Secondary 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. 

In the photochemical procedure, addition product can be minimized in keeping the relative NBS concentration as small as possible. In addition, substrate concentrations should be optimized with regard to the exitance of the chosen light source to avoid secondary recombination reactions. Under these conditions 4-bromomethyl-5-methyl-l,3-dioxol-2-one can be prepared with only minor impurities (bromine addition and multiple allylic bromination reactions (Eq. 23)) [34]. 

The added scavengers compete with the secondary recombination between CeHgO and e aq according to the square root of concentration law. 

PET across membranes is extensively discussed in the first contribution with emphasis on primary photochemical charge separation processes and secondary recombination reactions. Mainly vesicles and planar bilayer membranes serve as models which allow the spatial separation of photochemically generated oxidants and reductants. 

This seems like a rather arbitrary definition, and indeed it is. In the present case it provides that the chance of primary recominnation of an (A,B) pair is no greater than the recombination of A and B with fragments from other A-B molecules. Depending on the competing processes, there will be a different definition of step 3. For a more exact treatment [see R. M. Noyes, J. Am. Chern. Soc.y 79, 551 (1957) 78, 5486 (1956) 77, 2042 (1955) J . Chem. Phys. 22, 1349 (1954)] we must consider the rather high probability of secondary recombination for a nascent pair A and B which have separated by only a few molecular diameters (i.e., the reverse of 3). 

As shown in Fig. 3-1, some radicals in separated radical pairs re-encounter their partners within the solvent cages, but others escape from the cages, forming "escape radicals". Since the time scales for the secondary recombination and the S-To conversion rates are 10 " 10 and 10 10 s, [3] respectively, the change in the S-To conversion rate by an external magnetic field and/or the HFC term can influence the yield of cage and escape products. 

The lifetime of encounter complexes between neutral reactants is on the order of 0.1 ns in solvents of low viscosity, that is, k d kd m. Random diffusive displacements of the order of a molecular diameter occur with a frequency of about 1011 s Subsequently, the fragments from a specific dissociation may re-encounter each other and undergo secondary recombination .59 If secondary recombination does not take place within about 1 ns, the fragments will almost certainly have diffused so far apart that the chance of a reencounter becomes negligible. The initial overall electronic multiplicity 2S + 1 of encounter complexes is thus important in determining the fate of the reactants, because their lifetime is usually insufficient to allow for intersystem crossing during an encounter. 

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. 

Part of the stimulus for research in this area comes from the possibility of probing the dynamics of such processes on short time scales by using picosecond lasers. The standard pulse-and-probe experiments will measure the entire time profile of the recombination and photodissociation processes. An interpretation of such results therefore requires a consideration of the dynamics on several potential energy surfaces for both the primary and secondary recombination processes. The very short time behavior is often obscured by experimental problems (laser rise times etc.), but the secondary recombination process is more easily studied. 

There is still a gap between our models of liquid-state reactions and the often bewildering complexity of real chemical systems. Progress in shortening the gap will probably come only from the application of a variety of methods to this problem. The full promise of picosecond spectroscopy techniques for studying the details of the dynamics of reactive events in liquids has yet to be realized. How deeply can these methods probe the dynamics Computer simulations, another source of experimental information in reacting systems, are only beginning to be exploited. "" The description by direct computer simulation of both primary and secondary recombination dynamics, for example, would yield a wealth of information that could be used to test theories. 

Because of the relatively slow rates of radical diffusion in polymer matrices, it seems likely that the probability of secondary recombination will depend very much on the separation achieved while the particles are moving apart with the original excess kinetic energy imparted in the primary dissociation step. This, in turn, should depend on the energy of the exciting photon. There is some evidence for this, even in small molecules in solution. For example, Slivinskas and Guillet (16) report a one-hundied-fold increase in the relative yields of Norrish type I radical products from simple aliphatic ketones, when the reaction is initiated by y-rays rather than ultraviolet light (Table 5). Similar increases were observed in polymeric systems such as in ethylene-CO copolymers (17). 

Cage escape to the solvent-separated radical pair opens further reaction channels such as secondary recombination and other reactions. 

Secondary recombination, which occurs within about 10 s. Diffusion out of the cage into bulk, however, can compete with the secondary recombination. 

This is a primary recombination process, whieh ean be distinguished from secondary recombination reactions, which occur after the two atoms have separated. 

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