Radicals decay

Time-resolved optical absorption spectroscopy experiments have shown that arenesul-fonyl radicals decay with clean second-order kinetics14 the values of 2 k,/a h where s2 is the extinction coefficient at the monitoring wavelength, increased linearly with decreasing viscosity of the solvent, further indicating that reaction 16 is clearly a diffusion-controlled process. 

The radical decay according to this equation is depicted by curve ABF in Fig. 17. Observation of the decay in the polymerization rate immediately following cessation of illumination offers an alternative method for determining r, for it follows from Eq. (53) that the slope of the ratio Rp)s/Rp plotted against t should equal 1/r,. 

The above factors impose a severe limitation on the use of in situ electrochemical epr as a possible means of establishing the kinetics and mechanism of radical decay. As a consequence, a great deal of effort has been expended in trying to improve the electrochemical behaviour of the epr cell and to design a system that allows the lifetimes and kinetic modes of radical decay to be determined, as well as the identity of the radical. Up until recently these objectives appeared mutually exclusive and led to two alternative methodologies ... 

The peroxyl and sulfonyl radicals absorb light in different regions of UV and visible light Amax(R02 ) = 260 nm and Amax(RS 02) = 360 nm. Such concentrations of the reactants were chosen to create the ratio of initial concentrations of radicals [R02 ]o [RS 02]o- Therefore, the kinetics of sulfonyl radicals decay obeys the first-order equation ... 

The cyclohexadienyl radicals decay by second-order kinetics, as proven by the absorption decay, with almost diffusion-controlled rate (2k = 2.8 x 109 M 1 s 1). The cyclohexyl radicals 3 and 4 decay both in pseudo-first-order bimolecular reaction with the 1,4-cyclohexadiene to give the cyclohexadienyl radical 5 and cyclohexene (or its hydroxy derivative) (equation 15) and in a second order bimolecular reaction of two radicals. The cyclohexene (or its hydroxy derivative) can be formed also in a reaction of radical 3 or... 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

The primary product of benzyl radical decay appears to be benzaldehyde [33,61] ... 

In l,l,l,3,3,3-hexafluoropropan-2-ol, the reaction of l,4-dimethoxy-2,3-dimethylbenzene with a deficit of nitrogen dioxide gives a high concentration of the aromatic cation-radical, which lives long enough and can be detected spectroscopically. In the presence of excessive amounts of N02, this cation-radical decays rapidly giving the 5-nitro derivative of the starting compound (Eberson etal. 1996). 

Radical decay kinetics have been shown to be 3/2 order, falling to first order, and also second order, falling with time deviations are apparently due to side reactions of 118. Radical half-lives are strongly influenced by the nature of the aryl substituents, being particularly short for ortho-substituted Ar because of inhibited delocalization. The corresponding compounds 39 have, accordingly, enhanced thermal stability, a factor useful in some commercial thermo- and photographic processes. 

Triphenylmethyl hydroperoxide appeared to react relatively slowly with ceric ion and the peroxy radicals decayed extremely rapidly, probably by the unimolecular decomposition faCOO — < 3C + 02 (3, 21, 28). Only traces of peroxy radicals could be detected even at high flow rates, and so the decay kinetics could not be examined. 

Figure 1. Variation in the concentration of radicals in poly(methyl methacrylate) irradiated at 77° and warmed progressively to 370°K. The numbers indicate the temperature range where different radicals decay.

Figure 10.9 Sector technique for the determination of free radical decay times ...

Recently, experiments have been reported where the time dependence of the radical survival probability has been measured. Not only is the (long time) escape or recombination probability measured, but also the time scale over which the initial concentration of radicals decays to the final radical concentration has been noted [266—68]. Such studies provide extremely valuable additional information, because the time scale for reaction is the time scale it takes for the radicals to diffuse together again and hence these experiments give some insight into the distribution of initial separation distances. For instance, radicals separated by r0 1 nm take rl/6D 0.16 ns to diffuse together in a solvent of diffusion coefficient 10 9 m2 s-1. Once the theory of radical recombination has been discussed in the remainder of this section, these time-dependent studies will be reconsidered in Sect. 3. 

In the absence of amine both D + and D " appear simultaneously with the decay of the triplet. The absorptions due to the radicals decay to zero within one millisecond. 

A.I. Mikhailov and V.A. Anikolenko, Low Temperature Electron Transfer at Ion Radicals Decay. Investigation of Electron Wave Function Damping, Inst. Chem. Phys. USSR Academy of Sciences, Chernogolovka, 1977 (in Russian). 

Flowing Solutions. When higher current densities are required, the mass transport can be supplemented by flowing solution through the cuvette. Dohrmann and Vetter [25] explored this arrangement to determine the minimum lifetime, reported as t/AH (k = 1/t is the rate for first-order radical decay and AH is the peak-to-peak EPR line width), of electrogenerated radicals that could be detected. Their approximate calculations yielded values from 10 2 to 10-4 s/mT, depending on considerations of flow rate, current density, electrode area, electrolyte conductance, and substrate concentration, for a spectrum of one line. 

The majority of FPTRMS investigations have been of the reaction of a free radical with an excess of a stable molecule, making the radical decay pseudo-first order. Of this class of reaction, the most frequently studied has been association with 02 to form a peroxy radical. These reactions are of importance in combustion and in the atmosphere. Bayes and coworkers have reported a series of FPTRMS investigations of radical/02 association reactions, following the decay of ions formed by photoionization of the radicals. 

Reaction of singlet cyanoanthracenes with t-1 or c-1 in polar solvents results in the formation of stilbene cation radicals (see Section VII.A). In the absence of oxygen, the t-1 cation radical decays by back electron transfer to the cyanoanthracene anion radical without undergoing isomerization. In contrast, the c-1 cation radical undergoes isomerization with concentration dependent quantum yields which can exceed 1.0 to yield a photostationary state consisting of 99% t-1 and 1% c-1 (27). The selective isomerization of c-1 but not t-1 is... 

The thermal decay process of the radicals is a bimolecular reaction. The decay rate increases with increasing temperature. At 100° C, a half-life of 12 minutes has been observed for methyl methacrylate popcorn radicals. A fast decay rate takes place when the dry popcorn is swollen in a liquid such as benzene. That means that during the proliferous growth process, when the polymer is swollen by the monomer, a decay process also occurs, and a stationary radical concentration in the growing polymer popcorn results. A liquid that does not swell the polymer (for example, methanol for polystyrene) does not influence the decay rate. A much higher rate of radical decay is obtained with a benzene solution of diphenylpicrylhydrazil. The reaction rate between the polymer radical and inhibitor radical may be measured. 

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