- 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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- 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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- 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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- Stopped-flow and spectrophotometric study on radical scavenging by tea catechins and the model compounds
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Radical scavenging of four tea catechins, (-)-epicatechin (EC), (-)- epigallocatechin (EGC), (-)-epicatechin gallate (ECg) and (-)- eppigallocatechin gallate (EGCg), and the model compounds of their partial structure was examined against the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical using stopped-flow and spectrophotometric methods. The number of DPPH radicals scavenged by a polyphenol molecule was larger than that of phenolic hydroxyl groups, suggesting that hydrogens which bond directly to the aromatic ring can also participate in radical scavenging. A model for the scavenging reaction was proposed in which the reaction proceeded with successive dehydrogenation from a polyphenol molecule. Analysis of the second order reaction rate constants and the activation parameters between DPPH and polyphenol at the early stage of the reaction showed that the values depended on the number of phenolic hydroxyl groups and their mutual position. Contribution of the A ring of catechins to the rate constants was estimated to be far smaller than that from the B ring. In the EGCg molecule, the B ring and the gallate group were not independent, but acted as a single group for DPPH radical scavenging.
- Senba, Yasushi,Nishishita, Tsukasa,Saito, Kieko,Yoshioka, Hiroe,Yoshioka, Hisashi
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p. 1369 - 1374
(2007/10/03)
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- Reduction potentials and kinetics of electron transfer reactions of phenylthiyl radicals: Comparisons with phenoxyl radicals
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The reduction potentials relative to the standard hydrogen electrode (SHE) for a number of para-substituted phenylthiyl radicals (Eo(p-XC6H4S./p-XC6H 4S-)) have been derived from pulse radiolytic studies of electron transfer equilibria which compare their values to those of radicals of known reduction potentials. A ladder combining the reduction potentials for both phenylthiyl and phenoxyl radicals has been established. These reduction potentials have been shown to be self-consistent and are intermediate between those of p-benzosemiquinone radical anion at 0.02 V and phenoxyl radical at 0.79 V. The reduction potential decreases as the electron donating power of the para substituent rises. The substituent effect is, however, much weaker for the phenylthiyl radicals than for their oxygen analogs. These observations demonstrate that the electronic interaction between the sulfur atoms and the aromatic ring system is much less than that which occurs with oxygen atoms. Examination of the rates of electron transfer in terms of the Marcus theory indicates that the reorganization energies of both p-XC6H4O. and p-XC6H4S. radicals are similarly affected by H, CH3, and CH3O substitution. However, the reorganization energies increase substantially for H2N and O- para substituents with the effect being much less for the p-XC6H4S. radicals than for the p-XC6H4O. radicals. These observations are in accord with structural information from spectroscopic and theoretical studies of the radicals which show that in the latter system the substituent groups interact strongly with the aromatic π system.
- Armstrong,Sun, Qun,Schuler
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p. 9892 - 9899
(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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- 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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- 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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- 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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- 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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