Hydrated electron kinetics

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 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. 

Figure 5 Decay kinetics of the hydrated electron. Experimental data ( ) [32] (----------[73].

Although its precise structure has not yet been settled, the hydrated electron may be visualized as an excess electron surrounded by a small number of oriented water molecules and behaving in some ways like a singly charged anion of about the same size as the iodide ion. Its intense absorption band in the visible region of the spectrum makes it a simple matter to measure its reaction rate constants using pulse radiolysis combined with kinetic spectrophotometry. Rate constants for several hundred different reactions have been obtained in this way, making kinetically one of the most studied chemical entities. 

I am net able to offer an entirely convincing explanation here. Wre are tempted to propose the hydrated electron suggested by Dorfman to account for the lack of kinetic isotope effect. 

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). 

In order to study the influence of electron concentration on the observed dynamics, we performed experiments with different laser power densities. As an illustration, the transient absorption signals recorded at 715 nm in ethylene glycol upon photoionisation of the solvent at 263 nm with three different laser power densities are presented in Fig.3. As expected for a two-photon ionization process, the signal intensity increases roughly with the square of the power density. However, the recorded decay kinetics does not depend on the 263 nm laser power density since the normalised transient signals are identical (Cf. Fig.3 inset). That result indicates that the same phenomena occur whatever the power density and consequently that the solvation dynamics are independent of the electron concentration in our experimental conditions i.e. we are still within the independent pair approximation as opposed to our previous work on hydrated electron [8]. 

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 solvated electron has been studied in a number of organic liquids, among which are the aliphatic alcohols (27, 28, 3, 2d, 2, 27), some ethers (25, 5), and certain amines (9, 22, 2). Of these systems, it is only in the alcohols, to which this paper is principally but not exclusively directed, that both the chemical reactivity and the optical absorption spectrum of the solvated electron have been investigated in detail. The method used in these studies is that of pulse radiolysis (22, 22), developed some five years ago. The way was shown for such investigations of the solvated electron by the observation of the absorption spectrum of the hydrated electron (6, 28, 19) and by the subsequent kinetic studies (2d, 22, 20) which are being discussed in other papers in this symposium. 

Thus, in THF, the band at 9000 A. is attributed to the trapped electron pair. The observations in water (12), relating to the electron-electron reaction, are of interest in this connection. Although our isotopic studies as well as the kinetics, support the occurrence of Reaction 4 in water, we may reach the following alternative conclusions regarding the species (eiff)2 (a) that if it is formed in water by Reaction 4, its lifetime is less than 0.2 /xsec., or (b) if this is not true, then the species does not absorb in the same region of the visible as does 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. 

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. 

Water, H20 + and Bronsted Acids. The most important reagent in the chemistry of e aq is obviously the solvent, water. Were it not for the relatively low reactivity of elq with H20 most of our information on hydrated electrons would be merely hypothetical. Fortunately the rate of the eaq + H20 - H + OH - reaction is slow enough to enable one to examine the kinetic behavior of any solute reacting with e aq at a rate over 106 Af-1 sec.-1... 

Pimblott SM, LaVerne JA (1998) On the radiation chemical kinetics of the precursor to the hydrated electron. J Phys Chem A 102 2967-2975... 

Hickel B, Sehested K (1985) Activation energy for the reaction H + OH" —> eaq. Kinetic determination of the enthalpy and entropy of solvation of the hydrated electron. J Phys Chem 89 5271-5274 Hoffman MZ, Hayon E (1973) Pulse radiolysis study of sulfhydryl compounds in aqueous solution. J Phys Chem 77 990-996... 

Visscher KJ, de Haas MP, Loman H, Vojnovic B, Warman JM (1987) Fast protonation of adenosine and of its radical anion formed by hydrated electron attack a nanosecond optical and dc-conduc-tivity pulse radiolysis study. Int J Radiat Biol 52 745-753 Visscher KJ, Spoelder HJW, Loman H, Hummel A, Horn ML (1988) Kinetics and mechanism of electron transfer between purines and pyrimidines, their dinucleotides and polynucleotides after reaction with hydrated electrons a pulse radiolysis study. Int J Radiat Biol 54 787-802 von Sonntag C (1980) Free radical reactions of carbohydrates as studied by radiation techniques. Adv Carbohydr Chem Biochem 37 7-77... 

The actual solubilization limit depends on the temperature, the nature of surfactant, the concentration of water, and on the nature of the acid. Irrespective of size or the specific properties of the solubilized molecules, very little is known about the thermodynamics or the kinetics of the solubilization process. The association of the solute with the interface can be checked using techniques capable of yielding detailed microscopic information at the molecular level (e.g. NMR, ESR, fluorescence, hydrated electrons). 

The discovery of hydrated electrons in the radiolysis of water is undoubtedly one of the outstanding events in chemistry in this decade. Hydrated electrons have been found to react with many organic compounds in aqueous solution, and the kinetics of the reactions have been measured. From these kinetic studies, as well as from the detection of intermediates and the identification of final products, sufficient information has accumulated to allow a comprehensive discussion of the mechanisms of these reactions. 

Hydrated-electron reactions are, by definition, electron-transfer processes, which are not very common in classical organic chemistry. The kinetic studies have shown, however, that the eleotron behaves analogously to a classical nucleophilic reagent and, although this analogy... 

The experimental methods used in the investigation of the hydrated electron include competition kinetics and product analysis, as well as pulse-radiolysis and flash-photolysis techniques. All these methods have... 

There has been a tendency among radiation chemists to use the information on the kinetic and stoicheiometric behaviour of dilute aqueous solutions containing biochemical solutes for interpreting mechanisms of molecular radiobiology. Such a comparison may possibly be justified for the reactions of OH radicals and H atoms. The analogous treatment of hydrated-electron reactions seems, however, to be a gross oversimplification that might easily result in erroneous conclusions. 

Chromium(n). The applications of chromium(n) salts in preparative organic chemistry have been reviewed.69 The kinetics of oxidation of Cr11 to Crm by halogen radical anions, a particularly simple one-electron oxidation scheme, have been determined.70 Hydrated electrons are formed71 during the photochemical oxidation of aqueous chromium(n). 

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. 

Fig. 17. Kinetic profile of hydrated electron at 700 nm recorded with the streak camera after subtraction of the Cerenkov light (average of 400 pulses).

Pin S, Hickel B, Alpert B, Ferradini C. (1989) Parameters controlling the kinetics of ferric and ferrous hemeproteins reduction by hydrated electrons. Biochim Biophys Acta 994 47-51. 

Fig. 5. Absorbance kinetics of the hydrated electron after a 1-ns pulse of of 1 GeV in water [result published in Ref 34].

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