Solvent cage, escape from

Application of low, moderate, and strong magnetic fields (MFs) affects the escape of free radicals from a (viscous) solvent cage or from a microheterogeneous compartment such as a micelle. The theory of magnetic field effects (MFEs) is well described in a number of review articles. ... 

For very fast reactions, the competition between geminate recombmation of a pair of initially fomied reactants and its escape from the connnon solvent cage is an important phenomenon in condensed-phase kinetics that has received considerable attention botli theoretically and experimentally. An extremely well studied example is the... 

An individual radical from die RP may be scavenged by a solvent or another chemical species to fonn diamagnetic products. Because the products are fonned following escape from the cage, they are known as escape or scavenging products. 

Quantum yields for the formation of symmetrical and unsymmetrical (mixed) products were determined as a function of solvent viscosity. If perchance expulsion of CO were concerted, yielding two benzyl radicals, formation of mixed combination products may reflect the ability of the radicals to escape from the solvent cage. If this were true, variation of the solvent viscosity should alter the rate of escape of these radicals and the ratio of symmetrical to unsymmetrical products should change. The results found in this study are presented in Table 4.6. The data in Table 4.6 indicate that the reaction is... 

This molecule will undergo many collisions with its nearest molecules before it escapes from the cage. In the case of two solute molecules hemmed in by solvent molecules, multiple collisions will occur before one or both of the solute molecules can diffuse out of the cage. In liquid solution then, the total number of collisions is comparable in magnitude to the number of gas phase collisions, but repeated collisions are favored over fresh collisions. 

The introduction of a free radical acceptor (scavenger) helps to measure the probability (e) of radical pairs to escape from geminate combination and diffuse out of the cage. The value of e for the fixed initiator or photoinitiator depends on the viscosity 17 of the solvent. The following empirical dependence for the photodecomposition of initiators was found... 

A further complication in the action of the solvent is connected with the possibility that the original ions will recombine and reverse the ionization reaction even before they can escape from the cage formed by the surrounding solvent molecules. This return of the original departing group to its parent molecule is called internal return or the... 

Fig. 1 for stepwise solvolysis of R-X is due to the increase in ks (s ), with decreasing stability of the carbocation intermediate, relative to the constant value of az (M s ) for the diffusion-limited addition of azide anion. The lifetime for the carbocation intermediate eventually becomes so short that essentially no azide ion adduct forms by diffusion-controlled trapping, because addition of solvent to R occurs faster than escape of the carbocation from the solvent cage followed by addition of azide ion (k Now, the nucleophile adduct must form through a... 

A possible mechanism for these reactions is shown in Scheme 21 for compound 101. The absence of a solvent polarity effect on the efficiency of photoreactions of 119 and 124 might be due to a very fast rearrangement of the radical-anion 126 within solvent cages (Scheme 21, path a). In cases in which this intermediate escapes from the cage before rearrangement occurs, a significant influence of the polarity of the solvent would have been observed. This is the situation in DMA-sensitized reactions of 101,117, and 118 (Scheme 21, path b). 

A quite different reaction course was observed with benzoyl peroxide. The increase in the decomposition rate on going from nonprotonated to protonated quinoline is relatively small. The high decomposition rate of decanoyl peroxide in the presence of protonated heteroaromatic bases was mainly ascribed to the nucleophilic character of the alkyl radicals, which allows the complete capture of the nonyl radicals escaping from the solvent cage and the consequently rapid induced decomposition. The... 

Since alkyl radicals can also attack protonated aromatic bases at the a- and y-positions, it therefore cannot be excluded that alkylation takes place by both mechanisms that is, cross-dimerization taking place within the solvent cage and the alkyl radicals escaping from the cage... 

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