Hydrated electron production

Table III. Correlation of Optical Hydrated Electron Production with Gat-Phase...

On the experimental side, evidence was accumulating that there is more than one kind of reducing species, based on the anomalies of rate constant ratios and yields of products (Hayon and Weiss, 1958 Baxendale and Hughes, 1958 Barr and Allen, 1959). The second reducing species, because of its uncertain nature, was sometimes denoted by H. The definite chemical identification of H with the hydrated electron was made by Czapski and Schwarz (1962) in an experiment concerning the kinetic salt effect on reaction rates. They considered four... 

The intensity of absorption gives the product G , where G is the observed yield and is the molar extinction coefficient. The absolute value of was determined by Fielden and Hart (1967) using an H2-saturated alkaline solution and an alkaline permanganate-formate solution, where all radicals are converted into Mn042. They thus obtained = 1.09 x 104 M- cm1 at 578 nm, which is almost identical with that obtained by Rabani et al. (1965), who converted the hydrated electron into the nitroform anion in a neutral solution of tetrani-tromethane. From the shape of the absorption spectrum and the absolute value of at 578 nm, one can then find the absolute extinction coefficient at all wavelengths. In particular, at the peak of absorption, (720)/ (578) = 1.7 gives at 720 nm as 1.85 X 104 M 1cm 1. 

The hydrated electron reacts with H202 with a diffusion-controlled rate (see Table 6.6), giving OH and OH-. An intermediate product of this reaction, H202, may be responsible for prolonged conductivity in pulse-irradiated water. The rate of this reaction is consistent with rates of similar one-electron reduction reactions of H202. 

The gas-phase lifetime of N20- is 10-3 s in alkaline solutions, it is still >10-8 s. Under suitable conditions, N20- may react with solutes, including N20. The hydrated electron reacts very quickly with NO (see Table 6.6). The rate is about three times that of diffusion control, suggesting some faster process such as tunneling. NO has an electron affinity in the gas phase enhanced upon solvation. The free energy change of the reaction NO + eh (NO-)aq is estimated to be --50 Kcal/mole. Both N02- and N03- react with eh at a nearly diffusion-controlled rate. The intermediate product in the first reaction, N02-, generates NO and... 

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. 

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. 

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. 

The hydrated electron is one of the main water decomposition products, and it is known that the absorption band of the hydrated electron shifts to longer wavelength with... 

The transient absorption spectra of duplexes with [2AP]A4GGAs are depicted in Fig. 5. At a delay time of 100 ns, the transient absorption spectrum is attributed to the superposition of the spectra of the 2AP(-H) and G /G (-H) radical products and the hydrated electrons. The structureless tail of the eh absorption in the 350-600 nm region decays completely within At<500 ns. The formation of G VG(-H) radicals monitored by the rise of the 310-nm absorption band and associated with the decay of the 2AP V 2AP(-H) transient absorption bands at 365 and 510 nm (Fig. 5) occurs in at least three well-separated time domains (Fig. 6). The prompt (<100 ns) rise of the transient absorption at 312 nm due to guanine oxidation by 2AP was not resolved in our experiments. However, the ampHtude, A((=ioo), related to the prompt formation of the G /G(-H) radicals (Fig. 6a) can be estimated using the extinction coefficients of the radical species at 312 and 330 nm (isosbestic point) [11]. The kinetics of the G VG(-H) formation in the yits and ms time intervals were time-resolved and characterized by two well-defined components shown in Fig. 6a (0.5 /zs) and Fig. 6b (60 /zs). 

The primary products of the pulse radiolysis 2. sis. si ) of jjy pulses of high-energy electrons (1-15 MeV) are hydrated electrons e and the free radicals OH besides smaller amounts of H, H2O2, H2, and H3O. ... 

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. 

In the century since its discovery, much has been learned about the physical and chemical properties of the ammoniated electron and of solvated electrons in general. Although research on the structure of reaction products is well advanced, much of the work on chemical reactivity and kinetics is only qualitative in nature. Quite the opposite is true of research on the hydrated electron. Relatively little is known about the structure of products, but by utilizing the spectrum of the hydrated electron, the reaction rate constants of several hundred reactions are now known. This conference has been organized and arranged in order to combine the superior knowledge of the physical properties and chemical reactions of solvated electrons with the extensive research on chemical kinetics of the hydrated electron. 

The hydrated electron, if the major reducing species in water. A number of its properties are important either in understanding or measuring its kinetic behavior in radiolysis. Such properties are the molar extinction coefficient, the charge, the equilibrium constant for interconversion with H atoms, the hydration energy, the redox potential, the reaction radius, and the diffusion constant. Measured or estimated values for these quantities can be found in the literature. The rate constants for the reaction of Bag with other products of water radiolysis are in many cases diffusion controlled. These rate constants for reactions between the transient species in aqueous radiolysis are essential for testing the "diffusion from spurs" model of aqueous radiation chemistry. 

The following review will summarize and systematize the available knowledge on the chemical reactivity of solvated electrons and the products of their reactions. Since most of the work was carried out with solvated electrons in aqueous solutions, we shall confine ourselves mainly to hydrated electrons. We do not intend to discuss the interaction of solvated electrons with their solvents since this will be covered in other chapters. 

Hydrated electrons are also formed as a product of the interaction of hydroxide ions with hydrogen atoms. This reaction was first established kinetically (4, 6, 81, 82, 99) and then corroborated spectro-photometrically using flash radiolysis (95, 96). It should be noted that the rate of the H + OH- - e aq reaction is only 1.8 X 107 M l sec.-1 (66) thus, this step may become rate determining in many reactions with reactive substrates. 

The Reactivity of Different Chemical Species toward Hydrated Electrons and the Products of these Reactions... 

A very fast second-order disappearance of hydrated electrons with the production of molecular hydrogen has been observed in water using pulse radiolysis (9). In our system this reaction would be expected to cause large deviations from first-order behavior at high water concentrations. The absence of such deviation shows that this reaction depends strongly upon the solvent structure and not merely upon the concentration of water molecules. 

Table I. Optical Production of Hydrated Electron from Aromatic Compounds in Aqueous Solution...

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