Recombination step

Benzylenol ethers rearrange in an apparently similar fashion via photolytic fission of the benzyl-oxygen bond and subsequent recombination steps. Irradiation in quartz of a cyclohexane solution of 3-benzyloxycholesta-3,5-diene (250) leads to 23% (251), 13% (252) [presumably formed from (251) during workup] and 10% (253). ... 

Since the recombination step (c) does not principally differ from a recombination of two H or D atoms to the respective hcmonuclear imole-cule there is no reason to assume a special activation barrier for a H and a D atom to recombine to the HD molecule. Therefore the rate of the HD production is solely determined by the rates of adsorption of H and D, respectively (as long as the reaction is adsorption-controlled, i.e., at hi enou tenperatures), or by the rate of desorption of HD (provided the reaction is desorpticai-oontrolled, i.e., at low temperatures). If wie deal with the first case only we may w/rite ... 

On a homogeneous surface, the rate of the atom recombination step will be proportional to the square of the atom surface concentrations ... 

A basic defect of these ideas is their failure to provide an explanation of the substantial effects of sofution composition, in particular the pH value, on the rate of the electrochemical reaction. Since hydrogen ions are not involved in the recombination step, the rate of this step according to Eq. (15.12) should not depend on solution pH. Yet in many cases the rate of hydrogen evolution at constant potential is proportional to the hydrogen ion concentration in solution. 

For sake of simplicity, the recombination steps of A - and O are not shown. 

Fig. 6. Computer simulation for hydrogen TDS from chemisorbed ethylene on Pt(lll) (a) First order process only, with activation energy - 15.0 Kcal/mole (dashed line) (b) same as (a) but including a hydrogen recombination step (solid line) (c) experimental data (crosses). See details in text.

Since reaction (3.21) is a recombination step requiring a third body, its rate decreases with increasing temperature, whereas the rate of reaction (3.17) increases with temperature. One then can generally conclude that reaction (3.17) will dominate at higher temperatures and lower pressures, while reaction (3.21) will be more effective at higher pressures and lower temperatures. Thus, in order to explain the limits in Fig. 3.2 it becomes apparent that at temperatures above 875 K, reaction (3.17) always prevails and the mixture is explosive for the complete pressure range covered. 

Competition experiment in which a limited amount (0.9 molar equiv) of Me2CuLi was allowed to react with a mixture of PhCOPh and an enone, Me2C=CHCHO, was carried out. In spite of the fact that both substrates have similar reduction potential, the reaction of Me2CuLi with the enone was at least 15 times as rapid as addition to PhCOPh. The results indicated that the rate of ET was not the sole controlling factor of the reactivity of these reactions. It was suggested that the recombination step to give the 1,2-adduct was too slow to allow appreciable concentration of the ketyl to accumulate in the reaction mixture . ... 

Clearly for the procedure outlined in Fig. 10.1 to work, we need to take care of several critical steps next to reasonable initial phase estimates required to formulate the initial restraints, we need a statistically valid procedure for the combination of the phases obtained by back transformation of the real space restrained map and the initial phase probability distribution. This recombination step is discussed below. 

In Figure 1.13 two recombination steps are clearly distinguishable, especially in the pump-probe data. These have been fitted previously by two separate stretched exponentials. Here, we used a single distribution of relaxation times, which accounted for both recombination steps. 

The study of hydrogen and deuterium electrosorption in palladium limited volume electrodes (LVE) was carried out by the same group in both acidic and basic solutions [124,130,134]. It was found that the hydrogen capacity, H (D)/Pd, measured electrochemically, depends significantly on sweep rate in cyclic voltammetric experiments and also on the thickness of the LVE. Two different mechanisms of hydrogen desorption, that is, the electrochemical oxidation and the nonelectrochemical recombination step, which take place in parallel within the Pd—LVE, have been postulated. 

Rate equations of considerable complexity can result from chain reactions, such as the reaction of bromine with hydrogen in the gas phase between 200 and 300 °C to form hydrogen bromide. These are reactions in which a chain carrier is created in an initiation step (here, a Br- atom from dissociation of Br2) and goes on to create more carriers (Br + H2 — HBr + H-, followed by H + Br2 -4 HBr + Br-, and so on) until a recombination step ends the chain. The rate equation for HBr formation has been shown to be ... 

The repair of stalled replication forks entails a coordinated transition from replication to recombination and back to replication. The recombination steps function to fill the DNA gap or rejoin the broken DNA branch to recreate the branched DNA structure at the replication fork. Lesions left behind in what is now duplex DNA are repaired by pathways such as base-excision or nucleotide-excision repair. Thus a wide range of enzymes encompassing every aspect of DNA metabolism ultimately take part in the repair of a stalled replication fork. This type of repair process is clearly a primary function of the homologous recombination system of every cell, and defects in recombinational DNA repair play an important role in human disease (Box 25-1). 

Steady state procedures will be carried out under stylized schemes ignoring the need for third bodies, assuming one recombination step to be dominant, and taking gas and surface terminations separately. This simplified analysis is sufficient to demonstrate that the order can change if gas or surface phase termination is dominant, or if both are significant. In reality, actual reactions are more complex. 

The decomposition of the diatomic molecule is the reverse of the recombination step, and so will also require a third body. The physical process involved is transfer of energy by collision to form an activated molecule which has enough energy to react. This is more efficient at high pressures and low temperatures. 

However, although recombination steps are very common termination steps, disproportionation reactions are also found. 

Fig. 3, and therefore reduce the quantum yield of C-P+-QT. However, this does not necessarily mean that the yield of the final C+-P-QT state will be similarly reduced. Indeed, it was found that addition of a second methylene spacer as in 5 increased the quantum yield of the final state by a factor of 1.44 [56], With 3 and 4 methylene groups (6 and 7) the yield decreased to 0.65 and 0.56 that of 4, respectively. This complicated distance dependence for the yield of the final state is in part a consequence of the fact that increasing the porphyrin-quinone separation not only reduces the rate of step 2, but also that of charge recombination (step 3). The rate of the second forward electron transfer step 4 is essentially unaffected, since this step does not involve porphyrin-quinone electron transfer. Thus, increasing the separation will decrease the quantum yield of step 2, but increase the ratio kjk3, which determines the efficiency of the second electron transfer step. With 5, the loss in quantum yield of step 2 is more than compensated for by the increase of efficiency of step 4, and the overall quantum yield increases. In 6 and 7, any increase in the efficiency of step 4 evidently cannot compensate for the decrease in quantum yield for step 2, and the overall quantum yield decreases. 

What is particularly important from a propulsion point of view, in this regard, is that the three-body recombination reactions are the major energy releasing reactions as well. Thus if the Bray freezing point criterion is applied to only one step, it must be applied to a controlling three body recombination step in the reaction scheme. 

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