The Hydrated Electron

The hydrated electron has an extensive chemistry, and it is clear that e q is a thermodynamic entity. Its redox potential is defined by the following cell  

The potential of this couple was first estimated as —2.7 V. Baxendale derived this value by using the following thermochemical cycle (34)  

The equilibrium constant for reaction (2), 3.9 x 10 5 M, was obtained from the rates of the forward and reverse reactions. (Note that in many discussions the concentration of water is included in the equilibrium constant.) Thus AfG° for e q is 55 kJ/mol greater than for H. Reaction (3) was estimated to have AG° = 0. The other data are available in standard 

Another approach to estimating E° for the hydrated electron is to use the equilibrium 

Initial reports of a second-order decay of eaq that leads to the electron dimer, (e q)2, have been disputed, but there is still evidence that e q forms some other precursor to H2 (313). The proposal for (eaq)2 has recently been revived (85). 

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. 

In this review, it will be convenient first to discuss results that have been obtained on the hydrated electron and subsequently to discuss other systems in which reactions of solvated electrons have been observed. A wealth of information concerning the hydrated electron has now been amassed and this has led to a good understanding of many of its properties. Our knowledge and understanding of the solvated electron in other systems, however, is much less complete. 

Besides providing invaluable data for checking present theories of the mechanisms of electron-transfer reactions, e is interesting in reacting with an extremely wide range of species. The reactions all follow the same basic scheme 

This has been rationalized in terms of the fact that the dissociation energy of a P-O bond in phosphate is considerably higher than that of an O-H bond.  

Perhaps the one outstanding result of the direct kinetic investigations of e, (as well as the earlier competition kinetics based on product analysis) has been that the electron as a hydrated species seems to behave more or less as expected. 

A feature of shock waves not yet considered is that there is inevitably a low pressure or rarefaction wave produced at the diaphragm at the same time as the shock wave. This moves initially in the opposite direction from the shock wave but is reflected by the back wall of the tube, and so eventually follows the main shock wave down the tube. Relative to laboratory coordinates this rarefaction wave travels with the local velocity of sound in the gas. This is considerably less than that of the shock wave because of the substantially lower temperature, but superimposed on it is the flow motion of the driver gas towards the low-pressure region. This has the result that the rarefaction wave tends to catch up with the shock wave. Because of the simplifications it allows, it is convenient to make the measurements on the shocked gas before the rarefaction arrives. This consideration is an important one in deciding on the relative positions of the diaphragm and observation points, and on the relative lengths of the high- and low-pressure areas . For a reason considered below, measurements are also sometimes made after the shock wave has been reflected from the front wall, but before the rarefaction wave has arrived. Such a situation is only used where absolutely necessary because it is now felt that the shock front is significantly distorted on reflection. 

In addition to the shock wave velocity it is necessary, in a system where the density does not uniquely determine the composition (and this includes all but the very simplest chemical systems of interest), to measure some concentration function in order to follow the reaction. This is one of the greatest experimental difficulties associated with the method, since the changes occur so rapidly. Where possible, the concentration change is followed spectrophotometrically. This concentration monitoring is the second function of the observation points in Fig. 3. Especially for species with line spectra, the small optical density change, coupled with the fast response-time necessary, excludes the use of a conventional spectrophotometer. An example of a detection system which has been used for the hydrogen/oxygen 

---

Fessenden R W and Verma N C 1976 Time resolved electron spin resonance spectroscopy. III. Electron spin resonance emission from the hydrated electron. Possible evidence for reaction to the triplet state J. Am. Chem. Soc. 98 243-4... 

Rossky, P.J., Schnitker, J. The hydrated electron quantum simulation of structure, spectroscopy and dynamics. J. Phys. Chem. 92 (1988) 4277-4285. 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

The rate parameters for the reactions of e (aq) with substrates are generally determined by monitoring the disappearance of the hydrated electron at 600-700 nm. The first order rate parameters are generally determined over a range of substrate concentrations and the second order rate parameter calculated from the resulting linear relation. The data available for such studies with Pu ions are presented in Table IV. 

The former investigation was motivated, in part by the fact that in a previous study (7) there had been a marked difference on the rates of reactions of e (aq) and U(VI) between homogeneous solutions and those containing micellar material. When the rate of disappearance of the hydrated electron is measured over a range of concentrations from 2 x 10-5 M to 8 x 10-lt M at pH = 9.7 in solutions formally 0.003 M Si02, the calculated second order rate parameter is 1.4 x 109 M-1s-1. This is a marked decrease from any of the previous measurements and emphasizes the point that the prediction of Pu chemistry in a natural water system must take cognizence of factors that are not usually deemed significant. 

Knowledge of the value of ij (abs) makes it possible to convert all relative values of electrode potential to an absolute scale. For instance, the standard electrode potentials of the oxygen electrode, the zero charge of mercury, and the hydrated electron, in the absolute scale are equal to -5.67,. 25, and 1.57 V, recpectively. ... 

Hart, E. J. Anbar, M. (1970). The Hydrated Electron. New York Wiley. 

In the case of UV-light, the reactions are (CHjljCO hv (CHjl CO (CHjl CO + (CHjljCHOH 2 (CHjljCOH. In the case of y-rays, hydrated electrons, H-atoms, and OH radicals are formed by decomposition of the solvent. The hydrated electrons react with acetone aq + (CHj)jCO -E -> (CHjljCOH, and the H and OH radieals with propanol-2 H(OH) + (CHjljCHOH H fHjO) -E (OHjl COH,... 

In these solutions, the hydrated electrons from the radiolysis of water produce CO by their reaction with COj, and the OH radicals attack the alcohol to form (CHjIjCOH radicals (see also footnote on page 117). 

A surprising observation was made in the first experiments on the flash photolysis of CdS and CdS/ZnS co-colloids Immediately after the flash from, a frequency doubled ruby laser (X = 347.2 nm photon energy, = 3.57 eV) the absorption spectrum of the hydrated electron was recorded. This species disappeared within 5 to 10 microseconds. More recent studies showed that the quantum yield increased... 

The hydrated electrons then react according to e + Cd Cd, and the Cd ions which have a strong absorption at 300 nm react with the colloidal particles after the pulse. It was observed that the same bleaching took place during this reaction as in the reaction of e with CkiS particles, and it was concluded from this result that Cd" transfers an electron to a CdS particle Cd" + (CdS), - Cd + (CdS) . These observations also are of interest for our understanding of the formation of Cd atoms in the photocathodic dissolution of CdS (see Sect. 3.4). Cd" cannot be the intermediate of the overall reaction 2e + Cd - Cd° as already pointed out in discussing the mechanism of Eqs. (35) and (36)... 

Debierne (1914) was the first to suggest a radical reaction theory for water radiolysis (H and OH). In various forms, the idea has been regenerated by Risse (1929), Weiss (1944), Burton (1947, 1950), Allen (1948), and others. Platzman (1953), however, criticized the radical model on theoretical grounds and proposed the formation of the hydrated electron. Stein (1952a, b) meanwhile had suggested that both electrons and H atoms may coexist in radiolyzed water and proposed a model in which the electron digs its own hole. Later, Weiss (1953, 1960) also favored electron hydration with ideas similar to those of Stein and Platzman. In some respects, the theoretical basis of these ideas is attributable to the polaron (Landau, 1933 Platzman and... 

It takes 10-u s, the normal dielectric relaxation time for water, to form the hydrated electron, and -10-9 s for the electron to disappear by reacting with the water molecule (the former is an overestimate, the latter an underestimate). 

The heat of solution of the hydrated electron, by analogy with ammonia, would be about 2 eV. 

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... 

FIGURE 6.2 Absorption spectrum of the hydrated electron. The spectrum is structureless, broad (half-width 0.84 eV), intense (oscillator strength 0.75), and has a single peak at 1.725 eV. (See text for details.)... 

First, we will review the stationary primary yield of the hydrated electron at neutral pH for low-LET radiation at a small dose. The primary species are eh, H30+, H, OH, H2, and H2Or Material balance gives... 

The pulse radiolysis technique gives a direct way for measuring the hydrated electron yield. To get the stationary yield, one can simply follow the electron absorption signal as a function of time and, from the known value of the extinction coefficient (Table 6.2), evaluate g(eh). Alternatively, the electron can be converted into a stable anion with a known extinction coefficient. An example of such an ion is the nitroform anion produced by reaction of eh with tetrani-tromethane (TNM) in aqueous solution ... 

TABLE 6.2 Summary of Physical Data for the Hydrated Electron at 23° ... 

The reaction H + OH— eh is undoubtedly responsible for the increase of G(eh) at high pH. Similarly, the reaction eh + H+—H must be responsible for the reduction of the hydrated electron yield in acid solution. The increase of total reducing yield and water decomposition yield at pH = 1.3 is not clearly understood, but it may also be due to secondary reactions. 

The first experimental measurements of the time dependence of the hydrated electron yield were due to Wolff et al. (1973) and Hunt et al. (1973). They used the stroboscopic pulse radiolysis (SPR) technique, which allowed them to interpret the yield during the interval (30-350 ps) between fine structures of the microwave pulse envelope (1-10 ns). These observations were quickly supported by the work of Jonah et al. (1973), who used the subharmonic pre-buncher technique to generate very short pulses of 50-ps duration. Allowing... 

FIGURE 6.3 Decay of the hydrated electron yield with time compiled from various experiments. There is relatively little decay from 30 ps to -1 ns and a fast decay from 1 to 10 ns. These results were found difficult to reconcile with diffusion theory. The error bars indicate experimental uncertainties. 

The absorption spectrum of ehis intense, has a broad peak at about 720 nm (halfwidth -1 eV), and is structureless (see Sect. 6.1 and Figure 6.2). It covers at least 220 to 1000 nm and possibly extends on either side. There is some evidence that the absorption rises somewhat in the UV, which has been interpreted as the water absorption perturbed by the hydrated electron (Nielsen et al, 1969,1976). 

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. 

For the hydrated electron, = 1.85 x 104 M cmr1. Taking the half-width as... 

Table 6.2 lists some of the physical data for the hydrated electron. Most of these data are experimental. The molar volume is calculated, as experimental measurements are not reliable. The oscillator strength and the natural lifetime against reaction with water molecules are lower bounds, whereas the salvation time is possibly an upper bound. 

---