Geminate recombination kinetic rate constants

Tanaka et al. studied the decay reactions of PVB radical anions produced by electron pulses in MTHF [47]. At low concentration ( < 0.05 base-mol dm - 3) of polymers the decay reaction followed a simple second-order kinetics. The charge neutralization reaction is responsible for the decay curve as is the case of biphenyl radical anions. However, the rate constant of the polymer anions was only a half or one-third of that of the biphenyl anion, because of the small diffusion coefficient of the polymer ion in solution. At high concentration of the polymer, a spike was observed in the time-profile of the PVB anion this was attributed to the retarded geminate recombinations within micro-domains where the polymers were entangled with each other. 

The mechanism can be best understood within the framework of the conventional theory of radical chain kinetics, provided that certain of the usual simplifying assumptions are omitted. A solution is given to the problem of steady-state polymerization rate as a function of monomer and initiator concentration, taking into account termination reactions of primary radicals and recombination of geminate chains arising from the same initiation event. This model is shown to account for the kinetic data reported herein. With appropriate rate constants it should be generally applicable to radical polymerizations. 

The first-order combination rate constants from in-cage processes, 3 x 10 s, were determined from the initial slopes of the fast components of the kinetic curves. They remain nearly constant in films with 30-100 wt% of PCV. The activation energy of Iq-i is very small, 8 kJ/mol, suggesting that geminal recombination in the PVC films is limited only by the rotational mobility of the radicals in their cages. Furthermore, was insensitive to the application of a strong external magnetic field... 

Other applications of laser flash photolysis have included (a) a study of the bimolecular rate constant for the reaction between singlet oxygen and several lipid-soluble substances,(b) an investigation of quenching, solvent, and temperature effects on the photolysis of indoles, (c) the time dependence of the quenching of aromatic hydrocarbons by tetramethylpiperidine TV-oxide, and d) the kinetics of the geminate recombination of aromatic free radicals. Finally, flash photolysis has been utilized in order to examine the feasibility of a tunable IF laser (479—498 nm) and an ICl laser (430 nm). ... 

It is interesting to compare O2, CO, and NO molecules in this respect (Blomberg et al. 2005 Friedman and Campbell 1987 Strickland and Harwey 2007). The CO recombination with heme is a single exponential process characterized by a slow rate constant (k 10 s ) at the ambient temperature and a low solvent viscosity (below viscosity of globin). This intrinsic (geminate) recombination rate is slower than both protein relaxation and CO escape, thus the recombination yield is very small (0.04) (Shikama 2006). The rebinding of the NO stable radical (doublet ground state, S = 1/2) is characterized by two-exponential kinetics with the rapid (ki 10 s ) and slow (k2 5 10 s ) rate constants under ambient conditions (Friedman and Campbell... 

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