hydrogen atoms. 

The decay of the transients was also followed at 330 n.m. The transients formed either from Au(CN)2" + N20 or from AuCIf -f CH3OH decay according to a second order rate law (see Figure 4) up to about 70% of the maximum optical density. The slopes of a number of such decay curves pertaining to both the two systems average around (2.1 0.4) X 104 sec."1, and they are the same at 25/xM and 50/xM gold concentrations which are much in excess of the free radicals or hydrated electrons produced per pulse. The second order decay indicates a disproportionation reaction... 

C(io(OH)i8 (10) Poly hydroxy lation (Scheme 4) of the hydrophobic [60]fullerene core enhances the water solubility of this carbon allotrope up to 4.0 x 10 M (67). The tt-radical anion, (Ceo )(OH)ig, generated by electron transfer from hydrated electrons and (CH3)2 C0H radicals, absorbs with maxima at 870, 980 and 1050 nm. The bimolecular rate constant for a reaction with hydrated electrons is 4.5 x 10 M s . Based on electron transfer studies with suitable electron donor / acceptor substrates, the reduction potential of the C6o(OH)ig/(C6o )(OH)i8 couple was estimated to be in the range between -0.358 V and -0.465 V versus NHE. 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

In these solutions, the hydrated electrons from the radiolysis of water produce CO by their reaction with COj, and the OH radicals attack the alcohol to form (CHjIjCOH radicals (see also footnote on page 117). 

Radiation chemistry highlights the importance of the role of the solvent in chemical reactions. When one radiolyzes water in the gas phase, the primary products are H atoms and OH radicals, whereas in solution, the primary species are eaq , OH, and H" [1]. One can vary the temperature and pressure of water so that it is possible to go continuously from the liquid to the gas phase (with supercritical water as a bridge). In such experiments, it was found that the ratio of the yield of the H atom to the hydrated electron (H/eaq ) does indeed go from that in the liquid phase to the gas phase [2]. Similarly, when one photoionizes water, the threshold energy for the ejection of an electron is much lower in the liquid phase than it is in the gas phase. One might suspect that a major difference is that the electron can be transferred to a trap in the solution so that the full ionization energy is not required to transfer the electron from the molecule to the solvent. 

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 ... 

Shortly after the discovery of the hydrated electron. Hart and Boag [7] developed the method of pulse radiolysis, which enabled them to make the first direct observation of this species by optical spectroscopy. In the 1960s, pulse radiolysis facilities became quite widely available and attention was focussed on the measurement of the rate constants of reactions that were expected to take place in the spurs. Armed with this information, Schwarz [8] reported in 1969 the first detailed spur-diffusion model for water to make the link between the yields of the products in reaction (7) at ca. 10 sec and those present initially in the spurs at ca. 10 sec. This time scale was then only partially accessible experimentally, down to ca. 10 ° sec, by using high concentrations of scavengers (up to ca. 1 mol dm ) to capture the radicals in the spurs. From then on, advancements were made in the time resolution of pulse radiolysis equipment from microseconds (10 sec) to picoseconds (10 sec), which permitted spur processes to be measured by direct observation. Simultaneously, the increase in computational power has enabled more sophisticated models of the radiation chemistry of water to be developed and tested against the experimental data. 

Reaction of the hydrated electron via Reaction 2 or 3 and of the hydrogen atom via Reaction 5 or 6 will ultimately yield an H02 radical. From known values of the rate constants (13)—viz., k2 = k3 = 2 X 1010 liters mole"1 sec."1—it can be calculated that under the experimental conditions of Table I, virtually all hydrated electrons react via Reaction 2. 

Transfer of radiation-induced electrons and holes (H20 ) from the hydration layer of DNA has been of considerable recent interest. Results from ESR experiments at low temperatures suggest that ionization of hydration water (reaction 4) results in hole transfer to the DNA (reaction 5) [4, 24-28]. Since the proton transfer reaction (reaction 6) to form the hydroxyl radical likely occurs on the timescale of a few molecular vibrations [29], it is competitive with and limits hole transfer to DNA [27]. 

Schmidt KH, Flan P, Bartels DM (1995) Radiolytic yields of the hydrated electron from transient conductivity improved calculation of the hydrated electron diffusion coefficient and analysis of some diffusion-limited (e )aq reaction rates. J Phys Chem 99 10530-10539 Schoneich C, Aced A, Asmus K-D (1991) Halogenated peroxyl radicals as two-electron-transfer agents. Oxidation of organic sulfides to sulfoxides. J Am Chem Soc 113 375-376 Schuchmann Fl-P, von Sonntag C (1981) Photolysis at 185 nm of dimethyl ether in aqueous solution Involvement of the hydroxymethyl radical. J Photochem 16 289-295 Schuchmann Fl-P, von Sonntag C (1984) Methylperoxyl radicals a study ofthey-radiolysis of methane in oxygenated aqueous solutions. Z Naturforsch 39b 217-221 Schuchmann Fl-P, von Sonntag C (1997) Heteroatom peroxyl radicals. In Alfassi ZB (ed) Peroxyl radicals. Wiley, Chichester, pp 439-455... 

Burney S, Niles JC, Dedon PC, Tannenbaum SR (1999) DNA damage in deoxynucleosides and oligonucleotides treated with peroxynitrite. Chem Res Toxicol 12 513-520 Burr JG, Wagner BO, Schulte-Frohlinde D (1976) The rates of electron transfer from ClUra " and CIUraH" top-nitroacetophenone. Int J Radiat Biol 29 433-438 Buxton GV, GreenstockCL, Helman WP, Ross AB (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals ( OH/ O-) in aqueous solution. J Phys Chem Ref Data 17 513-886... 

The irradiation of starch in aqueous gels, sols, and solutions leads to somewhat different results because of the operation of different mechanisms. Although all reactions in the solid state involve free-radical processes, those in aqueous solutions may involve hydrated electrons, which with oxygen give O2 ions. Kochetkov et al. 90 have assumed that deoxy sugars result from the involvement of Oj ions alone, whereas the formation of deoxy ketoses is assisted by hydroxyl radicals. Stockhausen et a/.191 have proposed the following sequences of reactions. Ionization of water leads to solvated electrons and further radicals. 

In the case of metal deposition at the ionic liquid plasma interface two possible reduction processes can conceivably take place. First, the metal cations of the dissolved metal salt can be reduced. Secondly, the cations of the ionic liquid can be reduced to neutral radicals, which can further react as described by Witkamp and as summarized above. As a first guess of which process is preferred, the rate constants of the reaction for example of silver ions (k > 3.2 x 1010 Lmol-1 s-1) and imidazolium ions (k < 4.3 xlO9 Lmoh1 s 1) with hydrated electrons, taken from the data collection of Buxton et al., can be considered [52]. Thus, as long as sufficient silver ions are still in solution the reduction of the imidazolium cations of the ionic liquid represent the minor reaction pathway and the ionic liquid should not decompose significantly. 

The radiolysis of water produces hydrated electrons (e q, 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 also yields H202 (G = 0.75), gaseous hydrogen (G = 0.45) and hydronium ions (H30+, 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 added7-10. 

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. 

In addition, the hydrated electron acts as a nucleophile, especially with organic molecules that contain halogen atoms (Eq. 6-16). This reaction results in rapid elimination of a halide ion from the initially formed negatively charged organic species. The reaction of Eq. 6-16 is of special interest for the degradation of per-halogenated saturated hydrocarbons that are usually not affected by hydroxyl radicals (Sun et al, 2000). 

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