Rate constants of hydrated electron

Hart, E.J., Sheffield, G., and Thomas, J.K., Rate constants of hydrated electron reactions with organic compounds, ]. Phys. Chem., 68(6), 1271-1274, 1964. 

Michael BD, Hart EJ. The rate constants of hydrated electron, hydrogen atom, and hydroxyl radical reactions with benzene, 1,3-cyclohexadiene, 1,4-cyclo-hexadiene, and cyclohexene. J Phys Chem 1970 74 2878-2884. 

Braams R. (1966) Rate constants of hydrated electron reactions with amino acids. Radiat Res 27 319-329. 

Buxton GV, Greenstock CL, Helman WP et al (1988) Critical review of rate constants of hydrated electrons, hydrogen atoms and hydroxyl radicals ( OH/ 0 ) in aqueous solutiorr J Phys Chem Ref Data 17 513-886... 

According to the Marcus theory [64] for outer-sphere reactions, there is good correlation between the heterogeneous (electrode) and homogeneous (solution) rate constants. This is the theoretical basis for the proposed use of hydrated-electron rate constants (ke) as a criterion for the reactivity of an electrolyte component towards lithium or any electrode at lithium potential. Table 1 shows rate-constant values for selected materials that are relevant to SE1 formation and to lithium batteries. Although many important materials are missing (such as PC, EC, diethyl carbonate (DEC), LiPF6, etc.), much can be learned from a careful study of this table (and its sources). 

Table 1. A compilation of specific bimolecular rate constants (/tc) for the reaction of hydrated electrons with Li battery related materials (61,62]...

Meissner and coworkers36 studied the pulse radiolysis of aqueous solutions of dimethyl sulfoxide. It was found that hydrated electrons react with DMSO with a rate constant of... 

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. 

Fig. 8-17. Electron state density in a semiconductor electrode and in hydrated redox particles, rate constant of electron tunneling, and exchange redox current in equilibrium with a redox electron transfer reaction for which the Fermi level is dose to the valence band edge.

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. 

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. 

Figure 1 Arrhenius plot of the rate constant of the hydrated electron with silver cation Ag. (From Ref. 50.)...

It is known that more than 30 reactions are needed to reproduce the radiation-induced reactions occurring in pure water. Intensive measurements with a pulse radiolysis method have been done at elevated temperature up to 300°C [25 2], and the temperature dependence of some reactions does not exhibit a straight line but a curved one in Arrhenius plot. These examples are the reactions of the hydrated electron with N2O, NOJ, NO2, phenol, Se04, 8203 , and Mn [33,35], and two examples, egq + NOJ and ejq -i- NOJ, are shown in Fig. 2. The rate constant for the reaction of hydrated electron with NOJ is near diffusion-controlled reaction at room temperature and is increasing with increasing temperature. Above 100°C, the rate does not increase and reaches the maximum at 150°C, and then decreases. Therefore the curve is concave upward in Arrhenius plot. 

Buxton, G. V., C. L. Greenstock, W. P. Helman, and A. B. Ross, Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms, and Hydroxyl Radicals (-OH/-LO ) in Aqueous Solution, J. Phys. Chem. Ref. Data, 17, 513-886 (1988). 

Oxidative reactions of 2AP(-H) have been studied in oxygen-saturated solutions because 2AP(-H) radicals do not exhibit observable reactivities towards O2 [10]. In contrast, O2 rapidly reacts with hydrated electrons (rate constant of 1.9x10 ° s ) [51] and hydrated electrons do not interfere in... 

Buxton GV, Greenstock C L, Helman WP, Ross A B (1988) Critical Review of Rate Constants for Oxidation of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (0H°/0° ) in Aqueous Solutions, Journal of Physical and Chemical Reference Data 17 513-886. 

There are also certain data on electron tunneling in electron transfer reactions in liquids. The ideas about electron tunneling have been used by Anbar and Hart [75] to interpret the anomalously large rate constants for the diffusion controlled reactions of hydrated electrons with some inorganic anions in aqueous solution. Table 5 represents the data on the largest values of the rate constants, ke, observed for the reactions of eaq with various inorganic anions and cations. Theoretical diffusion rate constants, kA, for... 

The rate constants of reactions of hydrated electrons with some accep-tors-anions substantially exceed the diffusion rate constants calculated with the help of the Debye equation [Chap. 2, eqn. (45)l(see Chap. 2, Sect. 4). This excess is usually attributed to the capture of electrons by acceptors via tunneling at distances exceeding the sum of the reagents [28,89,111,1201- In this case, the tunneling distance can be estimated from experimental rate constants for reactions of eaq with acceptors [109] by means of the expression... 

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 absolute rate constants were determined for a variety of reactions of the solvated electron in ethanol and methanol. Three categories of reaction were investigated (a) ion-electron combination, (b) electron attachment, and (c) dissociative electron attachment. These bimolecular rate constants (3, 27, 28) are listed in Table III. The rate constants of four of these reactions have also been obtained for the hydrated electron in water. These are also listed in the table so that a comparison may be made for the four rate constants in the solvents ethanol, methanol, and water. 

It now seems reasonably definite that an entity such as the hydrated electron exists. Further, the rate constants of reaction of e aq with a large number of species have now been measured using the technique of pulse radiolysis. This paper describes some of the properties of e aq and discusses the rate constants of reaction of e aq with the other species produced in the pulse radiolysis of water. These rate constants are significant for any diffusion theory model of the radiolysis of water. 

The rate constants for 8 and 9 were determined by pulse radiolysis by adding known amounts of excess acid or H20-2 and measuring the pseudo-first order decay of hydrated electron absorption (15, 25). The rate constant for recombination of OH radicals (Reactions 19) was deter-... 

Finally, the development of pulse radiolysis enabled a direct observation of e aq, and a direct distinction between e aq and H could easily be made. Matheson (37) (with spectroscopic data obtained by Keene) suggested that e ag has optical absorption in the visible. Hart and Boag (26) used spectrographic plates and studied this absorption. The effect of solutes, which were known as electron scavengers led to the conclusion that the absorption was due to e aq. It was confirmed later, that the absorption belonged to unit negatively charged species by means of a salt effect (20), as well as by conductivity measurements (49). Many more papers on the absorption spectrum and rate constants of the hydrated electron have since appeared (16). 

Our observation of the hydrated electron band at a 5 /xsec. delay cannot be attributed to the thermal reaction H + OH - e aq + H20, because the rate constant of 1.8 X 107Af 1 sec. 1 (21) permits only a negligible conversion of H atoms below pH 10. Therefore, the cases indicated as (+) and (+ + ) are taken as definite proof of photoionization. The cases indicated as (f) are less certain, although a photographic density difference of proper lifetime was measured densitometrically because the weak absorptions made the delineation from other transients, such as short-lived triplets, less certain. The absence of the hydrated electron... 

As well as for substances of exclusively biological importance, it is necessary, for a full understanding, to know the reactivity of hydrated electrons with substances such as H+, 02, CO, water, etc. which are equally present in biological systems. Many of these rate constants are known, but unfortunately the rate constant for reaction with water itself is still not entirely certain, although it has been investigated recently by two independent groups (4, 14). 

Buxton, G.V., Greenstock, C.L., Helman, W.P., and Ross, A.B., Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solution, /. Phys. Chem. Ref. Data, 17(2), 513-886, 1988. 

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