Primary cage effect

Figure 3. Calculated efficiencies. (1) From the cage effect model and no primary radical termination (Case I) (2) From the assumption of an overall efficiency and no primary radical termination (Case II) (3) From the assumption of an overall efficiency and primary radical termination (Case III) ( l) Calculated from equation (A) with fo - 0.663.

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. 

Initiator decomposition studies of AIBN in supercritical C02 carried out by DeSimone et al. showed that there is kinetic deviation from the traditionally studied solvent systems.16 These studies indicated a measurable decrease in the thermal decomposition of AIBN in supercritical C02 over decomposition rates measured in benzene. Kirkwood correlation plots indicate that the slower rates in supercritical C02 emanate from the overall lower dielectric constant (e) of C02 relative to that ofbenzene. Similar studies have shown an analogous trend in the decomposition kinetics ofperfluoroalkyl acyl peroxides in liquid and supercritical C02.17 Rate decreases of as much as 30% have been seen compared to decomposition measured in 1,1,2-trichlorotrifluoroethane. These studies also served to show that while initiator decomposition is in general slower in supercritical C02, overall initiation is more efficient. Uv-visual studies incorporating radical scavengers concluded that primary geminate radicals formed during thermal decomposition in supercritical C02 are not hindered to the same extent by cage effects as are those in traditional solvents such as benzene. This effect noted in AIBN decomposition in C02 is ascribed to the substantially lower viscosity of supercritical C02 compared to that ofbenzene.18... 

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... 

Thus in the condensed phases the collision frequency, cage effect, primary excitation delocalization, and rapid excitation energy transfer will all tend to negate the chemical effects of excitation in saturated hydrocarbons. 

The highly excited states of molecules produced by high-energy radiation that arc chemically important are mainly the ionic states because of the rapidity of internal conversion processes. Primary excitation is relatively unimportant while secondary excitation is quite common. In the condensed phases energy dissipation is very rapid because of colli-sional deactivation, the cage effect, and excitation energy transfer processes all of which act to negate the chemical effects of secondary excitation,... 

The quantum yield for the primary photochemical process differs from that of the end product when secondary reactions occur. Transient species produced as intermediates can only be studied by special techniques such as flash photolysis, rotating sector devices, use of scavengers, etc. Suitable spectroscopic techniques can be utilized for their observations (UV, IR, NMR, ESR, etc.). A low quantum yield for reaction in solutions may sometimes be caused by recombination of the products due to solvent cage effect. 

The least known of these reactions is chain initiation together with the efficiency of the primary radicals. This group consists of at least three elementary reactions. The so-called cage effect certainly plays an important role. On the other hand, the cage effect in its classical form cannot explain all phenomena sufficiently. 

However, the spatial inhomogeneity in the distribution of reagents is not the only reason why the radiolysis of substances in the condensed state is different from that of gases. As we have already mentioned in Section VIII, as we pass from the gaseous state to the condensed one, at the primary stage of radiolysis we already observe a redistribution of yields of primary active particles (resulting in the increase of the yield of ionized states). Also different are the subsequent relaxation processes, as well as the processes of decay of excited and ionized states.354 Another specific feature of processes in a condensed medium is the cage effect, which slows down the decay of a molecule into radicals.355 Finally, the formation of solvated electrons is also a characteristic feature of radiation-chemical processes in liquids.356... 

For this reeuson, any si)ecific "polymer effects", if indeed they do occur, must be attributed to processes occurring outside the primary cage. Secondary cage recombination, for example, will be affected by the rate of diffusion in the polymer matrix. This might be expected to reduce the number of radicals which can escape the region associated their primary partners and become true "free" radicals. 

Carbon tetrachloride could be formed by the abstraction of a chlorine atom from a hexachloroacetone molecule by a trichloromethyl radical tetrachloroethylene could then result from the dimerisation of dichlorocarbene radicals produced from the dissociation of pentachloroacetonyl radicals. Haszeldine and Nyman identified trichloroacetyl chloride and octachloropropane as products of the liquid phase photolysis, suggesting a primary step of Type 3 involving rupture of the carbon-halogen bond. Photolysis in the liquid phase was found to be very slow, and this has been attributed to cage effects and recombination of radicals formed in the primary step. 

In order to inhibit degradation by the radical scavenging, two approaches were studied one is to let an excipient to play the role ofa radical scavenger, so protecting the drug from the radical attack, the second is to freeze the solution into a solid state where the cage effect favours the recombination of primary radicals ofthe solvent prior to the reaction with the drug. 

In fact, energy, necessary for primary formation of radicals at polymer photolysis, is greater than the energy of bond dissociation, that is explained by the presence of cage effect in polymers [8]. 

Practical free-radical polymerizations often deviate from Eq. (6.126) because the assumptions made in the ideal kinetic scheme are not fuUy satisfied by the actual reaction conditions or because some of these assumptions are not valid. For example, according to the ideal kinetic scheme that leads to Eq. (6.126), the initiation rate (Rf) and initiator efficiency (/) are independent of monomer concentration in the reaction mixture and primary radicals (i.e., radicals derived directly from the initiator) do not terminate kinetic chains, thoughrifi reality R may depend on [M], as in the case of cage effect (see Problem 6.7) and, at high initiation rates, some of the primary radicals may terminate kinetic chains (see Problem 6.25). Moreover, whereas in the ideal kinetic scheme, both kp and kt are assumed to be independent of the size of the growing chain radical, in reahty k[ may be size-dependent and diffusion-controlled, as discussed later. 

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