Recombination, radical Cage effects

Radical cage effect and coupling (recombination) Radical coupling reactions do not dominate free radical chemistry as most radicals have very short lifetimes and are present in very low concentrations. Consequently, if short-lived radicals are to contribute to useful synthetic procedures by way of a radical coupling, all the events leading up to the coupling must take place in a solvent cage. 

Recombination reactions between two different macroradicals are readily observable in the condensed state where molecular mobility is restricted and the concentration of radicals is high. Its role in flow-induced degradation is probably negligible at the polymer concentration normally used in these experiments (< 100 ppm), the rate of radical formation is extremely small and the radicals are immediately separated by the velocity gradient at the very moment of their formation. Thus there is no cage effect, which otherwise could enhance the recombination efficiency. 

The efficiency of the intitiator is a measure of the extent to which the number of radicals formed reflects the number of polymer chains formed. Typical initiator efficiencies for vinyl polymerisations lie between 0.6 and 1.0. Clearly the efficiency cannot exceed 1.0 but it may fall below this figure for a number of reasons, the most important being the tendency of the newly generated free radicals to recombine before they have time to move apart. This phenomenon is called the cage effect . 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

The influence of pressure on the cage effect was studied by Neuman and colleagues [95-98]. They measured the influence of pressure on the cage effect for competition between recombination and diffusion for the 1,1-dimethylethoxy radical pairs generated from bis(l,l-dimethylethyl)hyponitrite. The empirical activation volume difference (AF(f for the... 

The cage effect was also analyzed for the model of diffusion of two particles (radical pair) in viscous continuum using the diffusion equation [106], Due to initiator decomposition, two radicals R formed are separated by the distance r( at / = 0. The acceptor of free radicals Q is introduced into the solvent it reacts with radicals with the rate constant k i. Two radicals recombine with the rate constant kc when they come into contact at a distance 2rR, where rR is the radius of the radical R Solvent is treated as continuum with viscosity 17. The distribution of radical pairs (n) as a function of the distance x between them obeys the equation of diffusion ... 

This is the result of cage effect. The cage of a solid polymer matrix is rigid (see earlier) and the most part of the forming radical pairs recombine in the cage. Hence, the probability of... 

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. 

Let us assume that fci is equal to k9, the rate constant for the gas phase decomposition (15), where no cage effect is expected. This assumption does not always hold (15, 18). For example, it is known (18) that di-f erf-butyl peroxide (DPB) decomposes about 30% slower in the gas phase than in solution. We can calculate from our value of k8 and the known value of kg, from the work of Szwarc (7, 21), a value for the fraction of acetoxy radical pairs recombining, fR, where... 

The cage effect, or the enhanced probability for recombination of two radicals formed in close proximity, was discussed by Franck and Rabinowitch in 1934, and Rabinowitch and Wood used a pinball illustration of this process (Fig. 5). The theory of this reaction was further developed by Noyes. ... 

The very slow rate of photolysis of hexachloroacetone in the liquid phase is attributed to cage effects and recombination of the primary radicals. This is supported by the fact that the rate of disappearance of hexachloroacetone is accelerated by the addition of oxygen which would combine with the radicals as they were formed. A second consequence of... 

It may be assumed that in absence of oxygen, a large fraction of. CCi3 radicals will recombine due to solvent cage effect to give the addition product trichloro-dihydroanthracene I. This compound is expected to be unstable and eliminates HC1 in a number of ways. 

With the classifications of spatially correlated reactant recombinations and the cage effect in mind, the effects discussed in Sect. 3.1 are largely due to the spatial correlation. Indeed, changing the solvent viscosity by applying pressure or changing the temperature, or the radical reactivity should have little effect on the potential of mean force and not much on the extent and range of hydrodynamic repulsion. 

Radicals initially formed in solution by a bond homolysis will be held together briefly in a cage of solvent molecules. Because radical recombinations and disproportionations are so fast, they can compete with diffusion of the radicals through the layer of solvent molecules that surround them, with the consequence that some of the radicals formed never become available to initiate other processes in the bulk of the solution.98 These recombinations are termed geminate recombinations, and the phenomena that arise from this behavior are cage effects. 

We suggest that the cyclodextrin cavity provides an environment wherein recombination of the geminate radical pair (from Type I) is favored and this results in lower yield of products from Type I process relative to Type II. Experiments are underway to test this cage effect with other examples. 

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