Out-of-cage processes

Micellar media created by sodium dodecyl sulfate or cetyltrimethylammon-ium bromide have also a positive effect on the ortho selectivity [262,269-271]. In a recent article, the PFR of 1-naphthyl acetate in aqueous solutions of an antenna polyelectrolyte like poly(sodium styrenesulfonate-co-2-vinylfluorene) has been reported, which, in addition to the micellar effect, also provides singlet sensitization by means of fluorene chromophores. In this particular case, the occurrence of in-cage versus out-of-cage processes can be clearly correlated with the different packing of the microdomains, which depends on the molar fraction of monomers in the antenna copolymer [272]. 

A detailed study of the photochemical reaction of naphthaquinone with aldehydes has shown that the reaction occurs via an in cage mechanism at low temperatures but at ambient temperatures a small part of the reaction arises from an out-of-cage process. ... 

The original concept of cage processes, due to Franck and Rabinowitsch (10), was invoked to explain the reduction in quantum yields which occur on the photolysis of simple molecules like acetone and iodine when carried out in solution as compared to the gas phase. Extensive studies were carried out by Noyes (11) during the 1950s and 1960s to develop a theoretical understanding. 

Further proof for an in-cage process came from the thermal generation of acyl radicals and consequent comparison of the product spectrum with that obtained from the photoreaction. In contrast to the photoacylation (and also to Schenck s thermal reaction involving chloranil, vide supra), thermal reactions of various acyl radicals with ground state 9,10-phenanthrenequinone (1) gave ca. 1 1 mixtures of the isomeric dimers 75 (Scheme 22), thus disproving a similar out-of-cage scenario in the photochemical case. ... 

The assumption that k values are constant over the entire duration of the reaction breaks down for termination reactions in bulk polymerizations. Here, as in Sec. 5.2, we can consider the termination process—whether by combination or disproportionation to depend on the rates at which polymer molecules can diffuse into (characterized by kj) or out of (characterized by k ) the same solvent cage and the rate at which chemical reaction between them (characterized by kj.) occurs in that cage. In Chap. 5 we saw that two limiting cases of Eq. (5.8) could be readily identified ... 

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. 

Photodissociation from the LMCT excited state produces the primary radical-ion pair within a solvent cage. Collisions among the primary products lead to recombination. The recombination competes with the diffusion out of the solvent cage. In this primary process, the fragments may never attain a separation of as much... 

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