Decreased radical recombination efficiency

FIGURE 7. The proposed effect of stress in the several stages of the decreased radical recombination efficiency hypothesis. 

HL Stress-Induced Changes in krecombmatumi the Decreased Radical Recombination Efficiency Hypothesis... 

Removal efficiency of phenol can also be expressed in terms of GD values. As the pH increases, the GD value decreases. At any given solute concentration and pH, the removal rate decreases with increasing dose, suggesting that the radical-radical recombination reaction involving OH, e, and H also increased. 

It was proposed that the temperature dependence of polymer 5 arises from the temperature dependence of the kA step. Specifically, it was suggested that the polymer segments to which the radicals are attached are conformationally stressed. There are two possible modes for the newly formed radicals to relax and become separated They can rotate or recoil away from each other (Scheme 9). These secondary motions of the polymer arise from the relaxation of unfavorable bond conformations that are formed during the polymer casting process. The increased thermal energy facilitates the rotation and recoil relaxation processes, which effectively increases the rate constant for diffusion of the radicals out of the cage, kA. This leads to decreased radical-radical recombination and consequently an increase in photodegradation efficiency. 

The key feature of Inisurfs is their surfactant behavior. They form micelles and are adsorbed at interfaces, and as such they are characterized by a critical micelle concentration (CMC) and an area/molecule in the adsorbed state. This influences both the decomposition behavior and the radical efficiency, which are much lower than those for conventional, low molecular weight initiators. Tauer and Kosmella [4] have observed that in the emulsion polymerization of styrene, using an Inisurf concentration above the CMC resulted in an increase in the rate constant of the production of free radicals. This was attributed to micellar catalysis effects as described, for example, by Rieger [5]. Conversely, if the Inisurf concentration was below the CMC the rate constant of the production of free radicals decreased with an increase in the Inisurf concentration, which was attributed to enhanced radical recombination. Also note that a similar effect of the dependence of initiator efficiency on concentration was reported by Van Hook and Tobolsky for azobisisobutyronitrile (AIBN) [6]. 

A distribution of oxidized bases similar to that obtained with y-rays is observed upon exposure of human cells to high LET particles such as heavy ions [11]. However, with both and Ar + ions beams, a decrease in the yield of all lesions was observed [12]. This observation is reminiscent of the effect of increasing LET on the radiolytic yield of OH that decreases as the result of efficient radical recombination in the particle track. A same trend has been observed for both single and double-strand breaks [13]. These results strongly suggest a major role ofthe hydroxyl radical chemistry in the formation of radiation-induced... 

Recently we observed the effect which supports the conclusion about the substantial role of the radical reaction outside of the catalyst grains. When a very efficient OCM oxide catalyst (10% Nd/MgO) was placed into the reactor together with an inactive metal filament (Ni-based alloy) the sharp increase of conversion accompanied by the selectivity shift from oxidative coupling to the formation of CO and H2 was observed [19]. Since the metal component has a low activity also with respect to ethane oxidation, this behavior is not due to successive oxidation or decomposition of C2 hydrocarbons on the metal surface, but should be attributed to the reactions of methane oxidation intermediates. Almost total disappearance of ethane (which is a product of CH3 radicals recombination) and acceleration of the apparent reaction rate by the addition of an "inert material indicate that the efficiency of methane oxidative transformations can be substantially increased if the radicals have a chance to react outside the zone where they formed and the role of reaction (-1) decreases. Although the second (metal) surface is not active enough to conduct the reaction of saturated hydrocarbon molecules (methane and ethane), the radicals generated by the oxide can react further on the metal surface. As a result, the fraction of the products formed from methane activated in the reaction (1) increases, and the formation of the final reaction mixture of different composition takes place. 

The relative efficiencies of different benzoin derivatives vary considerably in photoinitia-tion. The nature of the monomer and the environment exert an effect as well. Also, the efficiency decreases when radical recombination is favored by the cage effect as a result of an increase in the viscosity of the medium. It is also possible that hydrogen abstractions in a viscous medium contribute to photoinitiations by benzoin and its derivatives. 

Increasing the temperature may favour the HCl diffusion out of the sample and decrease the autocatalytic effects [2182]. The initiation, propagation and termination steps in the thermal degradation of poly(vinyl chloride) at higher temperatures (190-220°C) are catalysed by HCl [121,954]. The low initiation efficiency results partly from the restricted mobility of the free radical fragments in a solid polymer irradiated at temperatures below (87°C), which favours radical recombination. By contrast, when poly(vinyl chloride)... 

Not all initiating radicals (/ ) succeed in initiating polymerization, recombination of these radicals in the solvent can decrease the efficiency (/) to a value lower than 1. Detailed kinetic treatment of photoinitiation processes are discussed by Oster and Yang [3]. 

That products of intermediate oxidation level can be detected in the photocatalytic reactions of hydrocarbons and fossil fuels is also consistent with a surface bound radical intermediate . Photocatalytic isotope exchange between cyclopentane and deuterium on bifunctional platinum/titanium dioxide catalysts indicates the importance of weakly adsorbed pentane at oxide sites. The platinum serves to attract free electrons, decreasing the efficiency of electron-hole recombination, and to regenerate the surface oxide after exchange. Much better control of the exchange is afforded with photoelectrochemical than thermal catalysis > ) As before, hydrocarbon oxidations can also be conducted at the gas-solid interface... 

The results for phenolate and naphtholate show that internal transition may lead to solvated electron formation from aromatic anions. The fact that the products are the negatively charged solvated electron and a radical which is neutral (and not a positively charged one) may be partly responsible for the increased efficiency of anions over the undissociated molecules. Primary recombination may decrease in the absence of coulombic attraction. Moreover the ionization potential of the anion is lower. 

Even more straightforward evidence in favor of recombination to the triplet state was obtained by heavy-atom substitution into the fluorescence quencher, which also enhances the rate of spin conversion. By measuring the transient absorption of both ion radicals and the triplet products of their recombination, it was demonstrated that the quantum yield of triplets increases when the charge separation yield 9(0) decreases as a result of heavy-atom substitution [225]. As was shown in Figure 3.76, triplet state is faster than to the singlet state, while the quantum yield of triplets produced from the singlet precursor only increases with ks. Hence these data also indicate that the triplet channel of recombination is the most efficient. 

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