Ions reaction hydrated electron

The transient univalent states of certain metals—e.g., Zn+, Ni+, Pb+, Cd+, Co", etc.—can be produced in aqueous solution by reducing the corresponding M2+ ions by hydrated electrons (15). These unstable cations absorb strongly in the ultraviolet, and their reactions can be followed conveniently by pulse radiolysis (2, 17). At neutral pH, the lifetimes of the transient absorptions depend upon the experimental conditions, but initial half-lives of 50 psec. or more can be obtained routinely. However, in the presence of other oxidizing cations, the lifetimes decrease considerably owing to the onset of electron transfer reactions of the type... 

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 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)... 

The formation of hydrated electrons by the photolysis of halide ions in solution may be envisaged in two steps. The first step is the CTTS absorption leading to (X -). The second step is a slow, thermal process releasing the electron in competition with degradation and recapture. In the presence of acid and alcohol, photolysis of halide solutions generates H2 with a yield that increases both with acid and alcohol concentrations (seejortner et al., 1962, 1963, 1964). At 25°, the limiting quantum yields are 0.98 for Cl- at 185 nm, 0.6 and 0.5 for Brat 185 and 229 nm, respectively, and 0.3 and 0.25 for I- at 254 and 229 nm, respectively. Since most of these yields are less than 1, the direct reaction of HsO and (Xaq-) is ruled out. Instead, it is proposed that eh is produced from the... 

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 ... 

Pig. 7-1. (a) Cathodic reaction and (b) anodic reaction M = metal electrode S = aqueous solution (electrolyte Mm = metal ion in metallic bonding state M., = metal ion in hydrated state Om = electron in metals. 

The reaction of electron transfer at electrodes in aqueous electrolytes proceeds either with hydrated redox particles at the plane of closest approach of hydrated ions to the electrode interface (OHP, the outer Helmholtz plane) or with dehydrated and adsorbed redox particles at the plane of contact adsorption on the electrode interface (IHP, the inner Helmholtz plane) as shown in Fig. 7-2. 

The second-order rate constant for the reaction of a hydrogen atom with a hydroxide ion to give an electron and water (hydrated electron) is 2.0 x 10 M s . The rate constant for the decay of a hydrated electron to give a hydrogen atom and hydroxide ion is 16M s. Both rate constants can be determined by pulse radiolytic methods. Estimate, using these values, the pA of the hydrogen atom. Assume the concentration of water is 55.5M and that the ionization constant of water is 10 M. 

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. 

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. 

Photolysis of ion pairs of cobalt(III) complexes with iodide ions leads to oxidation of iodide and reduction of the complex.55,63-86 Under the normal experimental conditions, however, most of the light is absorbed by free iodide and the reduction of the complex is effected by hydrated electrons produced as in reaction (36).86... 

Nucleophilic addition to C=0 (contd.) ammonia derivs., 219 base catalysis, 204, 207, 212, 216, 226 benzoin condensation, 231 bisulphite anion, 207, 213 Cannizzaro reaction, 216 carbanions, 221-234 Claisen ester condensation, 229 Claisen-Schmidt reaction, 226 conjugate, 200, 213 cyanide ion, 212 Dieckmann reaction, 230 electronic effects in, 205, 208, 226 electrons, 217 Grignard reagents, 221, 235 halide ion, 214 hydration, 207 hydride ion, 214 hydrogen bonding in, 204, 209 in carboxylic derivs., 236-244 intermediates in, 50, 219 intramolecular, 217, 232 irreversible, 215, 222 Knoevenagel reaction, 228 Lewis acids in, 204, 222 Meerwein-Ponndorf reaction, 215 MejSiCN, 213 nitroalkanes, 226 Perkin reaction, 227 pH and, 204, 208, 219 protection, 211... 

This chapter is devoted to the important relationship between electrode potentials and the changes in Gibbs energy (AO ) for half-reactions and overall reactions. In discussions of the properties of ions in aqueous solution it is frequently more convenient to represent changes in Gibbs energy, quoted with units of k.I mol-1, in terms of electrode potentials, quoted with units of volts (V). The electrochemical series is introduced. The properties of the hydrated electron are described. 

The first element, hydrogen, has an Allred Rochow electronegativity coefficient of 2.1, and an electronic configuration Is1. The atom may lose the single electron to become a proton, which exists in aqueous solutions as the hydroxonium ion, H30+(aq), in which the proton is covalently bonded to the oxygen atom of a water molecule. The ion is hydrated, as is discussed extensively in Chapter 2. The reduction of the hydrated proton by an electron forms the reference standard half-reaction for the scale of reduction potentials ... 

Upon ejection from an ion or molecule by photoionization or high energy radiolysis, the electron can be captured in the solvent to form an anionic species. This species is called the solvated electron and has properties reminiscent of molecular anions redox potential of —2.75eV and diffusion coefficient of 4.5 x 10-9 m2 s-1 (Hart and Anbar [17]) in water. Reactions between this very strong reductant and an oxidising agent are usually very fast. The agreement between experimental results and the Smoluchowski theoretical rate coefficients [3] is often close and within experimental error. For instance, the rate coefficient for reaction of the solvated (hydrated) electron in water with nitrobenzene has a value 3.3 x 10+1° dm3 mol-1 s-1. 

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. 

Encounter radii, R, for reaction between hydrated electrons and various ions [17], the corrected encounter radius, i H, incorporating the hydrodynamic effect, and the sum of the hydrated electron and ion crystallographic radii, Rc... 

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. 

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