Reactions of the Hydrated Electron

As has been seen, the hydrated electron is a very powerful reductant. It reacts with a wide range of both inorganic and organic compounds with rate coefficients ranging from the lowest with water of 16 1 mole sec to the diffusion controlled limit of — 10 1 mole  

Because the reaction of the hydrated electron with water is so slow, its lifetime is sufficiently long for it to be possible to observe its absorption spectrum and to study its rates of reaction with added solutes. It is interesting to calculate the lifetime under the typical conditions of pulse radiolysis. The concentration of hydrated electrons produced is 10 M. Under these conditions, bimolecular decay of hydrated electrons is negligible and at pH 9, reaction with water is the only observed process. The half life is given by the equation 

It is clear that pulses of 10 sec are required for significant observations to be made. At lower pH, the lifetime of the hydrated electron is, of course, much less because of the high rate coefficient for the reaction with hydrogen ion 

Now the hydrated electron reacts with all the other species produced except OH and Hj. These reactions are all very rapid, except that with water, as may be seen from Table 1. 

In the study of specific reactions of the hydrated electron it is therefore necessary to eliminate as many of these. other reactions as possible. Thus, by working at high pH, it is possible to convert all hydrogen atoms to hydrated electrons. 

By variation of ligand X (= CN, H, NO, NCS, OH, Ns, Cl, or I) a uniform variation of the rate of reaction with the energy of the d- d electronic transition has been observed, and it may be that the electron is reacting via an outer-sphere mechanism, the primary co-ordination shell of the aquated reductant being considered to involve four tetrahedrally disposed water molecules. In the corresponding reaction with the pentacyanocobaltate(u) ion, the transient cobalt(i) complex ion Co(CN)6 has been identified. This ion further reacts with water to yield the hydridopentacyanocobalt-ate(m) species  

In the reaction of sodium with alcohols, the electron is generated without the formation of other reactive species, thus providing a means of examining 

---

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. 

The reaction of the hydrated electron would be considered as a charge transfer reaction and elementary processes can be described as below [44]. 

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. 

Measurements of the rate of reaction of the hydrated electron with water and the reverse process have given a value for the change in Gibbs energy for the equilibrium ... 

REACTIONS OF THE HYDRATED ELECTRON WITH DILUTE ELECTROLYTES... 

Hart and An bar [17] have tabulated many rate coefficients for reactions of the hydrated electron. While many reactions are not diffusion-limited at all, of those reactions with ions, some clearly seem to be diffusion-limited. Using the Debye—Smoluchowski rate coefficient [68], eqn. (51), Hart and Anbar [17] deduced the encounter radii of reaction. 

Experimentally measured and calculated rate constants for the reactions of the hydrated electron with certain anions and cations [75]... 

Reactions of the Hydrated Electron with Substances of Biological Importance... 

Re-examination of the radiolysis of aqueous solutions of alanine (absence of oxygen) shows that electrons react rapidly with the cationic form, less rapidly with the zwitterion, and much less rapidly with the anionic form. These conclusions have been confirmed by pulse radiolysis. Rate constants for amino acids, peptides, proteins, and numerous other substances have been obtained. Critical evaluation of these and correlation with molecular properties is now well under way. In living systems the reactions of the hydrated electron vary with the part of the cell concerned, with the developmental stage of the cell, and possibly with the nature of any experimentally added substances. 

The effect of ionic form on the reaction of the hydrated electron with amino acids has been examined. The cationic form could not be examined since appreciable amounts of H + would have to be present, and with currently available techniques the electron would disappear too rapidly. But by making the solutions alkaline it has been possible to study the anionic form. For glycine (Table I), and several other amino acids and peptides (7), it has been shown that the amino acids are less reactive in the anionic form, agreeing with the conclusion drawn by Garrison. The results for glycine however cannot be interpreted on the basis of the known pK together with assumed rate constants for zwitterion and anion. Other factors are evidently present, and further work is required. 

Among other reactions of the hydrated electron may be mentioned the reaction with methylene blue. Methylene blue may be regarded as the prototype of easily reducible biological substances such as NAD, cytochrome-c, etc. The preliminary value for the rate constant for reaction with the hydrated electron (5) has now been shown to be too high, and the more reasonable value of 2.5 X 1010 M-1 sec.-1 has been obtained (19). It is thus no longer necessary to attribute special properties to methylene blue. 

Reactions of the hydrated electron possibly may be somewhat relevant to the action of dose-modifying agents such as 02, NO, C02, and sulfhydryl compounds. It can safely be assumed that these exert their influence at the radiation-chemical level, and it is notable that many of them react rapidly with hydrated electrons. Table II, taken from a paper by Braams (6), compares the rate constant for reaction with the hydrated electron with the concentration at which certain compounds have been used as protective agents. It can be seen that, at the concentrations used in biological systems, those substances which are effective as protectors can compete favorably with oxygen for hydrated electrons. Penicillamine was not a good protector at the concentration used and did not compete as favorably as the other substances for hydrated electrons. Higher concentrations of penicillamine could not be... 

Hart EJ, Gordon S, Fielden EM (1966) Reaction of the hydrated electron with water. J Phys Chem 70 150-156... 

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. 

Several reactions of aromatic compounds have been investigated for their energies of activation. These include p-bromophenol, phthalate, benzoate, henzensulphonate ions, benzyl alcohol, phenylalanine and phenyl acetate, the specific rates of which range from 3-7 x 107 to 1-2 x 1010 m—1 sec-1. The energies of activation of all these reactions were found to be the same, namely, 3-5 + 0-5 kcal mole-1 (Anbar et al., 1967). This corroborates the conclusion that the rate-determining step in e a-reactions with aromatic compounds involves one and the same process, namely, the accommodation of an electron into the aromatic substrate. The subsequent reactions discussed above may be fast or slow but are not involved in the rate-determining step of the reaction of the hydrated electron. 

Tab. 6.5 Representative examples of reactions of the hydrated electron (eaq) (Hart and Anbar, 1970)...

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... 

Thomas JK, Gordon S, Hart EJ. (1964) The rates of reaction of the hydrated electron in aqueous inorganic solutions. J Phys Chem 68 1524-1527. 

Faraggi M, BetteUieim A. (1977) The reaction of the hydrated electron with amino acids, peptides, and proteins in aqueous solutions 111. Histidyl peptides. RadiatRes7 311-324. 

The discovery in 1962 of the intense absorption band of eaq (Amax 720 nm, Cmax 1900 m mor ) [56] in pulse radiolysis experiments on aqueous solutions was made almost simultaneously at Mount Vernon Hospital [57] and Manchester [58], and provided an extremely useful method for measuring the rate constants for the reaction of this species with a variety of compounds. As mentioned in the Introduction, reactions of the hydrated electron are electron-transfer reactions. The first paper dealing with this type of measurement appeared in 1963 [59] and contained the rate constants for the reactions of Caq with H, H2O2, and O2. Many other rate constants for the reactions of Caq were determined in the following years. The NDRL/ NIST Solution Kinetics Database, Version 3, which covers the literature up to 1994, contains nearly two thousand entries for this type of reaction, almost all of them obtained by means of pulse radiolysis [7a]. In many cases, the rate constant for a given reaction has been determined more than once for example, the rate constants for the reaction of Caq with H+, NOs , C6H5NO2, Ag+, Cu +, and MV + (l,l -dimethyl-4,4 -bipyridinium) have been determined 19, 16, 14, 11, 10, and 8 times, respectively [7a]. 

The production of H2 in the radiolysis of water has been extensively re-examined in recent years [8], Previous studies had assumed that the main mechanism for H2 production was due to radical reactions of the hydrated electron and H atoms. Selected scavenger studies have shown that the precursor to the hydrated electron is also the precursor to H2. The majority of H2 production in the track of heavy ions is due to dissociative combination reactions between the precursor to the hydrated electron and the molecular water cation. Dissociative electron attachment reactions may also play some role in y-ray and fast electron radiolysis. The radiation chemical yield, G-value, of H2 is 0.45 molecule/100 eV at about 1 microsecond in the radiolysis of water with y-rays. This value may be different in the radiolysis of adsorbed water because of its dissociation at the surface, steric effects, or transport of energy through the interface. 

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. 

---