Cage formation from solvent

Figure 3.2 An enthalpy profile for a unimolecular reaction in solution, involving the formation of a radical pair inside a solvent cage. Adapted from [61],...

It is convenient to label the relative slowness of encounter pair reaction as due to an activated process and to remark that the chemical reaction (proton, electron or energy transfer, bond fission or formation) can be activation-limited. This is an unsatisfactory nomenclature for several reasons. Diffusion of molecules in solution not only involves a random walk, but oscillations of the molecules in solvent cages. Between each solvent cage in which the molecule oscillates, a transformation from one state to another occurs by passage over an activation barrier. Indeed, diffusion is activated (see Sect. 6.9), with a typical activation energy 8—12 kJ mol-1. By contrast, the chemical reaction of a pair of radicals is often not activated (Pilling [35]), or rather the entropy of activation... 

One important discrepancy should be noted between photochemical and chemical ion radical reactions. In the photochemical mode, an oxidized donor and a reduced acceptor remain in the same cage of a solvent and can interact instantly. In the chemical mode, these initial products of electron transfer can come apart and react separately in the bulk solvent. For example, one-electron oxidation of phenylbenzyl sulfide results in formation of the cation radical both in the photoinduced reaction with nitromethane and during treatment with ammoniumyl species. Sulfide cation radicals undergo fragmentation in the chemical process, but they form phenylbenzyl sulfoxide molecules in the photochemical reaction. The sulfoxide is formed at the expense of the oxygen atom donor. The latter comes from the nitromethane anion radical and is directly present in the solvent cage. As for the am-... 

The activation parameters for the purely diffusion controlled formation of the collisional cage pair from the free radicals (Scheme 1, kj), AH d, AS d) are relatively well established " . For nonpolar radicals in non-associating alkane solvents, the value of AH D should be very close to the Andrade energy [E, equation (6)] for viscous flow of the solvent as predicted by the... 

In discussing mechanism (5.F) in the last chapter we noted that the entrapment of two reactive species in the same solvent cage may be considered a transition state in the reaction of these species. Reactions such as the thermal homolysis of peroxides and azo compounds result in the formation of two radicals already trapped together in a cage that promotes direct recombination, as with the 2-cyanopropyl radicals from 2,2 -azobisisobutyronitrile (AIBN),... 

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... 

After quenching with DzO or Bu OD, analysis of the products from the Grignard reagents formed from PhCHXMe (X = Cl, Br, I) in the optically active solvent -(R)-2-methoxypentane leads to the conclusion that Grignard reagent formation occurs on the Mg surface within a solvent cage by a one-electron transfer mechanism.1... 

Further evidence for the formation of alkene radical cations derives from the work of Giese, Rist, and coworkers who observed a chemically induced dynamic nuclear polarization (CIDNP) effect on the dihydrofuran 6 arising from fragmentation of radical 5 and electron transfer from the benzoyl radical within the solvent cage (Scheme 6) [67]. 

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