A re-investigation of the reactions between superoxide anion and metal picolinate complexes

Rate constants for the reactions of superoxide with the α-picolinate ion and its complexes with copper(II), iron(III) and zinc(II), and for the reaction of α-picolinate with the hydrated electron, were measured using pulse radiolysis. The rate constant for the reaction of superoxide with copper(II)picolinate at pH 9 [(4.1 ± 0.4) × 107l mol−1 s−1] was an order of magnitude higher than that determined previously (W. H. Bannister, J. V. Bannister, A. J. F. Searle and P. J. Thornally, Inorg. Chim. Acta, 78, 139 (1983)) using a less direct competitive inhibition method. The corresponding rate constant for iron(III)picolinate [(7.5 ± 1.5) X 103 l mol−1 s−] was an order of magnitude lower than a previous pulse radiolysis determination (same reference as above). We are not able to reconcile these two values for iron(III)picolinate, although a possible source of spuriously high results is contamination with the kinetically active copper(II) complex. The likely roles of iron(III)picolinate and other low molecular weight iron complexes as potential catalysts of an in vivo superoxide-driven Fenton reaction are discussed in the light of present measurements

Decay kinetics of the ultraviolet and visible luminescences emitted by electron-irradiated crystalline H2O ice

Electron pulse irradiated samples of high purity, crystalline H2O ice at 88 K showed three kinetically distinguishable regions of luminescence emission at 280–340 nm (band I); 320–600 nm (band II); and 450–600 nm (band III). Band II emission was assigned to the A 2∑+→X 2Π transition of OH, the gas phase peak being shifted from 306.4 to ∼385 nm by the ice lattice. The decay half‐life of the band II emission resulting from a single, ∼0.05 Mrad electron pulse, was 25±3 ns and increased steeply to 210±10 ns for the second pulse and then steadily decreased to 140±10 ns after 20 pulses. Band II emission from the second or later pulses was resolved into a short lived component with a decay half‐life of ∼30 ns and a longer lived component with a half‐life of ∼400 ns. The latter decay fitted a second order homogeneous rate equation in which the initial concentrations of the two reactants were in the ratio (2.6±0.1):1 and was attributed to the formation of excited OH by electron–ion recombination in the bulk ice. The short lived band II emission was also attributed to excited OH and probably arose from a mixture of a fast intraspur recombination reaction with some other process of different reaction order. Dose accumulation (memory) effects were attributed to the accumulation of OH radicals and lattice vacancies in the irradiated ice. The band III emission had a half‐life of 25±5 ns and its decay kinetics were consistent with emission from species such as excited OH− or H3O produced when electrons tunnel from a trapping site to a geminate partner

INVESTIGATION OF THE LUMINESCENCE EMITTED BY PULSE-IRRADIATED D2O ICE

De la glace de D2O purifiée a été irradiée par des pulses électroniques de 0,53 MeV et les distributions spectrales de la luminescence ainsi que la durée de vie ont été enregistrées entre 200 et 800 nm. Les pics ont été observés à 390 et 540 nm et il apparaît qu'aucune de ces émissions n'est due aux impuretés. L'émission est plus intense dans le cas de la glace D2O que dans celui de la glace H2O pour tout le domaine de longueur d'onde et les durées de vie dans le cas de la glace D2O varient de 170 ns à 390 nm jusqu'à 60 ns à 540 nm et sont plus grandes que les valeurs moyennes correspondantes, 150 ns et 25 ns pour la glace H2OPurified D2O ice was irradiated with pulses of 0.53 MeV electrons and the spectral distributions of the luminescence intensity and half-life were recorded between 200 and 800 nm. Emission peaks were observed at 390 and 540 nm and it was shown that none of these emissions was caused by luminescent impurities. The emission from D2O ice was more intense than from H2O ice at all wavelengths above 370 nm. The half-lives of the luminescence decay in D2O ice varied from 170 ns at 390 nm to 60 ns at 540 nm and were longer than the corresponding values of ca. 140 ns and 25 ns for H2O ice

Chain-length-dependent termination rate processes in free-radical polymerizations. 3. Styrene polymerizations with and without added inert diluent as an experimental test of model

Experiments were performed to test a model for the kinetics of free-radical polymerization systems, including the dependence of the termination rate coefficients on the lengths of both chains involved. The model has few adjustable parameters, the values of which are moreover confined within fairly narrow limits. The data comprised the rate of polymerization in a seeded emulsion polymerization of styrene, with and without benzene as diluent, with initiation by persulfate and by gamma-radiolysis. The latter can be switched off instantly, providing relaxation data which are sensitive to termination kinetics. Data from a single relaxation at a fixed weight-fraction polymer (omega(p)) were fitted to fur the unknown parameters, of which the only significant one is the probability p of reaction between two radicals upon encounter, incorporating the effect of spin multiplicity; this must lie between 0.25 and 1. Modeling using the value so obtained then successfully fitted (a) relaxation data at the same omega(p) but with 15 mol % benzene diluent, (b) relaxation data with and without diluent over the range 0.5 less than or equal to omega(p) less than or equal to 0.8, and (c) chemically initiated data over the same omega(p) range. This provides convincing evidence for the correctness of the termination model, which calculates the termination rate coefficients between two chains from the Smoluchowski equation, incorporating p, with diffusion coefficients (as a function of chain length and of omega(p)) obtained from a ''universal'' scaling law inferred from NMR data, and where the interaction distance for termination is the van der Waals radius of a monomeric unit; contributions from ''reaction diffusion'' (whereby a chain end moves by propagating) are also important at high conversion. The data also support a model for initiator efficiency in emulsion polymerization, this model being based on competition between aqueous-phase propagation (to a sufficient degree of polymerization for surface activity) and termination

Desorbed free radicals in emulsion polymerizations: Effect of aqueous-phase spin trap

Radical loss events by desorption (exit) in seeded styrene emulsion polymerization systems are studied by observing (1) the relaxation behavior when polymerization is initiated by γ radiolysis and then rapidly removed from the radiation source and (2) the same radiolysis relaxation experiment with the addition of an aqueous-phase free-radical trap (Fremy's salt). The results are quantitatively consistent with desorbed free radicals being rapidly terminated by Fremy's salt before there is any opportunity for them to reenter another particle and with the transfer-diffusion model for exit. The Fremy's salt technique is a general one which should be able to be used to establish the fate of free radicals in a wide range of emulsion polymerization systems