Caged radicals, reactions

If the external surface contains sites similar to those in the cages, radical reaction could occur more effectively because of better accessibility of the gas phase. However, I doubt that an irregular external surface could hold Co2+ in surface positions because of the tendency of Co2+ to find tetrahedral or octahedral positions. The present spectra, of course, show only complexes formed on internal surfaces because there is a linear correspondence between the absorption coefficient and concentration and because all ions observed on Type A are surface ions. 

The cation undergoes homolytic decomposition to afford typical products of solvent-cage radical reactions. ... 

In studies of radical-radical reactions, radicals are typically generated pairwise and the products come from both cage and encounter (non-cage) reactions. 

The results were interpreted on the basis of a mechanism that starts with the photolytic formation of a radical cage consisting of an aryldiazenyl and and arylthiyl (Ar - S ) radical, followed by diffusion of both radicals out of the cage. Three reactions of the aryldiazenyl radical are assumed to occur bimolecular formation of the azoarene and N2, or of biphenyl and N2 (Scheme 8-37), the monomolecular dediazoniation (Scheme 8-38), and recombination with the thiyl radical accompanied by dediazoniation (Scheme 8-39). In addition, two radicals can react to form a di-phenyldisulfide (Scheme 8-40). 

A subsequent detailed analysis of the permanganate oxidation of the tertiary hydrogen atom of 4-phenylvaleric acid in 2.5 M potassium hydroxide solution supports the caged radical mechanism. The reaction order is two overall, A h/ d is ca. 11.5, ring substitution has little elfect on the rate (p 0) and the oxidation proceeds with a net 30-40 % retention of optical configuration. 

The formation of hydrogen peroxide as the product of peroxyl radical reactions in the cage [187-192],... 

The steric factor P is high enough for the bimolecular reaction to occur when two radicals met in the cage. This reaction is limited only by translational diffusion of the reacting radicals and depends on the viscosity of the solvent. The rate constant of such reaction is close to the frequency of encounters of radicals, namely, k = a, kn = 0.25 x 47rrABZ)AB = RT 100007] where spin-statistical factor, rAB is the sum of radii of reactants A and B, and DAB is the sum of their diffusion coefficients. 

The observed rate constant is kobs = kkn(k + vD)-1. For the fast reactions with k vD the rate constant is kobs = kI). In the case of a slow reaction with k vD the rate constant is k0bs = kx KAb, where KAB = k y vn is the equilibrium constant of formation of cage pairs A and B in the solvent or solid polymer matrix. The equilibrium constant KAB should not depend on the molecular mobility. According to this scheme, the rate constant of a slow bimolecular reaction kobs = kKAB(kobs kD) should be the same in a hydrocarbon solution and the nonpolar polymer matrix. However, it was found experimentally that several slow free radical reactions occur more slowly in the polymer matrix than in the solvent. A few examples are given in Table 19.1. 

Hence, the phenomena of the low reaction rate in the polymer matrix cannot be explained by the limiting rate of reactant orientation (rotational diffusion) in the cage. This result becomes the impetus to formulate the conception of the rigid cage of polymer matrix [16-20]. In addition to the experiments with comparison of the rate constants in the liquid phase and polymer matrix, experiments on the kinetic study of radical reactions in polymers with different amounts of introduced plasticizer were carried out [7,9,15,21], A correlation between the rate constant of the reaction k and the frequency of rotation vOT of the nitroxyl radical (2,2,6,6-tetramethyl-4-benzoyloxypiperidine-/Y-oxyI) was found. The values of the rate constants for the reaction... 

Thus the quantum yield for acid production from triphenylsulfonium salts is 0.8 in solution and about 0.3 in the polymer 2 matrix. The difference between acid generating efficiencies in solution and film may be due in part to the large component of resin absorption. Resin excited state energy may not be efficiently transferred to the sulfonium salt. Furthermore a reduction in quantum yield is generally expected for a radical process carried out in a polymer matrix due to cage effects which prevent the escape of initially formed radicals and result in recombination (IS). However there are cases where little or no difference in quantum efficiency is noted for radical reactions in various media. Photodissociation of diacylperoxides is nearly as efficient in polystyrene below the glass transition point as in fluid solution (12). This case is similar to that of the present study since the dissociation involves a small molecule dispersed in a glassy polymer. 

Each ion-radical reaction involves steps of electron transfer and further conversion of ion-radicals. Ion-radicals may either be consnmed within the solvent cage or pass into the solvent pool. If they pass into the solvent pool, the method of inhibitors will determine whether the ion-radicals are prodnced on the main pathway of the reaction, that is, whether these ion-radicals are necessary to obtain the hnal prodnct. Depending on its nature, the inhibitor may oxidize the anion-radical or reduce the cation-radical. Thns, quinones are such oxidizers whereas hydroquinones are reducers. Because both anion and cation-radicals are often formed at the first steps of many ion-radical reactions, qninohydrones— mixtures of quinones and hydroquinones—turn out to be very effective inhibitors. Linares and Nudehnan (2003) successfully used these inhibitors in studies on the mechanism of reactions between carbon monoxide and lithiated aromatic heterocycles. 

Where X represents the surface of the catalyst. It was also proposed Ramirez et al. 2007a) that the radicals can also be generated in the surface of the solid so they are actually caged in the solid structure, subsequently reacting with the adsorbed reagents without radicals generation. Besides the indicated steps many other radical reactions can occur similar to those described above (see reactions 3 and 4 among others). In the presence of a substrate Fenton chemistry is very complex and can involve reactions like those represented by equations (14), (15) and (16). 

Both radical reaction products are believed to be solvent cage reactions for the following reasons ... 

Clear-cut evidence on the intermolecular nature of the excitation step and the need for in-cage radical annihilation has been gained from a study on the solvent-cage effect in the peroxyoxalate reaction. The viscosity effect on the singlet quantum yields was verified using the binary solvent system toluene-diphenyhnethane. A rise in solvent viscosity from 0.50 cP (toluene) to 2.67 cP (diphenylmethane) leads to an up to tenfold increase in the quantum yields, demonstrating the intermolecular nature of the excitation step. 

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