Cage reaction effect

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

The special salt effect is a constant feature of the activation of substrates in cages subsequent to ET from electron-reservoir complexes. In the present case, the salt effect inhibits the C-H activation process [59], but in other cases, the result of the special effect can be favorable. For instance, when the reduction of a substrate is expected, one wishes to avoid the cage reaction with the sandwich. An example is the reduction of alkynes and of aldehydes or ketones [60], These reductions follow a pathway which is comparable to the one observed in the reaction with 02. In the absence of Na + PFg, coupling of the substrate with the sandwich is observed. Thus one equiv. Na+PFg is used to avoid this cage coupling and, in the presence of ethanol as a proton donor, hydrogenation is obtained (Scheme VII). 

The theory of CIDNP depends on the nuclear spin dependence of intersystem crossing in a radical (ion) pair, and the electron spin dependence of radical pair reaction rates. These principles cause a sorting of nuclear spin states into different products, resulting in characteristic nonequilibrium populations in the nuclear spin levels of geminate (in cage) reaction products, and complementary populations in free radical (escape) products. The effects are optimal for radical parrs with nanosecond lifetimes. 

Charged colloids and water-in-oil microemulsions provide organized environments that control photosensitized electron transfer reactions. Effective charge separation of the primary encounter cage complex, and subsequent stabilization of the photoproducts against back electron transfer reactions is achieved by means of electrostatic and hydrophobic interactions of the photoproducts and the organized media. 

If we are to use the radical pair theory to explain the effects of micellization on the cage reaction probability as well as the magnetic field effect, it is mandatory that we be able to observe CIDNP in these systems. In addition, since CIDNP is sensitive to events on the time scale of the radical pair lifetime, detailed analysis of the CIDNP can often lead to mechanistic insight to the dynamics of the radical pair. Below we describe one such result. 

Since the cage effect is less than 100 % in most cases (Table 1), some of the radical pairs exit the micelle and become water solubilized free radicals. At some later time, the free radicals recombine to form DPE. This has been verified by Cu2+ quenching experiments (Fig. 6)13,19). The disappearance of DBK is not dependent upon the concentration of Cu2+. However, the yield of DPE drops very rapidly with increasing copper concentration and then levels off. The leveling off region is directly related to the amount of cage reaction. The products formed by reaction with copper are benzyl chloride (4), and benzyl alcohol (5) (Scheme V)20). 

In a bimolecular reaction in solution reactants diffuse through the assembly of solute and solvent molecules (Scheme 1) and collide to form an encounter complex within a solvent cage. Reaction is not possible until any necessary changes occur in the ionic atmosphere to form an active complex and in solvation (such as desolvation of lone pairs) to form a reaction complex in which bonding changes take place. The encounter complex remains essentially intact for the time period of several collisions because of the protecting effect of the solvent surrounding the molecules once they have collided. The products of the subsequent reaction could either return to reactants or diffuse into the bulk solvent. 

An immediate result follows from an examination of the ratio (n)(C)/(n)(RW) for the three Cartesian shells considered here. For N N = 218, and N = 386, the ratios are n) C)/ n) RW) = 0.44, 0.50, and 0.54, respectively. Since it was proved analytically (see earlier discussion) that ind = 2 this ratio is unity in the limit Af oo, it is evident that short-range focusing associated with a cage/chemical effect can have a dramatic influence on the efficiency of the underlying diffusion-reaction process in finite, compartmentalized systems. Different assumptions on the spatial character of a local cage or a specific realization of the short-range quantum-chemical effect can influence further the above ratios. 

If it is reasonably assumed that AH + for the reverse, recombination reaction is small (ca. 2-3 kcal/mol), then the absolute Ln R bond enthalpy can be directly calculated from AH+ for the forward reaction (37-42). Radical cage diffusion effects cannot be ignored in such kinetic analyses, however they appear only to be important in more viscous media. Good agreement has recently been noted between... 

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