Keith P. Madden

The role of the DMPO-hydrated electron spin adduct in DMPO-OH spin trapping

Time-resolved in situ radiolysis ESR (electron spin resonance, equivalently EPR, electron paramagnetic resonance) studies have shown that the scavenging of radiolytically produced hydroxyl radical in nitrous oxide-saturated aqueous solutions containing 2 mM DMPO is essentially quantitative (94% of the theoretical yield) at 100 micros after the electron pulse [1]. This result appeared to conflict with earlier results using continuous cobalt-60 gamma radiolysis and hydrogen peroxide photolysis, where factors of 35 and 33% were obtained, respectively [2,3]. To investigate this discrepancy, nitrogen-saturated aqueous solutions containing 15 mM DMPO were cobalt-60 gamma irradiated (dose rate = 223 Gy/min) for periods of 0.25-6 min, and ESR absorption spectra were observed approximately 30 s after irradiation. A rapid, pseudo-first-order termination reaction of the protonated DMPO-hydrated electron adduct (DMPO-H) with DMPO-OH was observed for the first time. The rate constant for the reaction of DMPO-H with DMPO-OH is 2.44 x 10(2) (+/- 2.2 x 10(1)) M(-1) s(-1). In low-dose radiolysis experiments, this reaction lowers the observed yield of DMPO-OH to 44% of the radiation-chemical OH radical yield (G = 2.8), in good agreement with the earlier results [2,3]. In the absence of the DMPO-H radical, the DMPO-OH exhibits second-order radical termination kinetics, 2k(T) = 22 (+/- 2) M(-1) s(-1) at initial DMPO-OH concentrations > or = 13 microM, with first-order termination kinetics observed at lower concentrations, in agreement with earlier literature reports [4].

Critical Review of Aqueous Solution Reaction Rate Constants for Hydrogen Atoms

Kinetic data published in the peer-reviewed literature over the period of 1988–2007 for H⋅ radical reactions with molecules and ions derived from inorganic and organic solutes in aqueous solution have been critically reviewed. Rate constants for over 250 reactions, as studied by pulse radiolysis, end-product analysis, and other methods, have been tabulated.

DMPO-Alkyl Radical Spin Trapping: An In Situ Radiolysis Steady-State ESR Study

Abstract Taniguchi, H. and Madden, K. P. DMPO-Alkyl Radical Spin Trapping: An In Situ Radiolysis Steady-State ESR Study. Short-lived free radicals formed in the reaction of 11 substrates and radiolytically produced hydroxyl radicals were trapped successfully with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in dilute aqueous solution. The in situ radiolysis steady-state ESR spectra of the spin adducts were analyzed to determine accurate ESR parameters for these spin adducts in a uniform environment. Parent alkyl radicals include methyl, ethyl, 1-propyl and 2-propyl (1-methylethyl). Hydroxyalkyl parent radicals were hydroxymethyl, hydroxyethyl, 2-hydroxy-2-propyl (1-methyl-1-hydroxyethyl), 1-hydroxypropyl and 2-hydroxy-2-methylpropyl. Carboxyl radical (carbon dioxide anion, formate radical) and sulfite anion radical were the sigma radicals studied. The DMPO spin adduct of 1-propyl was identified for the first time. For most spin adducts, g factors were also determined for the first time. In DMPO spin adducts of hydroxyalkyl radicals, nitrogen and C2-proton hyperfine coupling constants are smaller than those of alkyl radical adducts; the hydroxyalkyl spin adducts possess larger g values than their unsubstituted counterparts. These changes are ascribed to the spread of π conjugation to include the hydroxyl group. Strong evidence of spin addend–aminoxyl group interaction can be seen in the asymmetrical line shapes in the hydroxyethyl and the hydroxypropyl spin adducts.

Computer controlled in situ radiolysis electron spin resonance spectrometer incorporating magnetic field‐microwave frequency locking

A lab‐built personal computer‐based electron spin resonance (ESR) spectrometer is described which incorporates a wide range magnetic field/microwave frequency lock as part of its magnetic field control subsystem. Instrument operation is accomplished by keyboard commands, with important experimental variables logged automatically for reference during subsequent data analysis. The spectrometer features both narrowband field modulation and direct‐detection time‐resolved ESR modes. The data acquisition system and field/frequency lock operate such that spectrum recordings consisting of multiple magnetic field sweeps are undistorted by long‐term klystron frequency drift. The spectrometer features low noise microwave preamplification, balanced mixer detection, automatic reference arm phase control, and a fast automatic frequency control system requiring no klystron frequency modulation. Natural abundance 13C and 33S studies of the terephthalic acid radical trianion and sulfite radical anion are presented as proo...

Spectra and structure of α-tocopherol radicals produced in anoxic media

Abstract Electron spin resonance and flash photolysis studies have been combined to determine, amid conflicting reports from radiolysis studies, the spectrum for the phenoxyl radical of α-tocopherol. Triplet state ketones were used to abstract the α-tocopherol phenolic hydrogen in order to obtain both the transient ESR spectrum and optical spectrum. Analysis of the ESR data yields g = 2.00469, a(H), 5-CCH3 = 6.04G, a(H,7-CCH3) = 4.51G, a(H, CH2) = 1.38G, a(H′, CH2) = 1.49G and a (H,8-CCH3) = 0.89G. The g factor shows a large spin population on oxygen in this radical demonstrates conclusively involvement of the phenoxyl radical. Both spectra were in agreement with those produced by radiolysis of α-tocopherol in N2 saturated EtOH. While the spectral characteristics of the phenoxyl radical now appear to be clarified, uncertainties remain concerning optical spectra for radiolysis of α-tocopherol in air- a nd oxygen-containing systems.

An in-situ radiolysis EPR study of spin trapping by 2-methyl-2-nitrosopropane : steric and electronic effects influencing the trapping of hydroxyalkyl radicals derived from pentanols and substituted pentanols

The spin adducts formed by reaction of bulky hydroxyalkyl radicals with the nitroso spin trap 2-methyl-2-nitrosopropane (MNP) were studied using in-situ radiolysis EPR. Parent hydroxyalkyl radicals were produced in aqueous solution either by hydroxyl-radical reaction with unsubstituted and methyl-substituted alcohols (propanols, pentanols and cyclohexanols) or by reaction of the corresponding ketone with the hydrated electron. The parent radicals included 1-hydroxypentyl, 1-hydroxy-1-methylbutyl, 1-ethyl-1-hydroxypropyl, 1-hydroxy-1-isopropyl-2-methylpropyl, 1-hydroxy-2,2-dimethylpropyl, 1,3-dihydroxy-2,2-dimethylpropyl, 1-hydroxycyclohexyl, 1-hydroxy-2-methylcyclohexyl, and 1-tert-butyl-1-hydroxy-2, 2-dimethylpropyl radicals. All but the bulkiest radicals reacted with MNP by addition at the nitroso nitrogen site to form the MNP–C(OH)RR′ spin adduct. In contrast with previous MNP spin-trapping studies using hydroxymethyl, hydroxyethyl, and 1-hydroxy-1-methylethyl parent radicals, steric interactions strongly modulated the yields of the spin adducts produced. Strongly reducing hydroxyalkyl radicals also reacted with MNP to produce the MNP–H adduct by direct reduction of MNP. Steric hindrance between the parent radical and MNP was sufficient in the most extreme case to shut off MNP–R production with concomitant production of MNP–H. Spin-adduct persistence was measured for the MNP–hydroxyalkyl and MNP–alkyl spin adducts. Hydroxyalkyl spin adduct lifetimes varied from seconds (MNP–1-hydroxy-1-methylbutyl) to one year (MNP-1-hydroxycyclohexyl), correlating with the level of aminoxyl function shielding afforded by its substituent groups. MNP spin adducts formed from other non-hydroxyalkyl alcohol radicals had short lifetimes of less than 18 hours.