Solvated electron scavengers

In case of electron scavenging (and no Ps lifetime quenching, as is true for both Cl" and Tl+), no other positron states are present than free e+ and Ps then, the intensities from PALS and from DB are the same. The p-Ps and o-Ps intensities are expected to decrease so that the fwhm of the DB spectra should increase with solute concentration (the narrow components are suppressed). The variations of fwhm with C can be completely calculated, knowing the intensities Ij from PALS and the Tj previously established for a given solvent. This is illustrated by the solid line in Figure 4 for Tl+ this ion, as expected from its high solvated electron scavenging rate constant, is thus shown to suppress Ps formation by electron capture. 

The reaction with halocarbons results in loss of halide, with concurrent Ibrmation of halocarbon-derived free radicals, R (eq 5) (47). Zepp and co-vorkers (12) used the photoproduction of chloride ions from the halocarbon, 2-chloroethanol, a known solvated electron scavenger, in solutions of NOM... 

Hence D amd R may be extracted sepaorately frcmi plots of t / In CA](t) vs. t /. This procedure hau3 been applied to two kinds of reactions, auLl studied by famt tramsient spectroscopy. The first kind of reaction is solvated electron scavenging. For example, Buxton et al.(l975) have applied equation (31) to the reaction of solvated electrons generated by pulse radiolysis with chromate ions, amd were thus able to show that the reaction e q + CrO is purely diffusion - controlled, amd to determine D amd R. The second kind of reaction concerns fluorescence quenching ... 

This is called the SrnI mechanism," and many other examples are known (see 13-3, 13-4,13-6,13-12). The lUPAC designation is T+Dn+An." Note that the last step of the mechanism produces ArT radical ions, so the process is a chain mechanism (see p. 895)." An electron donor is required to initiate the reaction. In the case above it was solvated electrons from KNH2 in NH3. Evidence was that the addition of potassium metal (a good producer of solvated electrons in ammonia) completely suppressed the cine substitution. Further evidence for the SrnI mechanism was that addition of radical scavengers (which would suppress a free-radical mechanism) led to 8 9 ratios much closer to 1.46 1. Numerous other observations of SrnI mechanisms that were stimulated by solvated electrons and inhibited by radical scavengers have also been recorded." Further evidence for the SrnI mechanism in the case above was that some 1,2,4-trimethylbenzene was found among the products. This could easily be formed by abstraction by Ar- of Ft from the solvent NH3. Besides initiation by solvated electrons," " SrnI reactions have been initiated photochemically," electrochemically," and even thermally." ... 

Steady photoemission currents can be realized when acceptors (scavengers) for the solvated electrons are present in the solution. A good scavenger should be nonelectroactive at the potenhal of interest, should react quickly with solvated electrons, and the reaction products should be either nonelectroactive or reducible. A reachon with acceptors implies that the current of reoxidation of the solvated electrons becomes lower, and thus a steady photoemission current appears. The acceptors most often used are nitrous oxide, N2O, and hydroxonium ions, HjO. In the former case, OH radical is produced in the scavenging process, which undergoes further reduction on the electrode, thus doubling the photocurrent ... 

Emission of electrons from the electrode into the solution with formation of solvated electrons and the subsequent reaction between the solvated electrons and the electron scavenger in solution. 

Of all the solvated electrons, eh is the most reactive several thousand of its reactions have been measured and, in many cases, activation energies, the effects of pH, and so forth, determined (Anbar and Neta, 1965 Anbar et al, 1973 Ross, 1975 CRC Handbook, 1991). Relatively few reactions of eamhave been studied because of its low reactivity. Rates of reactions of the solvated electron with certain scavengers are also available in alcohols, amines, and ethers. 

The rate of uracil photolysis is affected by the fraction of oxygen content in the solution,7,291 64 and is also affected by dissolved N20.65 The data of Burr and Park are shown in Table III. The effect of N20 upon the rate suggests the intervention of solvated electron intermediates in the reaction mechanism if N20 is, as is commonly supposed, a scavenger specific for electrons. 

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

The review by Rice and Pilling [39] discussed some of the experimental studies of reactions of solvated electrons with scavengers and other more recent work has been included in Chap. 3, Sect. 2. For completeness, some of the more important studies are presented below. 

Effective encounter distances for reaction of solvated electrons with electron scavengers at room temperature compared with crystallographic encounter distances Unless otherwise noted, the solvent is water (containing an inert electrolyte in some cases). Corrections for ionic interactions according to eqn. (106) were applied and reaction rate coefficient were extrapolated to zero ionic strength (Chap. 3, Sect. 1.6 and 1.7). Many of these studies have been mentioned in Chap. 3, Sect. 2... 

Effective encounter distances for reactions of solvated electrons with electron scavengers at low temperatures, compared with crystallographic encounter distances, from a fit between experiment and eqn. (105)... 

Many of the fast chemical reactions discussed in the preceding sections involve at least one reactant which is of low symmetry. The reactions of the solvated electron with nitrate, naphthalene or pyrene are instances where the oxidant has a mirror plane (in the molecular plane) in the accepting orbital. Hence, reaction of the solvated electron with such a scavenger when both are contained in this plane should be slower than in other configurations. Similarly, the contact quenching of fluorescence from naphthalene or 1,2-benzanthracene by carbon tetrabromide [7], or... 

A distinction has been made in the literature between those ion-pairs which recombine and those which escape. They are called geminate and free ion-pairs, respectively. Hummel [69] has noted in many experimental studies that the yield of products G(P) from the scavenging of one ion (usually the solvated electron) increases linearly with the square root of the scavenger concentration... 

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