Electron scavenger hydrated

FIGURE 7.6 Effect of hydrated electron scavenger on the scavenging yield. See text for explanation. Reproduced from Pimblott and Green,1995, with permission of Elsevier . 

The conduction band electron of liquid water, which cotild be produced by photoelectron emission fi m metal electrodes, is very unstable with its lifetime being 10 seconds it is readily captured by water molecules to form a hydrated electron. The hydrated electron is also very unstable being rapidly absorbed in electron scavenger particles such as H3O, NO 2 and O2. The level of the hydrated electron has been estimated at 0.3 to 0.5 eV below the conduction band edge [Battisi-Trasatti, 1977 Watanabe-Gerischer, 1981]. 

Coyle et al. [94] and Dainton and Logan [113] have studied the reactions of hydrated electrons in aqueous solution of two electron scavengers, one of which was usually N20. Since the rate coefficient for the reaction... 

Encounter radii for reactions of the solvated and hydrated electrons with various electron scavengers, corrected for electrolyte screening... 

A similar "shuttle mechanism of electron scavenging was also suggested [81 for the reaction of a hydrated electron, eaq, with p-nitrobenzoatopen-taamine Co(III),... 

Absorption Spectrum of e aq. The absorption spectrum of the hydrated electron is shown in Figure 1. The evidence that this spectrum is that of eaq is at least four-fold. First, the spectrum is suppressed by known electron scavengers, such as H30+, 02, N20 (4, 18). Second, it resembles in form the absorption bands of the solvated electron in liquid ammonia and methylamine (4, 18). Third, the rate constants calculated from the decay of this absorption in the presence of scavengers... 

The behavior of perchlorate would at first appear to be anomalous. Thus although in this case, H-atoms are expected to be formed and stabilized in the same way as in S04-2, HP04 2, P04 3, and C03 2, yet it is found (Table II) that the H atom yield is decreased in the presence of electron scavengers. To explain this behavior Kevan, Moorthy, and Weiss, (32) suggested that here the H atoms must be formed in the hydration shell of the anion by reaction of the electron with H +, the concentration of which in the hydration shell could possibly be higher than in the bulk. An alternative mechanism has been recently proposed by Kevan (31), according to whom the electron reacts with the anion to give an... 

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). 

The water radical cation, produced in reaction (1), is a very strong acid and immediately loses a proton to neighboring water molecules thereby forming OH [reaction (3)]. The electron becomes hydrated by water [reaction (4), for the scavenging of presolvated (Laenen et al. 2000) electrons see, e.g., Pimblott and LaVerne (1998) Pastina et al. (1999) Ballarini et al. (2000) for typical reactions of eaq, see Chap. 4], Electronically excited water can decompose into -OH and 11- [reaction (5)]. As a consequence, three kinds of free radicals are formed side by side in the spurs, OH, eaq , and H . To match the charge of the electrons, an equivalent amount of ED are also present. 

Jonah CD, Miller JR, Matheson MS. (1977) The reaction of the preorrsor of the hydrated electron with electron scavengers. J Phys Chem 81 1618-1622. 

Chitose N, Katsmnura Y, Domae M, Zuo Z, Murakami T. (1999) Radiolysis of aqueous solutions with pulsed helium ion beams — 2. Yield of S04" formed by scavenging hydrated electron as a function of S2082-concentration. Rjid Phys Chem 54(4) 385-391. 

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. 

When solutions of CdS colloids containing no additional electron and hole acceptor in the solution, are exposed to a high intensity laser flash, a rather large absorption of an intermediate is observed around 700 nm, similarto that described for the laser excitation of Ti02 in the previous section. The absorption spectrum of the intermediate is given in Fig. 9.17 [52]. It is not due to trapped electrons and holes but it is identical with to the well-known spectrum of hydrated electrons as proved by radiolysis experiments [52]. The half-life of the hydrated electrons is a few microseconds. In the presence of typical hydrated electron scavengers, such as oxygen, acetone or cadmium ions, the decay of the intermediate became much faster. 

The absorption spectrum is broad with a maximum at 720 nm. Solutions of the hydrated electron are consequently blue. The spectrum is similar to that of the solvated electron observed in the blue solutions of alkali metals in liquid ammonia (see Section 3.1). Typical electron scavengers, e.g. H3 0, NjO, suppress the spectrurh. 

Specific electron scavengers suppress the transient absorption obtained in the pulse radiolysis of water. By measurement of the efficiency of this process, it is possible to determine the rate coefficient for the reaction of the hydrated electron with the scavenger. Solutes are restricted to those which do not absorb significantly in some region of the hydrated electron spectrum. Computer programs have been developed for kinetic analysis of the oscilloscope traces and it is thus possible to obtain rate coefficients... 

The preparation of solutions, irradiation of the samples, and analysis of decay curves by Chloe follow our previously described techniques (24). Hydrated electron scavengers were removed from the hydrogen-saturated matrix by its pre-irradiation before injection of the solute in cases where concentrations of the order of 10/xM solute were tested. The sources of supply of the chemicals used appear in Tables I and II. 

Nitrate as Electron Scavenger. In the case of nitrate, a linear relationship between the D37 dose and the initial concentration of AA was found, as shown in Figure 3. The curve passes through the origin, and its slope corresponds to a G-value for total degradation nearly twice that in oxygen, namely G(-AA) = 2.2. At neutral pH, hydrated electrons and OH-radicals are both known to be formed with G-values of about 2.25. The present result would therefore suggest that hydrated electrons are transformed by nitrate into a radical which behaves as if it were stoichiometrically equivalent to OH. It may be assumed that this radical... 

The primary quantum yield for electron formation, determined on a very short time-scale in laser experiments, is a measure of the electrons initially formed on interaction with light (Eq. 9). The primary quantum yield for electron formation has been measured at 355 nm by laser flash photolysis. Quantum yields were 4.6 x 10 to 7.6 X 10 for purified humic substances from several different natural waters and 1.7 x 10 to 4 x 10 for two commercial humic acids (normalized for carbon concentration) [85]. The caged pair generated in Eq. (9) can either collapse back or eject an electron and form the hydrated electron, e q, free in solution (Eq. 10). The steady-state yield, measured with electron scavengers under continuous irradiation, is a measure of the electrons which escape the DOM matrix and are free in solution. The electron thus occurs trapped within the DOM matrix and/or free in solution. 

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