Second-order processes radical ions

An increase in the cA-stilbene concentration favors the chain propagation and decreases the probability of termination when the DCNA anion-radicals react with the stilbene cation-radicals. A decrease in the irradiation intensity has a similar effect The chain propagation is the first-order process, whereas termination of the chains is the second-order process. A temperature rise accelerates the accumulation of the stilbene cation-radicals. In this system, the free energy of electron transfer is -53- —44 kJ moD (the cation-radical generation is in fact an endothermal process). If a polar solvent is substituted for a nonpolar one, the conversion of the cii-stilbene cation-radical into the trani-stilbene cation-radical deepens. Polar solvents break ion pairs, releasing free ion-radicals. The cA-stilbene cation-radicals isomerize more easily on being released. The stilbene cation-radical not shielded with a counterion has a more positive charge, and therefore, becomes stabilized in the... 

The biperoxy radical produced by the ceric ion oxidation of 2,5-di-methylhexane-2,5-dihydroperoxide decays rapidly with first-order kinetics [k = ioio.e exp( -11,500 1000)/RT sec.1 = 180 sec."1 at 30°C. (30)]. After the first-order decay has run to completion, there is a residual radical concentration (—4% of the initial hydroperoxide concentration) which decays much more slowly by a second-order process. The residual second-order reaction cannot be eliminated or changed even by repeated recrystallization of the dihydroperoxide. This suggests that a small fraction of the biperoxy radicals react intermolecularly rather than by an intramolecular process and thus produce monoperoxy radicals. The bimolecular decay constant for this residual species of peroxy radical is similar to that found for the structurally similar radical from 1,1,3,3-tetra-methylbutyl hydroperoxide. Photolysis of the dihydroperoxide gave radicals with second-order decay kinetics which are presumed to be 2,5-hydroperoxyhexyl-5-peroxy radicals. 

In these circumstances, where routine kinetic measurements are uninformative and direct measurements of the product-forming steps difficult, comparative methods, involving competition between a calibrated and a non-calibrated reaction, come into their own. Experimentally, ratios of products from reaction cascades involving a key competition between a first-order and a second-order processes are measured as a function of trapping agent concentration. Relative rates are converted to absolute rates from the rate of the known reaction. The principle is much the same as the Jencks clock for carbenium ion lifetimes (see Section 3.2.1). However, in radical chemistry Newcomb prefers to restrict the term clock to a calibrated unimolecular reaction of a radical, but such restriction obscures the parallel with the Jencks clock, where the calibrated reaction is a bimolecular diffusional combination with and the unknown reaction a pseudounimolecular reaction of carbenium ion with solvent. Whatever the terminology, the practical usefulness of the method stems from the possibility of applying the same absolute rate data to all reactions of the same chemical type, as discussed in Section 7.1. 

Neutralization processes of ions in the radiolysis of ethane or ethylene with SFg have been shown to lead to the formation of the SF5 radical. Compounds of the type RSF5 are formed as a result of the recombination reactions with hydrocarbon radicals. A kinetic e.s.r. study of the self-reaction of SF5, and a spectroscopic and kinetic e.s.r. study of its reaction with 1,1-di-t-butylethylene, have been reported. The radical undergoes self-reaction by a second-order process and adds to 1,1-di-t-butylethylene to give BU2CCH2SF5, which decomposes by a first-order process. 

If the EDA and CT pre-equilibria are fast relative to such a (follow-up) process, the overall second-order rate constant is k2 = eda c e In this kinetic situation, the ion-radical pair might not be experimentally observed in a thermally activated adiabatic process. However, photochemical (laser) activation via the deliberate irradiation of the charge-transfer absorption (hvct) will lead to the spontaneous generation of the ion-radical pair (equations 4, 5) that is experimentally observable if the time-resolution of the laser pulse exceeds that of the follow-up processes (kf and /tBet)- Indeed, charge-transfer activation provides the basis for the experimental demonstration of the viability of the electron-transfer paradigm in Scheme l.21... 

Increasing the solvent polarity results in a red shift in the -t -amine exciplex fluorescence and a decrease in its lifetime and intensity (113), no fluorescence being detected in solvents more polar than tetrahydrofuran (e = 7.6). The decrease in fluorescence intensity is accompanied by ionic dissociation to yield the t-17 and the R3N" free radical ions (116) and proton transfer leading to product formation (see Section IV-B). The formation and decay of t-17 have been investigated by means of time resolved resonance Raman (TR ) spectroscopy (116). Both the TR spectrum and its excitation spectrum are similar to those obtained under steady state conditions. The initial yield of t-1 is dependent upon the amine structure due to competition between ionic dissociation and other radical ion pair processes (proton transfer, intersystem crossing, and quenching by ground state amine), which are dependent upon amine structure. However, the second order decay of t-1" is independent of amine structure... 

Radiolysis Mechanisms. In the earlier work (34) with water vapor a number of parameters relevant to understanding the experimental system were discussed in detail. These are summarized in Table I for H20 and D20. Briefly, it can be calculated that given a pressure in the irradiated vapor stream of — 0.05 torr, second- or third-order processes of radical decay cannot occur in the vapor phase on a time scale < 10 r> sec. On the other hand, ion-molecule reactions with a rate constant k — 5X 10"10 cc./molecule-sec. (16), could have reaction half-lives of this order and may go to completion before condensation at 77 °K. However, it is possible for radical-scavenger reactions to occur in this system. For example, it was shown that 0.14 mole % CH3I reacts readily with electrons produced from irradiated water vapor to give CHS radicals (34). We suggested that this reaction may occur in the transient liquid phase immediately before solidification at 77 °K. Irrespective of the phase where such a reaction occurs, it can be used for detecting the presence of electrons. 

The interaction of OH radical with seawater yields oxidized bromine species. The identity of the oxidized bromine species ahd its subsequent reactions have been studied by extensive flash photolysis experiments and also by pulse radiolysis. OH appears to give a nearly quantitative yield of the dibromide ion-radical in seawater. This radical decays by parallel first- and second-order reactions, the latter process being well-known but irrelevant in nature. 

Thus the formation of OH in seawater seems on careful examination to lead to very rapid oxidation of bromide, but this reactivity is in turn dissipated within a millisecond by reactions with other seawater constituents, apparently involving the carbonate system in a pH-dependent manner. This process, not the second-order self-decay due to Rll observed at high light intensities, undoubtedly predominates in natural sunlight. However, no reactions of the dibromide ion-radical with carbonate species have been reported, and the nature of this interaction is not clear. The simplest plausible reaction might be ... 

When the radical cation and radical anion decay completely after laser pulse, the generated radical ion pair returns to the corresponding neutral ground state by the back electron-transfer process [60]. When the solvent is highly polar, the generated radical ions are solvated as free radical ions thus, the back electron transfer obeys second-order kinetics [Eq. (6)]. On the other hand, in the less-polar solvents, the radical ions are present as geminate ion pairs thus, the back electron transfer obeys first-order kinetics [Eq. (7)] ... 

By analysis of the data of the dependence of signal on time during electrolysis it is possible by means of equations for homogeneous chemical kinetics to calculate the reaction order, the reaction rate constant, and the half-life for the primary process involved in cleavage of the radical ions [76]. It is thus possible to determine kinetic parameters for the second-order cleavage of radical anions with half-lives not shorter than 1 sec at an initial concentration of Co = 540 mole/liter. 

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