Primary recombination

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

The rate of reaction of methyl radicals is in excellent agreement with the predictions of the Smoluchowski theory (see Chap. 2, Sect. 2.6). Consequently, it appears that geminate radicals move towards and away from each other at a diffusion-limited rate. Once an encounter pair is formed, reaction is very rapid (primary recombination). Furthermore, the encounter pair is held together for a considerable time (< 0.1ns in mobile solvents) because the surrounding solvent molecules hinder their separation (solvent caging). There is much evidence which lends some support for this view the most important influences on the recombination probability are listed below. 

Reaction 27 is similar to the H atom forming Reaction 4 in the case of the acids and acid anions discussed previously. An expression similar to Equation 11 would result for the yields of the M +1 ions formed in Reaction 27. At low concentration of the solute the primary recombination of the electrons and holes is predominant compared to their reactions with the solute ions and the yield of the monopositive ions is given by ... 

From a detailed consideration of the kinetics of reactions of electrons and holes in 7-irradiated frozen aqueous solutions (45), it is inferred that any model proposed to describe the nature of these species and their interactions has to meet the following requirements in order to be able satisfactorily to explain the different experimental results the electrons and holes normally undergo a primary recombination process and experi-... 

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. 

It would appear that the possibilities of obtaining photoinduced charge separation with trinuclear systems containing the Ru(bpy)22+ chromophoric unit are bound to the possibility of (i) using two bridges entailing different degrees of adiabaticity for primary recombination and secondary electron transfer (electronic... 

Using the calculation technique described, with k, fe, 7 and tei g taking the values in Table 30, a survey of a number of published flame recombination investigations in both rich and lean systems leads to the assessment, shown in Table 32, of the relative importance of the net contributions of the three primary recombination steps at approximately the centre of each range of measurement. Clearly, results with sufficiently fuel-rich flames should be capable of providing reliable values of fei7, while in lean flames recombination is principally by way of HO2 formation. On the other hand, fe,g is always more difficult to measure reliably, since reaction (xviii) is not the exclusive recombination step in any system. The recombination in lean flames depends also on the fate of the intermediate HO2. This in turn depends on the rate coefficients fe 4, feg, and k2o k2 2-... 

The light-generated electron-hole pair may undergo primary recombination through radiative and non-radiative pathways (process 1, Figure 1), or may be... 

In view of these solvent structure effects, it is convenient to classify the recombination events into primary and secondary processes. Primary recombination processes are those in which the recombination takes place before the atoms separate to a distance roughly equal to the first maximum in the mean potential (i.e., recombination in the solvent cage ). Secondary recombination involves the recombination of solvent-separated-atom pairs. 

Primary recombination can occur if the diffusion of radical fragments is impeded in the solution and a cage effect leads to... 

This cage effect, also known as the Franck-Rabinowitch effect, has other important consequences. In a photochemical reaction in solution, for example, a pair of free radicals produced initially may, owing to their being caged in by the surrounding molecules, recombine before they can separate from each other. This effect is known as primary recombination. 

Physical and chemical tests have not detected radicals , and the low probability of primary recombination of radical-ions of like charge would not lead to quantitative and intramolecular rearrangement. Radical-ions may be formed in certain compounds (e.g. 92, B = 4) when rearrangement (and dissociation) are structurally inhibited, but with longer intemitrogen bridges a normal type of migration occurs (reaction 102) . ... 

Nijenhuis K (2007) On the nature of crosslinks in thermoreversible gels. Polym Bull 58(l) 27-42 Norisuye T, Takeda M, Shibayama M (1998) Cluster-size distribution of cross-linked polymer chains across the gelation threshold. Macromolecules 31(16) 5316-5322 Norrish RGW (1937) On the principle of primary recombination in relation to the velocity of thermal reactions in solution. Trans Faraday Soc 33 1521-1528 Nuzzo RG (2001) The future of electronics manufacturing is revealed in the fine print Proc Natl Acad Sci 98(9) 4827-4829... 

Primary recombination, which occurs in a period of the order of a vibration, i.e. 10 s. This recombination is so rapid that free radicals cannot escape from the cage or diffuse into the bulk. 

Dj (the diffusion constant of the initiator) decreases, the fraction of initiator radicals escaping from primary recombination decreases and, hence, the overall initiator efficiency, f, decreases. The secondary termination process [33,34] (a process whereby free radicals that have separated from an encounter subsequently in the course of diffusion, re-enter each other and undergo geminate recombination) is not operative at high conversions while the microdomains with the radicals are separated and their diffusion to reach each other has a low probability. 

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