Decay Kinetics Of Radicals Research Articles

Structure–reactivity studies on the crosslinking of poly(vinyl methyl ether) in aqueous solutions: a pulse radiolysis study

Pulse radiolysis techniques have been utilized to generate, characterize and study the kinetics of crosslinking reactions of the radicals of a temperature-sensitive polymer poly(vinyl methyl ether)(PVME) in aqueous solutions. The transient species formed on reaction with ˙OH has an absorption maximum at 310 nm with molar absorptivity (ε)= 1060 ± 330 dm3 mol–1 cm–1. The rate constant for the reaction of ˙OH with PVME has been determined as 3.2 × 108 dm3 mol–1 s–1 by competition kinetics at 296 K. The decay profile of the transient species has been found to vary with the polymer concentration and/or dose rate indicating different modes of recombination under various conditions of irradiation. These results have been interpreted in terms of intra- and inter-molecular crosslinking reactions of polymer radicals and it has been shown that when the intramolecular crosslinking reaction predominates, the decay kinetics deviates from the classical second order and shows a time-dependent behaviour. Since the polymer undergoes a coil–globule transition above 307 K, the bimolecular rate constant for the reaction of ˙OH with the polymer and the decay kinetics of radicals formed have also been investigated at 318 K.

Effect of Surfactant Nature on Decay Kinetics of Radicals from 1,2-Dihydro-2,2,4,6-tetramethylquinoline in Micellar Solutions

Effect of Surfactant Nature on Decay Kinetics of Radicals from 1,2-Dihydro-2,2,4,6-tetramethylquinoline in Micellar Solutions

Kinetics of Radical Decay and Molecular Dynamics in Oriented High Modulus Polyethylene

Abstract The kinetics of decay of macroradicals in irradiated ultradrawn (draw ratio 200) high modulus polyethylene, single crystal mats of polyethylene and polyethylene with extended chain crystals was studied comparatively by the ESR technique. The results were analyzed from the viewpoint of correlations between the microstructure and temperature-dependent chain dynamics of the polymers. The actual absence of appreciable radical decay in the temperature range 200–300 K and a remarkable stability of considerable fraction of initial radicals at higher temperatures in high modulus sample were explained by extremely restricted motion of the fragments of unfolded tie molecules in trans-conformations predominating in the amorphous region of this polymer. In addition, direct evidences for drastic conformational changes in amorphous regions of polyethylene as a result of ultradrawing obtained by IR-spectroscopy are reported.

Fourier transform-EPR studies of photoinduced electron transfer in homogeneous and micellar solutions

Fourier transform (FT) EPR was used to study the pulsed-laser-induced electron transfer from porphyrins to quinones in homogeneous and micellar solutions. By monitoring the EPR signal of the quinone anion radicals as a function of delay time (τd) between laser and microwave pulses, with τd ranging from nanoseconds to 1 millisecond, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. FT-EPR spectra of electron transfer products generated in micellar solution provide evidence for the generation of long-lived spincorrelated radical pairs.

FT-EPR study of electron spin polarization in the acetone ketyl radical formed by photolysis of tert-butylperoxide in 2-propanol

The kinetics of formation and decay of acetone ketyl radicals produced by photocleavage of tert-butylperoxide in 2-propanol and the generation of electron spin polarization has been investigated with Fourier-transform electron paramagnetic resonance (FT-EPR). It is found that the evolution of the FT-EPR spectra over a time domain ranging from nanoseconds to 500 μs can be completely explained in terms of: (1) kinetics of radical formation and decay, (2) relaxation of tert-butoxy radical precursors — born with zero spin polarization via the singlet excited state of the peroxide — to thermal equilibrium, (3) spin polarization created by the radical pair mechanism involving random encounters of ketyl radicals (F-pair CIDEP), and (4) spin-lattice relaxation of the ketyl radicals. A detailed numerical analysis of the time dependence of the amplitudes of the central three hyperfine components in the spectrum of the ketyl radical gives a value of 85±30 ns for the spin—lattice relaxation time of the tert-butoxy radical. The analysis also gives the value of the pseudo-first-order hydrogen abstraction rate constant (2.5 × 10 7 s −1) and the magnitude of the F-pair spin polarization.

Determination of diffusion coefficient of oxygen into polymers by using electron spin resonance spectroscopy. II. Poly(vinyl acetate)

AbstractElectron spin resonance spectroscopy has been used to follow the kinetics of radical decay during postirradiation oxidation of poly(vinyl acetate). The oxygen uptake of γ‐irradiated spherical poly(vinyl acetate) has been determined from changes in the intensity of the ESR signal. The diffusion coefficient (D) of oxygen determined by this technique showed an inverse relation with dose. The value of the diffusion coefficient of oxygen obtained in this study, after correcting for dose and size effects is 4.5 × 10−8 cm2/s, and is in very good agreement with the literature value of D = 5.1 × 10−8 cm2/s.

Fourier transform-electron paramagnetic resonance study of the photochemical reaction of acetone with 2-propanol

Fourier transform-electron paramagnetic resonance (FT-EPR) was used to study the pulsed-laser induced reduction of acetone with 2-propanol. By monitoring the EPR signal of the acetone ketyl radical as function of delay time (τd) between laser pulse and microwave pulse, with τd ranging from nanoseconds to 100 μs, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. It was found that the spectra obtained with τd settings ranging from 0 to 400 ns contain a dispersive signal contribution that is due to the presence of spin-correlated radical pairs (SCRP) at the time of the microwave pulse. For acetone(D6) in 2-propanol(D8) the rate constants of formation and decay of the SCRP are found to be 7.5±3.7×106 and ∼5×107 s−1, respectively. The SCRP lifetime in 2-propanol(D8) at room temperature corresponds to what would be expected for diffusion controlled cage escape. The rate constant for ketyl(D7) radical formation is found to be 5.8±0.5×106 s−1. The decay rate constant kdN(0), where N(0) is the maximum radical concentration, is found to be 1.0×105 s−1. The rate constant for radical formation increases to 1.1×107 s−1 for the acetone(H6)/2-propanol(H8) system. The pronounced isotope effect on radical formation is consistent with a transition state involving C–H bond stretching. The FT-EPR spectra show the effects of radical pair CIDEP. However, a detailed analysis of signal growth and decay kinetics shows that there is no triplet mechanism CIDEP contribution. This finding is in agreement with the relative magnitudes of acetone triplet spin lattice relaxation and radical formation rate constants. The results also demonstrate that the rate constant for hydrogen abstraction is triplet sublevel independent.

Radiolysis of linear model compounds of polyamides

Six oligomers of e-aminocaproic acid (ACA) from dimer (K2) to heptamer (K7), gamma-irradiated in an oxygen-free atmosphere, were investigated by the EPR method. The oligomers were synthesized and irradiated with60Co gamma-rays in the dose range from 0 to 6.5 kGy. The formation of −CH2−CONH−CH−CH2− radical was established. The quantitative determinations of total concentrations of spins were carried out. On the basis of the present and earlier results, the competition of the reactions of detachment of hydrogen and of breaking of −CONH− bond is discussed. The particular resistance of amide bond in dimer seems to be confirmed by the measurements of kinetics of radical decay. The mass spectrometry of gamma-unirradiated ACA and ACA oligomers K2-K4 was carried out. The results indicate a minute contribution of ionic processes in the radiolysis of ACA oligomers in solid phase.

The decay kinetics of spin adducts formed by nitrosodurene with pentacarbonylrhenium(0) radical

The decay of the anilino and nitroxide radicals that form by the addition of nitrosodurene to pentacarbonylrhenium(0) radical was studied by EPR spectroscopy. The anilino radical decays by first order kinetics with Δ H ≠ 20.5 kJ/mole and Δ S ≠ − 196 J/ mole K. The nitroxide radical decay kinetics are second order with Δ H ≠ 3.92 kJ/mole and Δ S ≠ −186 J/mole K. The isomerization of the anilino to the nitroxide form was not observed in the limited temperature range in which the anilino could be detected.

Radiation chemistry of poly(ethyl- methacrylate)—II. Kinetics of free radical decay

Kinetics of free radical decay in γ-irradiated poly(ethyl methacrylate) is followed by ESR spectroscopy. Decay of components contributing to five and four line spectra was investigated between 20° and 60° C. At temperatures well below the glass transition and close to that of glass transition, decay was found to be described by second order kinetics. Activation energy of free radical decay for the quintet and quartet part of the spectrum was determined to be the same, namely 30.1 ± 5.7 kcal/mol.

Chlorination of irradiated polyethylene single crystals: 1: Analysis of alkyl radical decay kinetics

The chlorination of electron beam-irradiated polyethylene (PE) single crystals was studied for a range of irradiation doses, temperatures and chlorine interaction times. The results of this work are presented in two parts: (1) Electron paramagnetic resonance (e.p.r.) analysis of irradiated PE without chlorination and (2) characterization of chlorine uptake in irradiated PE by use of X-ray energy of spectroscopy (XES) and electron spectroscopy for chemical analysis (e.s.c.a.). Spectra were obtained for 75 – 600 Mrad doses over 136 to 333 K temperature range. Alkyl radical concentrations and decay kinetics were derived from e.p.r. spectra and provided a basis for the chlorination analysis in Part II. A first-order alkyl radical decay constant of 4.3 × 10 −5s −1 at 298 K was determined in agreement with previous research. Alkyl radical migration was modelled as a one, two or three-dimensional unsteady-state Fickian diffusion process. E.p.r. alkyl radical concentration data was used to estimate alkyl radical diffusion coefficients yielding values of 10 to 4 × 10 −18cm 2−1 at 298 K for the three models evaluated, respectively. These values are consistent with a diffusivity estimated by Seguchi and Tamura based on a 3-D model. A three-dimensional diffusion model seems to be the most realistic since it permits a zig-zag radical trajectory due to the staggered arrangement of −CH 2- units in crystalline PE. Although a 3-D model would allow diffusion in all directions, it is still consistent with the observation that consecutive migration steps are most likely intermolecular.

Transient spectra and decay kinetics of semithionine radicals in sodium dodecylsulphate micellar media

The transient spectra and decay kinetics of semithionine species produced by the photoreduction of thionine by ferrous ions were studied in sodium dodecylsulphate (SDDS) micellar media. The results were compared with data obtained for a homogeneous aqueous medium. It was found that, whereas in the homogeneous aqueous medium there is evidence for the interaction of the semithionine species with ferrous ions, particularly at higher pH values and high ferrous ion concentrations, this is not the case in the micellar medium. Further, the decay of the semithionine species which is a kinetically second-order dismutation process in the homogeneous aqueous medium becomes pseudo first order in the SDDS micellar medium and is attributable to re-oxidation of semithionine by ferric ions adsorbed on the same micelle surface. From a comparison of the spectra in the two media at different pH values it is inferred that the species is more likely to be the doubly protonated semithionine TH. ̇ 3 2+ form in the micelle in the pH range 0 – 2.5. Addition of high concentrations of electrolytes such as NaCl had no effect on the spectrum of the species in the micellar medium, but the decay reverted to second order. This has been interpreted in terms of displacement of the ferric ions from the micelle surface into the bulk aqueous phase such that dismutation becomes the only pathway available for semithionine decay. The dismutation rate was found to decrease with increases in the concentration of the micellized surfactant in qualitative agreement with the pseudophase model of micellar kinetics.

Free radical decay kinetics in PMMA bone cement.

Electron-spin resonance has been used to measure the decay of the concentration of polymerization radicals in polymethyl methacrylate (PMMA) bone cement and the kinetics of the decay determined. Thermal annealing at various temperatures has shown that the logarithm of the concentration of radicals varies linearly with time, but with a nonzero intercept. These results have been analyzed by including both first- and second-order decay processes. The first-order process has an activation energy of 39 +/- 2 kcal/mol and is probably due to a diffusion-limited termination. The second-order process has an activation energy of 36 +/- 5 kcal/mol and is probably due to bimolecular termination.

Decay kinetics of hydroxyl radicals in frozen aqueous systems

Second- and first-order kinetic equations, respectively, with time-dependent rate constant of the form k(t) = Bt α-1, 0 < α ⪕ 1 , were found to be adequate to describe the decay of OH radicals at temperatures below and above the onset of structural relaxation, at about 100 K, in the frozen aqueous systems (polycrystalline ice and amorphous water imbedded in frozen cross-linked dextran) γ-irradiated at 77 K.

Kinetics of free radical decay in irradiated L-leucine

Kinetics of free radical decay in irradiated L-leucine

LIGHT‐INDUCED ELECTRON TRANSFER REACTIONS BETWEEN CHLOROPHYLL AND QUINONE IN LIPOSOMES: RADICAL FORMATION AND DECAY IN NEGATIVELY AND POSITIVELY CHARGED VESICLES*

Abstract— The incorporation of relatively small amounts (≤ 20 mol%) of a negatively charged surfactant into otherwise electrically neutral phosphatidylcholine vesicles containing chlorophyll in the presence of benzoquinone has been shown to produce large effects on radical formation and decay as measured by laser flash photolysis. When salt ions are present in the aqueous phase, increasing the level of negative surfactant leads to a small increase in radical yield, followed by a larger decrease in radical yield. When the salt concentration is low, increasing the negative surfactant concentration leads to a suppression of the fast radical recombination process, an increase in slow radical decay and, at the highest surfactant concentration, an approximately 35% increase in total radical yield. An analysis of these effects is given in terms of the influence of a negative electrostatic field on radical pair stabilization and recombination, radical pair separation and expulsion of the acceptor radical anion from the vesicle.The incorporation of relatively small amounts (≤ 20 mol%) of a positively charged surfactant into egg phosphatidylcholine (EPC) vesicles containing chlorophyll and benzoquinone also produces large effects on radical formation and decay. When the electrolyte concentration in the suspending aqueous medium is high, radical yields are decreased as the surfactant concentration is increased, without any appreciable effect on decay kinetics. When deionized water is used, the slow recombination component of the decay is specifically suppressed by the presence of the positive surfactant, whereas the fast decay component decreased and then increased in amount as the surfactant concentration is increased. In all cases, however, the total radical yield is less than in pure EPC vesicles. These results can be understood in terms of the influence of a positive electrostatic field on radical pair separation and acceptor radical anion mobility. When equimolar amounts of both positively and negatively charged surfactants are incorporated into EPC vesicles, the radical yields and decay kinetics are relatively unaffected, but a large effect is observed on the radical difference spectrum. This may be a consequence of clustering of oppositely charged molecules within the bilayer surface.

Diastereomeric anisochronism in the EPR spectra of phosphoranyl radicals

1. Diastereomeric anisochronism was detected in the EPR spectra of phosphoranyl radicals. 2. The decay kinetics of phosphoranyl radicals, derivatives of 1,3,2-oxazaphospholanes, were studied.

Structure and reactivity of aminophosphoranyl radicals, derivatives of orthophenylene(alkylamino) phosphites

1. The ESR spectra and decay kinetics of aminophosphoranyl radicals, 3,5-di-tert-butylpyrocatechol derivatives, with various substituents at the nitrogen atom have been studied. 2. It was established that the aminophosphoranyl radicals formed decay by different pathways (recombination, α-cleavage), depending on the substituent at the nitrogen atom.

Kinetics of free radical decay reactions in irradiated isotactic polypropylene

The kinetics of free radical decay reactions above and below the glass transition in isotactic polypropylene have been investigated for the decay reactions occurring both in a vacuum and in the presence of 550 torr of hydrogen gas. As expected the decay rate in vacuo is less below T/sub g/ at -26/sup 0/C than above T/sub g/ at -4/sup 0/C. Hydrogen markedly catalyzes the free radical decay at -4/sup 0/C, but less so at -26/sup 0/C, as expected because of the reduced hydrogen solubility at that temperature. The decay follows neither simple first-order nor second-order kinetics, nor is the decay a composite first-order reaction in contrast to the alkyl radical decay kinetics in bulk polyethylene. Instead, the data can be accurately represented by a new kinetic equation derived on the assumptions of a fraction of the initial free radicals decaying by a second-order mechanism and the remainder being converted by a first-order process into an identical free radical but in a location where it cannot react further.

Electron spin resonance of 136XeF and 84KrF: new results for polycrystalline spectra

ESR spectra of 136XeF· and 84KrF· in polycrystalline XeF 2, XeF 4 and KrF 2 were carefully analysed to obtain additional information on the structure and dynamic parameters of radical sites. A set of spectra with slightly different magnetic parameters was found in XeF 4 and KrF 2 and attributed to distortion of the local crystalline field in different sites of the matrix. A qualitative correlation between the dispersion of magnetic parameters and “polychromatic” kinetics of radical decay is demonstrated for the first time.