Hydrated electrons atoms

Irradiation of dilute aqueous solutions results in the interaction ofthe ionizing radiation with water molecules. The radiolysis of water produces hydrated electrons (eaq ", G = 2.8), hydrogen atoms (G = 0.6) and hydroxyl radicals (G = 2.8) which react with the molecules of the solutes. The use of special scavengers can convert one species to another, e.g. 

In the case of UV-light, the reactions are (CHjljCO hv (CHjl CO (CHjl CO + (CHjljCHOH 2 (CHjljCOH. In the case of y-rays, hydrated electrons, H-atoms, and OH radicals are formed by decomposition of the solvent. The hydrated electrons react with acetone aq + (CHj)jCO -E -> (CHjljCOH, and the H and OH radieals with propanol-2 H(OH) + (CHjljCHOH H fHjO) -E (OHjl COH,... 

The complexed halide atoms are produced by high energy radiation in solutions of colloids that contain halide anions X and are saturated with nitrous oxide. Hydrated electrons formed in the radiolysis of the aqueous solvent react with NjO according to NjO -f e -f H O - Nj -t- OH -I- OH to form additional OH radicals. Ions X are oxidized by OH, the atoms X thus formed react rapidly with X to yield XJ radicals. 

The hydrated electrons then react according to e + Cd Cd, and the Cd ions which have a strong absorption at 300 nm react with the colloidal particles after the pulse. It was observed that the same bleaching took place during this reaction as in the reaction of e with CkiS particles, and it was concluded from this result that Cd" transfers an electron to a CdS particle Cd" + (CdS), - Cd + (CdS) . These observations also are of interest for our understanding of the formation of Cd atoms in the photocathodic dissolution of CdS (see Sect. 3.4). Cd" cannot be the intermediate of the overall reaction 2e + Cd - Cd° as already pointed out in discussing the mechanism of Eqs. (35) and (36)... 

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. 

Misik V, Riesz P (1997) Effect of Cd2+ on the H atom yield in the sonolysis of water. Evidence against the formation of hydrated electrons. J Phys Chem A 101(8) 1441-1444... 

Debierne (1914) was the first to suggest a radical reaction theory for water radiolysis (H and OH). In various forms, the idea has been regenerated by Risse (1929), Weiss (1944), Burton (1947, 1950), Allen (1948), and others. Platzman (1953), however, criticized the radical model on theoretical grounds and proposed the formation of the hydrated electron. Stein (1952a, b) meanwhile had suggested that both electrons and H atoms may coexist in radiolyzed water and proposed a model in which the electron digs its own hole. Later, Weiss (1953, 1960) also favored electron hydration with ideas similar to those of Stein and Platzman. In some respects, the theoretical basis of these ideas is attributable to the polaron (Landau, 1933 Platzman and... 

The primary yield of H,02 may be obtained by measuring the H202 yield in a system containing excess oxygen. In this case, hydrated electrons and H atoms are converted into O,- and H02, respectively, with an equilibrium between these species ... 

FIGURE 6.5 Schematic of the structural model of the solvated electron. The electron is considered trapped at the center of the tetrahedron, whereas for the hydrated electron, the vertices are occupied by O atoms. Arrows indicate the direction of molecular dipoles that may differ from cell to cell. 

Discovery of the hydrated electron and pulse-radiolytic measurement of specific rates (giving generally different values for different reactions) necessitated consideration of multiradical diffusion models, for which the pioneering efforts were made by Kuppermann (1967) and by Schwarz (1969). In Kuppermann s model, there are seven reactive species. The four primary radicals are eh, H, H30+, and OH. Two secondary species, OH- and H202, are products of primary reactions while these themselves undergo various secondary reactions. The seventh species, the O atom was included for material balance as suggested by Allen (1964). However, since its initial yield is taken to be only 4% of the ionization yield, its involvement is not evident in the calculation. 

In the presence of 10 pM peroxide, the yields of H2, H202, and of H + eh are about the same in neutral and 0.4 M acid solutions. Since H atoms produced by the reaction of acid with hydrated electrons have different reaction rates and sequences of reaction, a much greater difference of the... 

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. 

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. 

In neutral water the radiation chemical yields G are 2.7 x 10 7 mol J-1 for the hydrated electron, 2.8 x ] 0 7 mol, 1 1 for the "OH radical and 6 x 10 x mol J-1 for the H atom. These values vary slightly with the solute concentration, due to increased reaction with the solute in the radiation spurs. In order to study the reaction of one radical without interference of the others, scavengers have to be added to the system. The best scavengers are those which will convert the unwanted radical to the studied one. This can be done with eaq, which can be converted to "OH or to H by the addition of N2O or H+, respectively (equations 3 and 4). 

Figure 9.20 Illustration of electron transfer between sulphide surface and hydration oxygen atom showing the mechanism of collectorless flotation of galena and pyrite...

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. 

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 ... 

Hydrated electron yields decrease with increasing MZ jE, but they do not seem to decrease to zero. Experiments have been performed on aerated and deaerated Fricke dosimeter solutions using Ni and ions [93]. One half of the difference in the ferric ion yields of these two systems is equal to the H atom yield. The Fricke dosimeter is highly acidic so the electrons are converted to H atoms and to a first approximation the initial H atom yield can be assumed to be zero (see below). There is considerable scatter in the data of the very heavy ions, but they seem to indicate that hydrated electron yields decrease to a lower limit of about 0.1 electron/100 eV. The hydrated electron distribution is wider than that of the other water products because of the delocalization due to solvation. This dispersion probably allows some hydrated electrons to escape the heavy ion track at even the highest value of MZ jE. 

Only two studies have determined H atom yields in neutral water with heavy ions [26,121]. H atom yields are extremely difficult to determine directly because of competition with hydrated electrons or OH radicals. Yields of H atoms are usually estimated from differences in molecular hydrogen yields using various scavengers for the H atom. Fig. 11 gives the H atom yields as a function of MZ jE. There are not many data points, but they seem to agree with each other and show a decrease with increasing MZ jE values. 

The measured H atom G-value is about 0.25 at MZ jE = 1, while the equivalent yield of hydrated electrons is found at MZ jE = 10. The persistence of the hydrated electron to higher MZ jE values suggests that it does not decrease to zero at an infinite value of MZ jE. Most H atoms are produced in conjunction with OH radicals in the core of the heavy ion track. The recombination rate constant is high so there is a small probability that H atoms will escape the track at high LET (MZ jE). H atoms can be formed by hydrated electron reactions and their yield cannot decrease to zero if hydrated electron yields do not. However, hydrated electron yields are low at high MZ /E values so the H atom yield can be considered negligible in this region. 

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. 

OMA 4) equipped with CCD detector. A 3D spectral data is reported in Fig.4. The transient near-IR band is the signature of electron-Cl atom pairs whose the complete disappearance occurs in less than 2 ps. On this figure, the long lived contribution of 1 s-like ground states of fully hydrated electrons is clearly observed in the red spectral region. 

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