- Direct EPR detection of a hydrogen-bonded complex between a semiquinone radical anion and a protonated amino acid, and electron transfer driven by hydrogen bonding
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Strong hydrogen bonding in a complex of a semiquinone radical anion and protonated histidine (His·2 H+), detected for the first time by EPR spectroscopy during photoinduced electron transfer from 10,10′-dimethyl-9,9′-biacridine (D) to 1-(p-toly
- Yuasa, Junpei,Yamada, Shunsuke,Fukuzumi, Shunichi
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- One-step versus stepwise mechanism in protonated amino acid-promoted electron-transfer reduction of a quinone by electron donors and two-electron reduction by a dihydronicotinamide adenine dinucleotide analogue. Interplay between electron transfer and hyd
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Semiquinone radical anion of 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ?-) forms a strong hydrogen bond with protonated histidine (TolSQ?-/His·2H+), which was successfully detected by electron spin resonance. Strong hydro
- Yuasa, Junpei,Yamada, Shunsuke,Fukuzumi, Shunichi
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Read Online
- The effect of the medium polarity on the mechanism of the reaction of hydroxybenzenes with hydrazyl radical in aprotic solvents
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Mechanisms of the reaction of di- and trihydroxybenzenes with 2,2′-diphenyl-1-picrylhydrazyl (stable radical) in aprotic media of different polarity have been elucidated by experimental and quantum-chemical methods. Kinetic, stoichiometric, and activation
- Belaya,Belyj,Zarechnaya,Scherbakov,Mikhalchuk,Doroshkevich
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p. 690 - 697
(2017/05/29)
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- Phenolic hydrogen abstraction by the triplet excited state of thiochromanone: A laser flash photolysis study
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Triplet ketones are known to oxidize biological substrates which can lead to damage of several biomolecules such as amino acids, nucleosides and DNA. As part of our systematic study on the interaction between carbonyl compounds and phenols, the triplet reactivity of thiochromanone (1) towards substituted phenols, in acetonitrile, was investigated employing the laser fash photolysis technique. The quenching rate constants ranged from (1.1 ± 0.1) × 108 L mol-1 s-1 (4-cyanophenol) to (5.8 ± 1.0) × 109 L mol-1 s-1 (hydroquinone). A Hammett plot for the reaction of triplet 1 with phenols containing polar substituents resulted in a reaction constant ρ =-0.90. This negative value observed for the reaction constant ρ is in accord with a mechanism in which the hydrogen transfer from phenols to the triplet carbonyl involves a coupled electron/proton transfer.
- Ribeiro, Alessandra M.,Bertoti, Ada Ruth,Netto-Ferreira, Jose? Carlos
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scheme or table
p. 1071 - 1076
(2010/10/21)
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- Kinetics and mechanisms of reactions of the nitrate radical (NO 3) with substituted phenols in aqueous solution
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Second order rate constants were obtained for the reactions of the nitrate radical (NO3) with substituted phenols in aqueous solutions at 298 K and pH = 0.5. The following compounds were investigated and the corresponding rate constants are rep
- Barzaghi, Paolo,Herrmann, Hartmut
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p. 5379 - 5388
(2007/10/03)
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- Photoreduction of p-Benzoquinones: Effects of Alcohols and Amines on the Intermediates and Reactivities in Solution
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The photochemistry of 1,4-benzoquinone (BQ) and alkyl-, Cl- and related derivatives, e.g. methyl-, 2,6-dimethyl-, chloro-, 2,5-dichloro-1,4-benzoquinone, duroquinone and chloranil, was studied in nonaqueous solvents by UV-vis spectroscopy using nanosecond laser pulses at 308 nm. The reactivity of the triplet state (3Q*) of the quinones with 2-propanol in the absence of water is largest for BQ and depends mainly on the quinone structure, whereas the rate constant of electron transfer from amines, such as triethylamine (TEA) or 1,4-diazabicyclo[2.2.2]octane, is close to the diffusion-controlled limit for BQ and most derivatives. Photoinduced charge separation after electron transfer from amines to 3Q* and the subsequent charge recombination or neutralization are supported by time-resolved conductivity measurements. The half-life of the decay kinetics of the semiquinone radical (.QH/Q.-) depends significantly on the donor and the medium. The photoconversion into the hydroquinones was measured under various conditions, the quantum yield, λirr = 254 nm, increases with increasing 2-propanol and TEA concentrations. The effects of quenching of 3Q*, the .QH/Q.- radicals and the photoconversion are outlined. The mechanisms of photoreduction of quinones in acetonitrile by 2-propanol are compared with those by TEA in benzene and acetonitrile, and the specific properties of substitution are discussed.
- Goerner, Helmut
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p. 440 - 448
(2007/10/03)
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- Diphenol radical cations and semiquinone radicals as direct products of the free electron transfer from catechol, resorcinol and hydroquinone to parent solvent radical cations
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In the pulse radiolysis of solutions of catechol, resorcinol and hydroquinone in n-butylchloride, dihydroxybenzene radical cations (40%) as well as semiquinone radicals (60%) are observed as direct synchronously formed products of the electron transfer from the solvent parent ions to the solute. This is explained in terms of free electron transfer succeeding in nearly every encounter of the reactants, which in the case of the studied dihydroxybenzenes involves femtosecond molecular dynamics effects. The rotation of the C-OH bond causes cycling of the molecule through transient conformations also exhibiting different electron distributions. From the more chemical point of view, the diphenol radical cations represent the first and till now unknown intermediates of oxidative semiquinone radical formation.
- Brede, Ortwin,Kapoor, Sudhir,Mukherjee, Tulsi,Hermann, Ralf,Naumov, Sergej
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p. 5096 - 5104
(2007/10/03)
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- The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds
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A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C+OH + R1COR2 → >C=O + R1R2C+OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C+OH + R1CH=CH2 → >C=O + R1C+ HCH3; >R1CH=CH2 + R2C+HCH2R3 → R2C+HCH3 + R2CH=CHR3. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (aldehydes, ketones, and quinones) from the C-H and O-H bonds were compared.
- Denisov
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p. 2110 - 2116
(2007/10/03)
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- Photoreduction of Triplet Benzophenone by Tertiary Amines: Amine Molecular Structure and Ketyl Radical Yield
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The photoreduction of triplet benzophenone by 14 tertiary amines was investigated. The ketyl radical yields do not correlate with the quenching rate constants, nor with the electron donor propensity of the amines. Individual structural features of the amines seem to determine the photoreduction yields.
- Von Raumer, Markus,Suppan, Paul,Haselbach, Edwin
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p. 719 - 724
(2007/10/03)
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- Radiolytic decomposition of 4-bromophenol and 4-chlorophenol in dilute aqueous solution
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The pH dependence of azide radical, hydroxyl radical and hydrated electron induced decomposition of 4-chloro- and 4-bromophenol was investigated by pulse radiolysis in airfree solutions under reducing and oxidizing conditions and also in air saturated sol
- Kovacs, Andras,Gonter, Katalin,Foeldiak, Gabor,Gyoergy, Istvan,Wojnarovits, Laszlo
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p. 453 - 475
(2007/10/03)
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- Formation of radical anions on the reduction of carbonyl-containing perfluoroaromatic compounds in aqueous solution: A pulse radiolysis study
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Radical anions are formed on addition of hydrated electrons to pentafluoroacetophenone (PFA) and pentafluorobenzaldehyde (PFB) in aqueous solutions. On the other hand, addition of hydrated electrons to pentafluorobenzoic acid (PFBA) leads to rapid fluoride elimination. The spectrum of the radical anion of PFA has λmax at 300 and 440 nm with absorption coefficient at 440 nm ε440 = 2100 L mol-1 cm-1. PFA?- decays with a rate constant of (7 ± 3.0) × 103 s-1. It has a pKa = 7.5 and the spectrum of the conjugate acid has λmax at 270 and 460 nm with ε460 = 900 L mol-1 cm-1. The spectrum of the radical anion of PFB has λmax at 285 and 430 nm with ε430 = 800 L mol-1 cm-1. PFB?- decays with a rate of (4 ± 2) × 103 s-1. It has a pKa = 7.2 and the spectrum of the conjugate acid has weak absorption at 330 nm. Evidence for the formation of the radical anion was obtained from intermolecular electron transfer from the radical anions of PFA and PFB top-benzoquinone (Q), methyl viologen (MV2+), and 9,10-anthraquinone-2-sulfonate (AQS-). Strong reductants derived from reduction of 2,2-bipyridine (BpyH?) and 1,10-phenanthroline (PhenH?) can reduce both PFA and PFB. From the kinetics of these electron transfer reactions the reduction potentials of PFA and PFB have been determined to be -0.86 ± 0.1 and -0.75 ± 0.1 V vs NHE at pH 9.4. Addition of OH? radical to the aromatic ring of these fluorinated compounds led to rapid HF elimination and the formation of phenoxyl radicals, and addition of H? atoms led to the formation of cyclohexadienyl radical.
- Shoute, Lian C. T.,Mittal, Jai P.
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p. 14022 - 14027
(2007/10/03)
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- Rate Constants for Reactions of Iodine Atoms in Solution
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Laser flash photolysis (at 248 or 308 nm) of aryl iodides in water or water/methanol solutions produces iodine atoms and phenyl radicals.Iodine atoms react rapidly with added I- to form I2- but do not react rapidly with O2 (k = 10s
- Alfassi, Z. B.,Huie, R. E.,Marguet, S.,Natarajan, E.,Neta, P.
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p. 181 - 188
(2007/10/02)
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- Triplet Mechanism of CIDNP in the Photolysis of Benzoquinones As Studied by Dynamic Nuclear Polarization
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CIDNP and CIDNP-detected ESR spectra are observed in the photolysis of benzoquinone and its methyl derivatives.In low concentrations of solutions, the sign of the CIDNP of the ring and methyl protons of benzoquinones is in accord with the prediction from
- Meng, Qing-Xiang,Suzuki, Kouei,Maeda, Kiminori,Terazima, Masahide,Azumi, Tohru
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p. 1265 - 1269
(2007/10/02)
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- DNP Spectrum Observed in the Photolysis of Benzoquinone and Some of Its Methyl Derivatives. The Triplet Mechanism in CIDNP
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CIDNP and dynamic nuclear polarization (DNP) are observed in the photolysis of benzoquinone and its methyl derivatives.In low concentrations of solutions, the signs of the CIDNP of the ring proton of benzoquinone and methyl protons of methyl derivatives o
- Meng, Q.,Yamakage, Y.,Maeda, K.,Azumi, T.
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- Iodine Atoms and Iodomethane Radical Cations: Their Formation in the Pulse Radiolysis of Iodomethane in Organic Solvents, Their Complexes, and Their Reactivity with Organic Reductants
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Pulse radiolysis of iodomethane in various organic solvents leads to formation of iodine atoms or iodomethane radical cations, which in turn form complexes with iodomethane or with the solvent.Radiolysis in cyclohexane gives CH3I*I, which exhibits an absorption peak at 390 nm, whereas radiolysis in benzene forms the solvent complex, C6H6*I, which exhibits an intense broad absorption centered at 490 nm.Radiolysis of iodomethane in acetone, benzonitrile, and halogenated hydrocarbons results in formation of the radical cation CH3I.+.In the former two solvents, this species forms a complex with another molecule of iodomethane to give (CH3))2+, which absorbs at 420 nm, in agreement with previous results in aqueous solutions, but in halogenated hydrocarbons it forms complexes with the solvents, absorbing at 320-360 nm, i.e. near the absorption of monomeric CH3I.+ in water.Complexes of I atoms oxidize phenol and triphenylamine relatively slowly whereas complexes of CH3I.+ react more rapidly.The reactivity of the CH3I.+*RX complexes increases in the order of RX = CH2Cl2, CHCl3, CH2Br2, CCl4, CH3I, and for each complex the reactivity with phenol increases with increase in electron donating power of substituents.Replacing the methyl group of iodomethane radical cation with ethyl or isopropyl decreases the reactivity, whereas trifluoromethyl increases the reactivity.These oxidation reactions proceed via an intermediate complex between the iodine species and the organic reductant.
- Shoute, Lian C. T.,Neta, P.
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p. 4411 - 4414
(2007/10/02)
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- Perfluorobutylperoxyl Radical as an Oxidant in Various Solvents
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Perfluorobutylperoxyl radicals were produced by pulse radiolysis of aerated solutions of perfluorobutyl iodide.The rate constants for reaction of this radical with several organic reductants, chlorpromazine, trolox, hydroquinone, and several other phenols, were determined in various solvents and were found to be in the range of 105 -109 M-1 s-1.By comparison with other haloalkylperoxyl radicals, C4F9OO. was found to be a much more powerful oxidant, whose reactions took place more rapidly and were less sensitive to solvent and substituent effects.The rate constants (k) for oxidation of a series of para-substituted phenols by C4F9OO. gave a good linear correlation between log k and the electrophilic substituent constant ?+, with a slope of ρ+ = -2.3, indicating formation of a positively charged transition state.Parallel experiments with CCl3OO. were limited to the most reactive phenols and gave a higher slope, ρ+ = -3.3.The rates of reaction of C4F9OO. with trolox and chlorpromazine were found to depend on solvent viscosity, but much less on solvent polarity and acid-base properties, probably because they were closer to the diffusion-controlled limit.The longer chain C10F21OO. was somewhat less reactive than C4F9OO. because of geometric factors.
- Nahor, G. S.,Neta, P.,Alfassi, Z. B.
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p. 4419 - 4422
(2007/10/02)
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- Effect of Ring Substitution on the Photochemistry of α-(Aryloxy)acetophenones
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The photochemistry of a series of 10 α-(aryloxy)acetophenones has been examined by quantum yield, product studies, and laser flash photolysis techniques.Triplet decay involves a competition of β-cleavage with intramolecular quenching by the β-aryl group, the latter being favored in all systems.Typical triplet lifetimes at room temperature range from 60 to 260 ps and are rather insensitive to the substitution pattern. β-Cleavage rate constants range between 1*107 and 3*109 s-1 and are strongly favored by electron releasing substituents, such as 4-methoxy.The determination of true quantum yields of β-cleavage requires the addition of thiophenol as a radical trap to prevent the efficient back reaction that takes place otherwise; under these conditions quantum yields range from 10-3 to 0.14, the highest value being obtained for the 4-methoxy and 4-hydroxy derivatives.
- Netto-Ferreira, J.C.,Avellar, I.G.J.,Scaiano, J.C.
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- Substituent Effects on Rates of One-Electron Oxidation of Phenols by the Radicals ClO2, NO2, and SO3(1-)
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Rate constants for the reactions of ClO2, NO2, and SO3(1-) radicals with several substituted phenoxide ions have been measured by pulse radiolysis.They vary from the immeasurably slow (4) to almost diffusion-controlled rates (>109 M-1 s-1) and depend on the redox potentials of the phenoxide ions and the inorganic radicals.With the weak oxidant SO3(1-) reverse reactions were observed in certain cases; i.e., the phenoxyl radical oxidizes sulfite ions.An attempt is made to correlate the rate constants with Hammett's substituent constants and the results are compared with those obtained previously for the reactions of various inorganic radicals with phenols and phenoxide ions.
- Alfassi, Zeev B.,Huie, Robert E.,Neta, P.
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p. 4156 - 4158
(2007/10/02)
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- One-Electron Redox Reactions in Aqueous Solutions of Sulfite with Hidroquinone and Other Hydrophenols
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The one-electron oxidation of hydroquinone and other hydroxyphenols by the sulfite radical, SO3-, and the preoxysulfate radical, SO5(1-), has been investigated along with the oxidation of sulfite by several phenoxy radicals.Rate constants for t
- Huie, Robert E.,Neta, P.
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p. 3918 - 3921
(2007/10/02)
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- Pulse Radiolysis of Aqueous Hydroquinone Solutions in the Presence of O2
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The pulse radiolysis of hydroquinone (H2Q) solutions containing O2 leads to the formation of the trihydroxycyclohexadienyl radical (H2Q-OH) which decays unimolecularly to the semiquinone radical (HQ).In 1M H2SO4, O2 saturated solution, G(HQ) ca. 3.8 but i
- Al-Suhybani, A. A.,Hughes, G.
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p. 229 - 238
(2007/10/02)
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- One-Electron Redox Potentials of Phenols. Hydroxy- and Aminophenols and Related Compounds of Biological Interest
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The rate constants for reversible electron transfer between a series of substituted phenolate ions and anilines and various substituted phenoxyl or anilino radicals in aqueous solution were measured by observing the formation or depletion of the radicals involved.Nonequilibrium concentrations of the radicals were produced in the presence of the corresponding phenols or anilines by using the pulse radiolysis technique.The relaxation of the system to equilibrium was monitored by optical detection methods.From the equilibrium constants for one-electron transfer, the one-electron redox potentials (E2) for 38 phenolic or anilino type compounds were determined, many of which are natural products.The redox potentials are strongly influenced by electron-donating or -withdrawing substituents at the aromatic system.
- Steenken, S.,Neta, P.
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p. 3661 - 3667
(2007/10/02)
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- Photochemical Dediazoniation of Arene Diazonium Salts Catalyzed by Quinones
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The rate constants of the hydrogen abstraction of triplet-excited p-benzoquinone (BQ), duroquinone (DQ), p-chlorophenyl-p-benzoquinone (PQ) and 2,5-di(p-chlorophenyl)-p-benzoquinone (P2Q) from n-heptane and acetonitrile are determined by laser flash photolysis as well as the rate constants for the reactions with oxygen. The semiquinones formed are in equilibrium with their respective semiquinone anions; at pH 3 and pH 7, respectively, it was possible in acetonitrile/water solution to determine separately the rate constants for their disproportionation affording quinones and hydroquinones and the rate constants for electron transfer to p-chlorbenzene diazonium tetrafluoroborate.It comes out that the electron transfer to the diazonium compound only competes favorably with the disproportionation in the case of the semiquinone anions.
- Becker, H. G. O.,Jirkovsky, J.,Fojtik, A.,Kleinschmidt, J.
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p. 505 - 511
(2007/10/02)
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- Reaction of tert-Butoxy Radicals with Phenols. Comparison with the Reactions of Carbonyl Triplets
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Tert-butoxy radicals generated in the photodecomposition of di-tert-butyl peroxide react efficiently with phenols to yield the corresponding phenoxy radicals.Typical rate constants in benzene at 22 deg C are 3.3x108 and 1.6x109 Msup
- Das, P. K.,Encinas, M. V.,Steenken, S.,Scaiano, J. C.
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p. 4162 - 4166
(2007/10/02)
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