Competitive electron scavenging

Finally, the presence of a competitive electron scavenger during photoionization prevents the appearance of methyl radicals. 

Rate Constants. The effect of electron scavengers on the H atom yields can be considered as a competition between Reactions 4 and 8 (S, electron scavenger). 

There is competition for the free electrons between the positrons and the holes (OH radicals in the case of water media). When OH competes for these free electrons the addition of OH scavengers is predicted to cause an increase in the yield of o-Ps, a phenomenon called enhancement . On the other hand, addition of electron scavengers will reduce the yield of o-Ps, an effect known as inhibition . The yield of o-Ps is usually denoted by I3 (where and I2 are the yields of free positrons and />-Ps, respectively, having a much shorter half-life than o-Ps). Duplatre and coworkers - found that the yield of o-Ps in aqueous solution in the presence of concentration C of allylamine is given by equation 10. The enhancement factor 1 -h )SC is due to the reaction of allylamine with OH radicals, while the 1 + C inhibition factor is due to the reaction with aqueous or free electrons. The enhancement by amines is one of the main pieces of evidence for the existence of the spur mechanism . Yet, it should be emphasized that it does not rule out the simultaneous existence of both mechanisms . Duplatre and coworkers found that the inhibition by a strong inhibitor such as NO3 is the same in the presence or absence of allylamine. 

Quantitative evaluation of the number of radicals shows that, although the nature of the spectrum depends on the concentration of ethyl mercaptan, the total radical yield remains constant this suggests that the protective effect of the additive is due to a competition between electron scavenging by the additive and recombination with cations in the matrix, leading to scission of the polymer chain. The mechanism... 

Let us digress to discuss certain problems which arise in handling data from competitive experiments. Since Gfi is small compared with the total yield of electrons scavenged in the region where competitive studies are usually carried out, it is convenient here to approximate Equation X by... 

Kinetic studies of the competitive reactions of other electron scavengers support this hypothesis (18, 20). In the radiolysis of solutions of nitrous oxide in alkanes, reactions with other intermediates must be considered. Radicals, hydrogen atoms, and positive ions can be eliminated (5, 20), but a reaction with excited molecules is possible. It has been reported... 

Electron scavenging by oxygen species in competition with eqn. (53a) was identified as the origin of the sensitivity of this photoassisted redox-type mechanism towards added oxygen. 

Hayon endeavored to prove that Ge is equal to 2.3 at low concentrations of electron scavenger, where keaq+s[S] < 107, and at higher concentrations follows G(e ) = 2.3 + f(ke +s[S]). The function (/(fc6 q+s[S] ) is given as an experimental curve (47), Hayon believes that the observed spread in Ge is caused only by the different values of fceaq+s[S] in the systems which were studied and proposes a theory (45). However, on checking his data, it can be shown that in these systems, an appreciable—if not the entire—effect of the decrease in Geaq as the concentration of S decreases, is caused by eaq competition with the irradiation products formed. 

Because the initial step in isomerization is electron donation from a substrate to SCC/r "), the isomerization reactions can be quenched readily by a competitive electron donor such as 1,4-diazobicycloctane, diisopropylamine, or 1,2,4-trimethoxybenzene [104]. The presence of methyl viologen dictation (a conduction-band electron quencher) or diphenylamine (an electron-hole relay) enhances the reaction efficiency [105]. Although oxygen is an effective scavenger for conduction-band electrons and capable of increasing the electron-hole... 

In alicyclic hydrocarbon solvents with aromatic solutes, energy transfer (vide infra) is unimportant and probably all excited solute states are formed on neutralization of solute cations with solute anions, which are formed in the first place by charge migration and scavenging in competition with electron solvent-cation recombination. The yields of naphthalene singlet and triplet excited states at 10 mM concentration solution are comparable and increase in the order cyclopentane, cyclohexane, cyclooctane, and decalin as solvents. Further, the yields of these... 

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