Radiation chemistry, hydrated electron

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

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. 

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. 

As would be expected, these results indicate that the thermalization distances and spatial distribution of the hydrated electron are key parameters in modelling the radiation chemistry of water. Although the stochastic approach is the more logical one to adopt, its present status does not appear to outweigh the advantages of using the simpler deterministic model to represent the essential features of water radiolysis over a wide range of conditions. 

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. 

Much of basic free-radical chemistry of DNA and its constituents have been elucidated with the help of radiation techniques. This requires one to address briefly the properties of the H atom and the hydrated electron, eaq , which are important intermediates in the radiolysis of water (Chap. 2.2). 

Extensive compilations have been made of the absolute rate coefficients for the reactions of the hydrated electron with a wide variety of substrates51. Many of these are extremely rapid reactions with rate coefficients as 1010 Af-1.sec-1, the rates being in some cases diffusion controlled. The results of such studies are important not only for radiation chemistry but for much wider areas of chemistry where the rate coefficients may lead to an understanding of the electronic structure of the scavenging molecule52. 

The review by Fendler and Fendler (1970) on the application of radiation chemistry to mechanistic studies in organic chemistry covers most of the relevant literature prior to 1970. Recently published books include The Hydrated Electron by Hart and Anbar (1970), The Radiation Chemistry of Water , by Draganic and Draganic (1971), Principles of Radiation Chemistry , by O Donnell and Sangster (1970), Introduction to Radiation Chemistry by Swallow (1973), and Einfuhrung in die Strahlenchemie by Henglein et al. (1969). Reviews can be found in the series Advances in Radiation Chemistry, edited by Burton and Magee (since 1969), Current Topics in Radiation Research, edited by Ebert and Howard (since 1965), Radiation Research Reviews, edited by Phillips and Cundall (since 1968), and in other review series more familiar to chemists. 

Following the realisation that the reactions of the hydrated electron played an important role in the radiation chemistry of liquid water it was not long before evidence was sought, and found, that the electron and the counter cation could be involved in chemical reactions in non-polar liquids before they underwent neutralisation. Scholes and Simic (1964(49)) showed that on irradiation of solutions of nitrous oxide in hydrocarbons nitrogen was formed in the dissociative attachment reaction analogous to reaction (6). Similarly, Buchanan and Williams (1966(50)) attributed the formation of HD in Y lrradiated solutions of C2H3OD in cyclohexane to the transfer of a... 

I think it is fair to say that these techniques have given tremendous information about the structure of the species, in particular the hydrated electron, that is generated by ionizing radiation, but little new information on the chemistry that evolves. 

This narrative echoes the themes addressed in our recent review on the properties of uncommon solvent anions. We do not pretend to be comprehensive or inclusive, as the literature on electron solvation is vast and rapidly expanding. This increase is cnrrently driven by ultrafast laser spectroscopy studies of electron injection and relaxation dynamics (see Chap. 2), and by gas phase studies of anion clusters by photoelectron and IR spectroscopy. Despite the great importance of the solvated/ hydrated electron for radiation chemistry (as this species is a common reducing agent in radiolysis of liquids and solids), pulse radiolysis studies of solvated electrons are becoming less frequent perhaps due to the insufficient time resolution of the method (picoseconds) as compared to state-of-the-art laser studies (time resolution to 5 fs ). The welcome exceptions are the recent spectroscopic and kinetic studies of hydrated electrons in supercriticaF and supercooled water. As the theoretical models for high-temperature hydrated electrons and the reaction mechanisms for these species are still rmder debate, we will exclude such extreme conditions from this review. 

The guanine moiety has the lowest ionization potential of any of the DNA bases or of the sugar-phosphate backbone. As a result, radiation-produced holes are stabilized as dG for hydrated DNA irradiated at 77 K There is an extensive literature describing the role of dG in the radiation chemistry of DNA as studied by pulse radiolysis, flash photolysis, and product analysis. In order to explicate the oxidative reaction sequence in irradiated DNA and to more firmly identify the relevant radical intermediates, ESR spectroscopy was employed to investigate y-irradiated hydrated DNA (T = 12 2). Some experiments were also performed on hydrated (T = 12 2) DNA in which an electron scavenger [thallium(ni) (TP )] was employed to isolate the oxidative path. Oxygen-17 isotopically enriched water was also used to confirm a proposed water addition step to G and the subsequent transformations that follow These experiments were run in oxygen-free samples under conditions for which indirect effects were unimportant. 

See reviews a) Edwards P.R, The electronic properties of metal solutions in liquid ammonia and related solvents, Adv. Inorg. Chem. Radiochem., i982,25, 135-185. b) Boag J.W., Pulse radiolysis a historical account of the discovery of the optical absorption spectrum of the hydrated electron, in Early developments in radiation chemistry", KrohJ. (Ed.), Royal Society ofChemistry, Cambridge, 1989, 7-20. 

A wealth of information on the reduction of metal ions in aqueous solutions has been obtained and a compilation was published in 1988 [20], However, alkali or alkaline earth metal ions such as Li Na or cannot be reduced by the hydrated electron in aqueous solution but can form an ion pair with the solvated electron in polar liquids. Among the various reactions of the solvated electron, the reduction of halogenated hydrocarbons is often used in radiation chemistry to produce well-defined radicals because of the selective cleavage of the carbon-halogen bond by the attack ofthe solvated electron. This reaction produces the halide ion and a carbon-centered radical, and is of great interest for environmental problems related to the destruction of halogenated organic contaminants in water and soil [21,22]. 

After the discovery of hydrated electrons extensive experimental data in radiation chemistry have been accumulated, which show that the solvated electron acts as a universal primary reducing agent in the processes that take place in the liquid bulk under the action of ionizing radiation. This provoked interest among researchers in the role of solvated electrons in other physico-chemical processes also, in particular in electrochemical processes. 

More recently, assisted by photochemistry, spectroscopy in its varied forms, chromatography, computers, and applied electronics, radiation chemistry is assaulting many of the problems associated with the properties of transient species at an unprecedented rate. Commonplace, already, is the study of intermediates lasting only milli- and micro-seconds. A rapidly developing subdivision is on the horizon—that of nanosecond and picosecond chemistry. Knowledge of the nature and rates of these reactions has been of inestimable aid in untangling reaction mechanisms in chemistry and biology. For example, the discovery of the hydrated electron and the determination of its rate constants has aided the interpretation of reactions in aqueous media. Recent studies on solvated and... 

The production of the hydrated electron by the interaction of ionizing radiation with water was one of the outstanding discoveries in chemistry in the 1960s. It thus became apparent that reactions of this species, produced as a result of cosmic bombardment of the earth s surface, must have been occurring from primeval times. It is, however, only relatively recently that it has been possible to study reactions of the hydrated electron in the laboratory. 

Following the discovery of the hydrated electron in radiation chemistry, the reexamination of some fields of aqueous chemistry gave rise to a new concept of primary reduction processes. This paper surveys aspects of these investigations in which it appears that e aq, as opposed to its conjugate acid (H atom), is invariably the precursor to H2 when water is reduced. Evidence is reviewed for the production of e aq (a) photochemically, (b) by chemical reduction of water, (c) electrolytically, (d) by photo-induced electron emission from metals, (e) from stable solvated electrons, and (f) from H atoms. The basis of standard electrode potentials and various aspects of hydrated electron chemistry are discussed briefly. 

A review, correlation, and discussion of hydrated electron chemistry - is justified in its inclusion in a radiation chemistry conference because of the impact upon aqueous chemistry that has resulted directly from the discovery of hydrated electrons. 

It would be superfluous to review here the story of e aq in the radiation chemistry of aqueous solutions. High energy radiations cause ionizations and the free electrons so generated dissipate their excess energy and are eventually trapped in solvation shells. The discovery of hydrated electrons showed that electrons in water were chemical entities (as distinct from possessing purely physical characteristics) in having diffusion properties, size and sphere of influence, associated ion atmosphere, and reaction rate parameters all of which are comparable to normal chemical reagents. 

Whereas much of the underlying mechanisms for the effects of radiation on materials were outlined using steady state radiation sources, the advent of pulse radiolysis on the heels of flash photolysis opened a window into direct observation of the intermediates. One of the early discoveries utilizing pulse radiolysis was the spectrophotometric detection of the hydrated electron by Boag and Hart (35,36). Since then thousands of rate constants, absorption spectra, one-electron redox potentials and radical yields have been collected using the pulse radiolysis technique. The Radiation Chemistry Data Center at the University of Notre Dame accumulates this information and posts it (at www.rcdc.nd.edu/) for the scientific community to use. They cover the reactions of the primary radicals of water and many organic radicals and inorganic intermediates. 

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