Electron Pulse Radiolysis Research Articles

Scavenging of the Precursor to the Hydrated Electron by the Selenate Ion

Electron pulse radiolysis experiments have been used to investigate the scavenging of the precursor to the hydrated electron in aqueous solutions of selenate, a relatively good scavenger of the prehydrated electron. The yields of the total electrons scavenged are compared to those obtained with cadmium, a hydrated electron only scavenger. At scavenging capacities greater than about 106 s-1 (1 mM selenate), the yield of total electrons is greater with selenate than with cadmium. The results are thought to be due to the scavenging of the prehydrated and hydrated electrons by the selenate anion, whereas the cadmium scavenges only the hydrated electron. Scavenging of the prehydrated electron occurs at relatively low concentrations of selenate, and other similarly reactive solutes may have noticeable effects on the outcome of water radiolysis.

Radiation-induced radical formation and crosslinking in aqueous solutions ofN-isopropylacrylamide

To investigate chain-initiating and crosslinking mechanisms, radical formation in dilute aqueous solutions of N-isopropylacrylamide (NIPAAm) and poly-NIPAAM was studied using electron pulse radiolysis with optical detection at room temperature. Several transients of NIPAAm generated by reactions with electrons, hydroxyl radicals and hydrogen atoms were observed. Electron attachment to the carboxyl group (k c = 9.0 x 10 9 dm 3 .mol -1 .s -1 ) forms the radical anion, which undergoes fast and reversible protonation (pK a = 7.8) at the carboxyl oxygen. At pH >> pK a , slow and irreversible protonation of the electron adduct at the vinyl group leads to the α-carboxyalkyl radical CH 3 ( . CH)CONHCH(CH 3 ) 2 , which is also formed by addition of H atoms to NIPAAm (k H = 7.3 x 10 9 dm 3 .mol 1 .s -1 ). Addition of OH radicals (k OH = 5.4 x 10 9 dm 3 .mol -1 .s -1 ) forms CH 2 (OH))( . CH))CONHCH(CH 3 ) 2 . Hydrogen abstraction was not observed in the case of NIPAAm monomer, but it was found for the reaction of OH radicals with thermally polymerized NIPAAm. Semi-empirical quantum chemical calculations support the assignment of the observed spectra to the radicals. A reaction mechanism for the formation of crosslinks is discussed.

Rate constant and arrhenius parameter determination for the reaction of the hydrated electron with iodomethane, iodoethane, 1-iodopropane and 2-iodopropane in aqueous solution

The techniques of electron pulse radiolysis and absorption spectroscopy have been used to directly measure rate constants for the hydrated electron reaction with CH 3I, C 2H 5I, 1-C 3H 7I and 2-C 3H 7I. At 25°C, specific values of (1.78 ± 0.05) ×x 10 10, (1.57 ± 0.07) × 10 10, (1.05 ± 0.06) × 10 10 and (1.05 ± 0.03) × 10 10 dm 3 mol −1 s −1 were determined for these compounds respectively, with corresponding overall activation energiesof 16.9 ± 1.3 (3.2–35.5°C), 19.5 ± 1.1 (3.3–51.6°C), 16.60 ± 0.65 (2.5–85.1°C) and 12.94 ± 0.44 (2.5–84.5°C) kJ mol −1. The slight curvature observed in the Arrhenius plots for the three larger iodoalkanes suggests that these rate constants are influenced by both diffusion and chemical processes. By fitting the measured temperature-dependent rate constants to the general equation 1/ k obs = 1 / k diff + 1/ k react, where k obs, is the measured rate constant, k diff is the encounter rate constant of the two reacting species and k react is the rate constant that would be measured if diffusion of the species was not rate influencing, specific Arrhenius parameters for only the chemical component have also been determined.

Generation of a Heterocyclic 1,3‐Cyclopentanediyl Radical Cation by Chemically Induced Electron Transfer Oxidation and Pulse Radiolysis

Instead of the 1,2-alkyl migration well-established for carbocyclic analogues, the 1,3-radical cation 1˙+, generated from the corresponding housane 1 by oxidation with (4-BrC6H4)3N˙+SbCl, undergoes an unexpected deprotonation to give the monoradical (1H)˙. In this process the proton is presumably transferred to one of the two carbonyl groups of the triazolidinedione ring.

Pulse-radiolytic one-electron reduction of anthraquinone and chloro-anthraquinones in aqueous-isopropanol-acetone mixed solvent

One-electron reduction of 9,10-anthraquinone and some chloro-anthraquinones and the characteristics of the semiquinones thus formed have been investigated in aqueous-isopropanol-acetone mixed solvent using electron pulse radiolysis technique. Spectroscopic characteristics, kinetic parameters of formation and decay, and the acid/base behaviour of the semiquinones have been investigated. The one-electron reduction potential of the quinones have been measured following electron transfer equilibria with a reference redox system (methyl viologen) and the values thus obtained have been compared with those of some other anthrasemiquinone systems. An analysis of the characteristics of the semiquinones shows that α-chloro substituents adjacent to the CO group act as electron withdrawing groups.

Arrhenius parameter determination for the reaction of the oxide radical, hydrated electron and hydroxyl radical with iodate in aqueous solution

The temperature dependence of the reactions of the primary water radiolysis radicals, O˙–, e–aq and ˙OH with IO3– has been determined using electron pulse radiolysis and absorption spectroscopy. The transient growth of the IO4˙2– species at 360 nm, formed by the oxide radical addition to iodate, was used to determine its reaction rate constant of (1.66 ± 0.04)× 109 dm3 mol–1 s–1(22.1 °C), with a corresponding activation energy of 14.60 ± 0.67 kJ mol–1. The equivalent values for hydrated electron reaction with iodate, obtained by directly monitoring the e–aq decay at 700 nm were (5.49 ± 0.15)× 109 dm3 mol–1 s–1(21.5 °C) and 8.86 ± 0.26 kJ mol–1, respectively. Arrhenius parameters for the reaction of the hydroxyl radical with iodate were determined using SCN– competition-kinetics and computer simulation, to give a rate constant of (1.08 ± 0.48)× 105 dm3 mol–1 s–1(22.0 °C) and activation energy of 26.5 ± 2.8 kJ mol–1.

Intramolecular Electron Transfer in Pentaammineruthenium(III)-Modified Cobaltocytochrome c

The iron in the heme group of horse-heart cytochrome c was replaced by cobalt according to established methods. The resulting cobalticytochrome c was subsequently modified at histidine-33 with a pentaammineruthenium group. Proof of correct derivatization was obtained by atomic absorption analysis of cobalt and ruthenium, differential pulse voltammetry, and enzymatic proteolysis analyzed by diode-array HPLC. Cobalt(II)-to-ruthenium(III) intramolecular electron transfer rates were measured as a function of temperature by electron pulse radiolysis. The azide radical (N3•) was used to oxidize the fully reduced form in order to generate the desired electron transfer precursor. The intramolecular electron transfer rate is 1.28 ± 0.04 s-1 at 25 °C (ΔH⧧ = 5.7 ± 0.2 kcal/mol, ΔS⧧ = −38.7 ± 0.5 cal/(deg mol)) for a driving force of 0.28 ± 0.02 eV. The results are compared with those for analogous pentaammineruthenium-modified, native iron, and zinc-substituted cytochromes c. The 0.4 eV increase in driving force for intramolecular electron transfer when iron is replaced by cobalt is largely compensated by an increase in reorganization energy.

Arrhenius Parameter Determination for the Reaction of Methyl Radicals with Iodine Species in Aqueous Solution

The techniques of electron pulse radiolysis and direct ESR detection have been used to determine Arrhenius parameters for the recombination reaction of methyl radicals and methyl radical reaction with iodine in aqueous solution. At 22.8 °C, rate constants of 2k7 = (1.77 ± 0.16) × 109 dm3 mol-1 s-1 and k1 = (2.75 ± 0.43) × 109 dm3 mol-1 s-1, with corresponding activation energies of 14.89 ± 0.87 and 13.10 ± 0.71 kJ mol-1 (5.7−39.6 °C), were obtained respectively for these two reactions. The analogous reaction of methyl radicals with iodide or iodate was found to be much slower, with the room temperature rate constant for both reactions estimated as k < 106 dm3 mol-1 s-1.

PK a of the hydrazinium ion and the reaction of hydrogen atoms with hydrazine in aqueous solution

The techniques of electron pulse radiolysis and paramagnetic resonance free induction decay (FID) attenuation measurements have been used to determine Arrhenius parameters for the reaction of the hydrogen atom with N2H5+ and N2H4 in aqueous solution. At 41.6 °C, a scavenging rate constant of (5.8 ± 1.9)× 106 dm3 mol–1 s–1 was directly measured for the N2H5+ reaction, with a corresponding activation energy of 61.4 ± 1.2 kJ mol–1 over the temperature range 41.6–82.1 °C. The pKa of the hydrazinium ion was determined by absorption spectroscopy, and over the temperature range 0–80 °C, this equilibrium was well described by pKa=(2600 ± 30)/T–(0.771 ± 0.099). Using these values, and FID attenuation measurements in borax buffered solution over the temperature range 0.2–83.0 °C, the hydrogen atom rate constant for N2H4 reaction was calculated to be (6.74 ± 0.23)× 107 dm3 mol–1 s–1(24.5 °C), with an activation energy of 16.28 ± 0.80 kJ mol–1. The reaction rate of ˙H with N2H4 is consistent with a hydrogen atom abstraction, giving H2 and ˙N2H3 radical as products, as in the gas phase. Based on the large activation enthalpy and entropy obtained for the reaction of hydrogen atoms with N2H5+, however, we suggest that this is more likely an addition–fragmentation process, to give ˙NH2, NH3 and (H+)aq as products.

Primary reactions in the electron-induced polymerization of acrylates

Using electron pulse radiolysis with optical detection the mechanism of the polymerization of tripropyleneglycol diacrylate (TPGDA) induced by electron-irradiation was studied at room temperature. The dose per pulse was 50 or 130 Gy corresponding to the electron pulse lengths of 5 and 15 ns, respectively. Short-lived transients, such as TPGDA radical cations and radical anions as well as different types of radicals were observed in neat TPGDA. The reactions of the different ions were studied separately in solutions of TPGDA in tetrahydrofuran and in n-butylchloride. TPGDA cations produced by charge transfer from n-butylchloride primary cations undergo both rapid deprotonation and ion-molecule reactions with TPGDA. Deprotonation leads to TPGDA vinyl-type radicals. In tetrahydrofuran TPGDA radical anions are produced by reaction of the solvated electron with the acrylate. Fast dimerization of the anions is observed. Proton abstraction from the solvent leads to alkyl-type radicals. Addition of the different radicals to TPGDA leads to the chain start of the radical polymerization. An upper bound of 10 5 dm 3mol −1s −1 was estimated for the starting reaction. In the presence of oxygen the formation of peroxyl radicals is observed and the polymerization is inhibited.

On the mechanism of the electron-initiated curing of acrylates

Using electron pulse radiolysis with optical detection the mechanism of the radiation induced polymerization of tripropyleneglycoldiacrylate (TPGDA) was studied in n-butylchloride (n-BuCl) solution at room temperature. Short-and long-lived transients, such as TPGDA radical cations and different types of radicals were observed.TPGDA cations produced by charge transfer from n-BuCl primary cations undergo both rapid deprotonation and ion-molecule reactions with TPGDA. Deprotonation leads to TPGDA vinyl-type radicals. As products two types of dimeric cations are observed: a covalently bonded dimer and a charge resonance stabilized structure. The covalently bonded dimer reacts with TPGDA and forms a species where excess charge and unpaired spin are separated by one molecular unit. Addition of the vinyl-type radicals to TPGDA leads to the chain start of the radical polymerization. An upper bound of 105 l·mol−1·s−1 was estimated for the starting reaction.In the presence of oxygen the formation of vinyl peroxyl radicals is observed and the polymerization is inhibited.

Excited states and electron transfer reactions of fullerenes

Abstract

Pulse radiolysis of lodate in aqueous solution

The reactions of primary water radiolysis radicals with IO–3 have been re-investigated using electron pulse radiolysis and absorption spectroscopy. Rate constants for iodate reaction with e–(aq)[(5.45 ± 0.45)× 109 dm3 mol–1 s–1], O˙–[(1.82 ± 0.04)× 109 dm3 mol–1 s–1] and ˙OH (< 105 dm3 mol–1 s–1) have been measured and compared with previously determined values. The observed iodate concentration dependence of the two transient absorptions produced by its reaction with the hydrated electron and the oxide radical were found to be due only to spur scavenging effects, and thus the dimerization equilibria proposed by previous workers is not necessary. The previously reported rate constants for the reaction of iodate with hydroxyl radicals have been shown to be due to just iodate reaction with the small concentration of O˙– present in equilibrium with the hydroxyl radical.

Excitation energy dependence of the photoionization of liquid water

Liquid water was ionized by UV two-photon excitation with intense subpicosecond laser pulses at 248.5 nm (two-photon energy 10 eV). Transient absorption signals, attributed to hydrated electrons, were monitored with probe pulses at 497 nm. It was found that about 30% of the hydrated electrons undergo geminate recombination with H[sub 3]O[sup +] ions or [sup [sm bullet]]OH radicals, which is considerably less than found in previous studies with excitation pulses at 310-312.5 nm (two-photon energy 8 eV) but still more than estimated for electron pulse radiolysis. Up to an excitation energy of 10 eV, the majority of ionization events is suggested to follow a concerted electron-transfer mechanism; in contrast to earlier conclusions, [open quotes]quasi-free[close quotes] electrons apparently play only a minor role at these excitation energies. 39 refs., 2 figs., 1 tab.

Radiation induced reactions in the ethanol-monochlorobenzene dosimeter solution

Using gamma radiolysis, electron pulse radiolysis, laser flash photolysis and endproduct measuring techniques the basic radiation chemical processes taking place in the ethanol-chlorobenzene dosimeter (composition: 24 vol% chlorobenzene, 4 vol% water, 0.04 vol% acetone, 0.04 vol% benzene and 71.92 vol% ethanol) were investigated. The HCl, used to measure the absorbed dose, forms by dissociative electron capture of chlorobenzene yielding Cl - ions (90%) and by Cl atom elimination from neutral excited chlorobenzene molecules (10%). In both reactions phenyl radicals are left behind, which predominantly abstract H atom from ethanol molecules producing α-hydroxy-ethyl radicals and benzene. The yield of the dimer, tetramer, etc. chlorobenzene molecules is small, which is important from the aspect of the linearity of the dose-HCl concentration relation.

Radiation stability of fullerenes: pulse radiolysis of C60 and C70 in hydrocarbon solvents

The transient optical absorption spectra obtained on electron pulse radiolysis of de-aerated solutions of C60 and C70 in benzene have been assigned to the triplet states with G(TC60)= 0.52 and G(TC70)= 0.48. Low G(–C60) and G(–C70) in the steady-state γ-radiolysis in benzene and hexane suggest their stability towards γ-radiation.

A fast-kinetic investigation of the redox chemistry of iridium chloride complexes using pulse radiolysis

Reactions initiated by OH radicals or e - aq in aqueous IrCl 3- 6 solutions were studied by electron pulse radiolysis using a 600 keV Febetron electron accelerator. Solutions of IrCl 3- 6 were made basic by adding Na 2CO 3; using the carbonate competition method, we find the rate constant for the reaction of OH with IrCl 3- 6 to be 4.7 × 10 9 M -1 s -1. The product IrCl 2- 6 disappears rapidly in N 2O-saturated basic solution or in neutral N 2-saturated solution (N 2O absent) but is nearly inert in neutral solution with H 2O present. We find that IrCl 2- 6 reacts rapidly with hydrogen peroxide in basic media, as confirmed on the benchtop and by stopped-flow kinetics. We therefore infer that reaction with HO - 2 may account for the loss of IrCl 2- 6 under basic conditions. Since e - aq reduces Ir(III) chloride to the Ir(II) state with a rate constant of 6.1 × 10 9 M -1 s -1, we suggest that loss of Ir(IV) in neutral deaerated solution without added N 2O may involve electron transfer from Ir(II). Loss of Ir(IV) in aerated solution is attributed to reduction by the superoxide ion, O - 2. Kinetic simulation of the system on the model described gives good agreement with our experimental results.

Electron transfer in linked viologen-quinone molecules: rate constant enhancement with increased chain length

We synthetized a homologous series of molecules (MVnn'Q) where a methylviologen (MV) and a aminochloronaphtoquinone (Q) are linked to each other via a flexible chain. Using the electron pulse radiolysis technique, we have measured time-resolved spectra and rate constant for intra and intermolecular electron transfer between donor and acceptor site of the MVnn'Q molecules in water and in SDS micellar solution.

Intra- and intermolecular electron transfer reactions in covalently linked donor-acceptor molecules

We synthesized a homologous series of molecules (MV nn′ Q) where a methylviologen (MV 2+) and an amino-chloronaphthoquinone (Q) are linked to each other via a flexible chain. Using the electron pulse radiolysis technique, we have measured time-resolved spectra and determined rate constants for intra- and intermolecular electron transfer (ET) between donor and acceptor site of the MV nn′ Q molecules in water and in sodium dodeeyl sulfate (SDS) micellar solution. For comparison, we also irradiated a solution containing a 1:1 mixture of methylviologen and amino-chloronaphthoquinone and measured spectra and intermolecular ET reactions between the separated electron donor and acceptor molecules. We found a remarkably slow intramolecular electron transfer from the reduced methylviologen moiety to the quinone site of all MV nn′ Q molecules both in water and in aqueous SDS micellar suspensions. The intramolecular rate constants measured in water increase with the number of intervening bonds, leading to the conclusion that electron transfer occurs by a through-space rather than through-bond mechanism. The intramolecular rate constants virtually lose their chain length dependence in SDS suspensions where, because of an extended configuration of the micellized MV nn′ Q molecules, through-space interaction is not favored.

The role of free radicals in photosensitized oxidation reactions induced by merocyanine 540

Abstract The formation of the anion radical of merocyanine 540 (MC − ) and its properties in methanol-aqueous media were studied using electron spin resonance (ESR), pulse radiolysis (PR) and laser flash photolysis (LFP). On illumination of MC, in the presence of ascorbate or catechol, characteristic ESR signals were observed due to ascorbyl or o -semiquinone radicals respectively. The durosemiquinone radical was detected in the illuminated samples that contained MC, reduced nicotinamide adenine dinucleotide (NADH) and duroquinone. The nature of the excited state of MC responsible for such electron transfer reactions was determined using LFP experiments. The LFP data indicated rapid quenching of the triplet excited state of MC with concurrent formation of MC − in the presence of ascorbate. In PR experiments, MC − was generated by an efficient interaction of the solvated electron with MC ( k = 6 × 10 9 M −1 s −1 ). It was found that MC − had an absorption maximum at 400 nm with ϵ = 5.9 × 10 4 M −1 cm −1 . In the absence of electron acceptors, MC − decayed probably via disproportionation. In the presence of oxygen or ferric ions (as in the complex with diethylenetriaminepentaacetic acid (DTPA)), the decay of MC − was greatly enhanced indicating efficient interaction between these species ( k = 1.7 × 10 9 M −1 s −1 and k = 6.5 × 10 8 M −1 s −1 for the reaction of MC − with O 2 and DTPA-Fe 111 respectively). Hence photoinduced electron transfer and the formation of MC − which may occur in MC-photosensitized systems could lead to the generation of oxygen radicals. The formation of hydroxyl radicals in such systems was shown indirectly by ESR spin trapping of methanol-derived radicals. It is postulated that the hydroxyl radicals are generated via a Fenton reaction driven by reducing radicals derived from photoinduced electron transfer reactions. An additional mechanism for the photodynamic efficiency of MC is proposed based on electron transfer reactions induced by light.