- Tris(trimethylsilyl)silane (TTMSS)-derived radical reactivity toward alkenes: A combined quantum mechanical and laser flash photolysis study
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(Figure Presented) The reactivity of the tris(trimethylsilyl)silane (TTMSS)-derived radical is studied through an approach combining laser flash photolysis and quantum mechanical calculations. The results obtained for TTMSS are compared both to a classica
- Lalevee, Jacques,Allonas, Xavier,Fouassier, Jean Pierre
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Read Online
- Direct Irradiation of Phenol and Para-Substituted Phenols with a Laser Pulse (266 nm) in Homogeneous and Micro-heterogeneous Media. A Time-Resolved Spectroscopy Study
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Direct irradiation of para-substituted phenols under N2 atmosphere in homogeneous (cyclohexane, acetonitrile, and methanol) and micellar (SDS) solution was investigated by means of time-resolved spectroscopy. After a laser pulse (266 nm), two transient species were formed, viz. the para-substituted phenol radical-cations and the corresponding phenoxy radicals. The radical-cations showed a broad absorption band located between 390 and 460 nm, while the phenoxy radicals showed two characteristic bands centered at 320 nm and 400-410 nm. The deprotonation rate constant of radical-cations (kH) of 105 s-1 and the reaction rate constant of the phenoxy radicals (kR) in the order of 109-1010 M-1·s-1 have been derived. The kH rate constants gave good linear Hammett correlation with positive slope indicating that electron-withdrawing substituents enhance the radical-cation acidity. The binding constants (Kb) of the para-substituted phenols with the surfactant were also measured, and NOESY experiments showed that phenols were located in the hydrophobic core of the micelle. Finally, computational calculations provided the predicted absorption spectra of the transients and nice linear correlations were obtained between the theoretical and experimental energy of the lower absorption band of these species.
- Siano, Gastón,Crespi, Stefano,Bonesi, Sergio M.
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p. 14012 - 14025
(2020/11/20)
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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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- Laser flash photolysis study of the reactivity of β-naphthoflavone triplet: Hydrogen abstraction and singlet oxygen generation
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The absorption spectra for β-naphthoflavone (1) reveal a solvatochromic red shift in polar solvents which is consistent with the π,π? character of the S0 → S1 electronic transition. The laser flash photolysis technique has been used to characterize and study the reactivity of the triplet excited state of 1. Excitation (355 nm) of degassed solutions of 1, in acetonitrile, resulted in the formation of its corresponding triplet excited state. Addition of hydrogen donors, such as 2-propanol and 1,4-cyclohexadiene, led to triplet quenching and formation of a new transient, which was assigned to the corresponding ketyl radical obtained from a hydrogen abstraction reaction by triplet 1. This ketyl radical was characterized by experiments with methylviologen. The triplet excited state of 1 was efficiently quenched by phenols and N-acetyl l-tryptophan methyl ester. In all cases new transients were formed in the quenching process, which were assigned to the corresponding radical pair resulting from an initial electron transfer from the quencher to the excited naphthoflavone, followed by a fast proton transfer. Singlet oxygen (1O2) is formed from the triplet of 1, and a quantum yield of 0.51 was measured. TDDFT calculations with implicit solvation (IEF-PCM) were used to calculate the ground state UV-vis absorption spectrum, from which the nature of the lowest energy transitions were characterized, and the triplet-triplet absorption spectrum consistent with the triplet transient generated by LFP. Excellent correlation of the calculated and experimental spectra was achieved using the conventional PBE0 hybrid functional.
- De Lucas, Nanci C.,Santos, Guilherme L.C.,Gaspar, Caio S.,Garden, Simon J.,Netto-Ferreira, José Carlos
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p. 121 - 129
(2015/02/19)
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- Structural and medium effects on the reactions of the cumyloxyl radical with intramolecular hydrogen bonded phenols. the interplay between hydrogen-bonding and acid-base interactions on the hydrogen atom transfer reactivity and selectivity
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A time-resolved kinetic study on the reactions of the cumyloxyl radical (CumO?) with intramolecularly hydrogen bonded 2-(1-piperidinylmethyl)phenol (1) and 4-methoxy-2-(1-piperidinylmethyl)phenol (2) and with 4-methoxy-3-(1-piperidinylmethyl)phenol (3) has been carried out. In acetonitrile, intramolecular hydrogen bonding protects the phenolic O-H of 1 and 2 from attack by CumO? and hydrogen atom transfer (HAT) exclusively occurs from the C-H bonds that are α to the piperidine nitrogen (α-C-H bonds). With 3 HAT from both the phenolic O-H and the α-C-H bonds is observed. In the presence of TFA or Mg(ClO 4)2, protonation or Mg2+ complexation of the piperidine nitrogen removes the intramolecular hydrogen bond in 1 and 2 and strongly deactivates the α-C-H bonds of the three substrates. Under these conditions, HAT to CumO? exclusively occurs from the phenolic O-H group of 1-3. These results clearly show that in these systems the interplay between intramolecular hydrogen bonding and Br?nsted and Lewis acid-base interactions can drastically influence both the HAT reactivity and selectivity. The possible implications of these findings are discussed in the framework of the important role played by tyrosyl radicals in biological systems.
- Salamone, Michela,Amorati, Riccardo,Menichetti, Stefano,Viglianisi, Caterina,Bietti, Massimo
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supporting information
p. 6196 - 6205
(2014/07/21)
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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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- Laser flash photolysis study of the photochemistry of thioxanthone in organic solvents
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The photoreactivity of the triplet excited state of thioxanthone (TX) was investigated employing the laser fash photolysis technique. The wavelength for the absorption maximum and the lifetime of the triplet excited state are solvent dependent. When hydrogen donor solvents were employed, a new band at 410 nm was observed in the triplet absorption spectrum, which was attributed to the ketyl radical derived from thioxantone. Quenching rate constants, kq, ranged from (1.7 0.1) × 106 L mol-1 s-1 for toluene to ca. 109 L mol-1 s-1 for phenol and its derivatives containing polar substituents, as well as for indole, triethylamine and DABCO.
- Rodrigues, Janaina F.,De Assis Da Silva, Francisco,Netto-Ferreira, Jose? Carlos
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scheme or table
p. 960 - 965
(2010/10/21)
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- Hydrogen hyperfine splitting constants for phenoxyl radicals by DFT methods: Regression analysis unravels hydrogen bonding effects
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DFT calculations using the B3LYP functional, medium-sized basis sets and empirical scaling of the results provide quantitative estimates of the hydrogen isotropic hyperfine splitting constants (hscs) in 2,6-di-alkyl phenoxyl radicals (1-11). Literature hs
- Amorati, Riccardo,Pedulli, Gian Franco,Guerra, Maurizio
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body text
p. 3136 - 3141
(2010/08/21)
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- A laser flash photolysis and theoretical study of hydrogen abstraction from phenols by triplet α-naphthoflavone
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The hydrogen abstraction (HA) reaction by the triplet of α-naphthoflavone (1) has been investigated experimentally by the use of laser flash photolysis (LFP) and theoretically with density functional theory (DFT) and atoms in molecules (AIM). The triplet
- De Lucas, Nanci C.,Fraga, Helena S.,Cardoso, Cristiane P.,Correa, Rodrigo J.,Garden, Simon J.,Netto-Ferreira
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experimental part
p. 10746 - 10753
(2011/05/19)
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- Unexpected acid catalysis in reactions of peroxyl radicals with phenols**
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Weak organic acids in millimolar concentrations increase the reactivity of peroxyl radicals with common phenolic antioxidants dramatically. This counterintuitive phenomenon relies on a substantially different reaction mechanism from that in the absence of an acid: rate-determining electron transfer occurs from the hydrogen-bonded phenol to the hydroperoxide cation radical present in equilibrium with the peroxyl radical under these conditions (see scheme).
- Valgimigli, Luca,Amorati, Riccardo,Petrucci, Silvia,Pedulli, Gian Franco,Hu, Di,Hanthorn, Jason J.,Pratt, Derek A.
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supporting information; experimental part
p. 8348 - 8351
(2010/02/28)
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- Absolute rate constants for some intermolecular reactions of α-aminoalkylperoxyl radicals. Comparison with alkylperoxyls
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(Graph Presented) Seven α-aminoalkylperoxyl radicals have been generated by 355 nm laser flash photolysis (LFP) of oxygen-saturated di-tert-butyl peroxide containing mono-, di-, and trialkylamines and a dialkylarylamine. All these peroxyls possess absorptions in the near-UV (strongest for the trialkylamine-derived peroxyls) which permits direct monitoring of the kinetics of their reactions with many substrates. The measured rate constants for hydrogen atom abstraction from some phenols and oxygen atom transfer to triphenylphosphine demonstrated that all seven α- aminoalkylperoxyls have similar reactivities toward each specific substrate. More importantly, a comparison with literature data for alkylperoxyls shows that α-aminoalkylperoxyls and these alkylperoxyls have essentially the same reactivities. The combination of LFP and alkylamines provides a quick, reliable method for determining absolute rate constants for alkylperoxyl radical reactions, an otherwise laborious task.
- Lalevee, Jacques,Allonas, Xavier,Fouassier, Jean-Pierre,Ingold
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p. 6489 - 6496
(2008/12/22)
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- Oxidation of phenols employing polyoxometalates as biomimetic models of the activity of phenoloxidase enzymes
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A kinetic study of the oxidation of substituted phenols with either vanadium(v) polyoxotungstate, [α-SiVVW11O 40]5- (viz. SiW11V), or manganese(iii) polyoxotungstate, [α-SiMnIIIW11/su
- Galli, Carlo,Gentili, Patrizia,Nunes Pontes, Ana Sofia,Gamelas, Jose A. F.,Evtuguin, Dmitry V.
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p. 1461 - 1467
(2008/02/13)
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- Bimolecular hydrogen abstraction from phenols by aromatic ketone triplets
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Absolute rate constants for hydrogen abstraction from 4-methylphenol (para-cresol) by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile solution at 23°C, and the results combined with previously reported data for roughly a dozen other compounds under identical conditions. The ketones studied include various ring-substituted benzophenones and acetophenones, α,α,α-trifluoroacetophenone and its 4-methoxy analog, 2-benzoylthiophene, 2-acetonaphthone, and various other polycyclic aromatic ketones such as fluorenone, xanthone and thioxanthone, and encompass n,π*, π,π*(CT) and arenoid π,π* lowest triplets with (triplet) reduction potentials (Ered*) varying from about -10 to -38 kcal mol-1. The 4-methylphenoxyl radical is observed as the product of triplet quenching in almost every case, along with the corresponding hemipinacol radical in most instances. Hammett plots for the acetophenones and benzophenones are quite different, but plots of log log kQ vs Ered* reveal a common behavior for most of the compounds studied. The results are consistent with reaction via two mechanisms: a simple electron-transfer mechanism, which applies to the n,π* triplet ketones and those π,π* triplets that possess particularly low reduction potentials, and a coupled electron-/proton-transfer mechanism involving the intermediacy of a hydrogen-bonded exciplex, which applies to the π,π* ketone triplets. Ketones with lowest charge-transfer π,π* states exhibit rate constants that vary only slightly with triplet reduction potential over the full range investigated; this is due to the compensating effect of substituents on triplet state basicity and reduction potential, which both play a role in quenching by the hydrogen-bonded exciplex mechanism. Ketones with arenoid π,π* states exhibit the fall-off in rate constant that is typical of photoinduced electron transfer reactions, but it occurs at a much higher potential than would be normally expected due to the effects of hydrogen-bonding on the rate of electron-transfer within the exciplex.
- Lathioor, Edward C.,Leigh, William J.
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p. 291 - 300
(2008/02/05)
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- Kinetic and thermodynamic parameters for the equilibrium reactions of phenols with the dpph. radical
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The kinetics and energetics of the reversible reaction of phenols with the dpph. radical have been studied; steric shielding of the divalent N by the o-NO2 in dpph. seems to be the main cause of the entropic barriers of this reaction. The Royal Society of Chemistry 2006.
- Foti, Mario C.,Daquino, Carmelo
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p. 3252 - 3254
(2008/10/09)
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- Abnormal solvent effects on hydrogen atom abstractions. 1. The reactions of phenols with 2,2-diphenyl-1-picrylhydrazyl (dpph?) in alcohols
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Rate constants, kArOH/dpph?,s, for hydrogen atom abstraction from 13 hindered and nonhindered phenols by the diphenylpicrylhydrazyl radical, dpph?, have been determined in n-heptane and a number of alcoholic and nonalcoholic, hydrogen-bond accepting solvents. Abnormally enhanced values of kArOH/dpph?,s have been observed in alcohols. It is proposed that this is due to partial ionization of the phenols and a very fast electron transfer from phenoxide anion to dpph?. The popular assessment of the antioxidant activities of phenols with dpph? in alcohol solvents will generally lead to an overestimation of their activities.
- Litwinienko, Grzegorz,Ingold
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p. 3433 - 3438
(2007/10/03)
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- Hydrogen-bonding effects on the properties of phenoxyl radicals. An EPR, kinetic, and computational study
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The effect of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) on the properties of phenoxyl radicals has been investigated. HFP produces large variations of the phenoxyl hyperfine splitting constants indicative of a large redistribution of electron spin density, which can be accounted for by the increased importance of the mesomeric structures with electric charge separation. The conformational rigidity of phenoxyl radicals with electron-releasing substituents is also greatly enhanced in the presence of HFP, as demonstrated by the 2 kcal/mol increase in the activation energy for the internal rotation of the p-OMe group in the p-methoxyphenoxyl radical. By using the EPR equilibration technique, we have found that in phenols the O-H bond dissociation enthalpy (BDE) is lowered in the presence of HFP because it preferentially stabilizes the phenoxyl radical. In phenols containing groups such as OR that are acceptors of H-bonds, the interaction between HFP and the substituent is stronger in the phenol than in the corresponding phenoxyl radical because the radical oxygen behaves as an electron-withdrawing group, which decreases the complexating ability of the substituent. In phenols containing OH or NH2 groups, EPR experiments performed in H-bond accepting solvents showed that the interaction between the solvent and the substituent is much stronger in the phenoxyl radical than in the parent phenol because of the electron-withdrawing effect of the radical oxygen, which makes more acidic, and therefore more available to give H-bonds, the OH or NH2 groups. These experimental results have been confirmed by DFT calculations. The effect of HFP solvent on the reactivity of phenols toward alkyl radicals has also been investigated. The results indicated that the decrease of BDE observed in the presence of HFP is not accompanied by a larger reactivity. The origin of this unexpected behavior has been shown by DFT computations. Finally, a remarkable increase in the persistency of the α-tocopheroxyl radical has been observed in the presence of HFP.
- Lucarini, Marco,Mugnaini, Veronica,Pedulli, Gian Franco,Guerra, Maurizio
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p. 8318 - 8329
(2007/10/03)
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- Development of novel antioxidants: Design, synthesis, and reactivity
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We are attempting to develop novel synthetic antioxidants aimed at retarding the effects of free-radical induced cell damage. In this paper we discuss the design strategy and report the synthesis of seven novel antioxidants, including six catechols and a
- Hussain, Helmi H.,Babic, Gordana,Durst, Tony,Wright, James S.,Flueraru, Mihaela,Chichirau, Alexandru,Chepelev, Leonid L.
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p. 7023 - 7032
(2007/10/03)
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- Localized electron transfer in nonpoiar solution: reaction of phenols and thiophenols with free solvent radical cations
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Free electron transfer (FET) is understood as the reaction of free and uncorrelated solvent parent radical cations with solutes characterized by a lower ionization potential than those of the solvent. We studied electron transfer from phenols and thiophen
- Brede, Ortwin,Ganapathi, Mahalaxmi R.,Naumov, Sergej,Naumann, Wolfgang,Hermann, Ralf
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p. 3757 - 3764
(2007/10/03)
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- Kinetic solvent effects on hydrogen-atom abstractions: Reliable, quantitative predictions via a single empirical equation
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The rate of hydrogen-atom abstraction from XH by a radical, Y·, can be solvent-dependent. In many cases, the kinetic solvent effect (KSE) is directly related to hydrogen-bonding interactions between XH and the solvent. The relative hydrogen-bond acceptor (HBA) properties of solvents are given by β2/H constants of Abraham et al. (Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Morris, J. J.; Taylor, P. J. J. Chem. Soc. Perkin Trans. 2 1990, 521-529). Room-temperature rate constants for hydrogen-atom abstraction, kXH/Y·/S, have been determined in a number of solvents, S, where XH refers to several substituted phenols, tert-butyl hydroperoxide or aniline and Y· is a tert-alkoxyl radical. In all cases, plots of log(kXH/Y·/S/M-1 s-1) versus β2/H gave excellent linear correlations, the slopes of which, MXH, were found to be proportional to the hydrogen-bond-donating (HBD)ability of XH, as scaled with α2/H parameters of Abraham et al. (Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Duce, P. P.; Morris, J. L.; Taylor, P. J. J. Chem. Soc., Perkin Trans. 2 1989, 699-711), with MXH = - 8.3α2/H. This leads to a general empirical equation which quantifies KSEs at room temperature: log kXH/Y·/S = log kXH/Y·O - 8.3α2/Hβ2/H, where kXH/Y·/O refers to the rate constant in a non-HBA solvent for which β2/H = 0, generally a saturated hydrocarbon. Since MXH depends only on XH, rate constants for hydrogen-atom abstraction from XH by any Y· can be accurately predicted in any of the several hundred solvents for which β2/H is known on the basis of one single measured rate constant, provided α2/H for XH is known or measured. HBA solvents can have profound effects on some of the reactions and thermodynamic properties of hydroxylic substrates including: (i) reaction product profiles (ii) antioxidant activities, (iii) Hammett-type correlations, and (iv) O-H bond dissociation enthalpies. Finally, literature data (Nielsen, M. F.; Hammerich, O. Acta Chem. Scand, 1992, 46, 883-896) on KSEs for two proton-transfer reactions are shown to be correlated by the same equation which correlates KSEs for hydrogen-atom transfers.
- Snelgrove,Lusztyk,Banks,Mulder,Ingold
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p. 469 - 477
(2007/10/03)
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- Free electron transfer from several phenols to radical cations of non-polar solvents
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Electron-transfer reactions from phenols to parent radical cations of solvents were studied using pulse radiolysis. Phenols bearing electron-withdrawing, electron-donating and bulky substituents were investigated in non-polar solvents such as cyclohexane, n-dodecane, n-butyl chloride and 1,2-dichloroethane. The experiments revealed the direct, synchronous formation of phenoxyl radicals and phenol radical cations in all cases and in nearly the same relative amounts. This was explained by two competing electron-transfer channels which depend on the geometry of encounter between the parent solvent radical cations and the solute phenol molecules. The mechanism is analysed at a microscopic level, treating diffusion as a slow process and the local electron transfer as an extremely rapid event. Furthermore, the effect of various phenol substituents and solvent types on the electron-transfer mechanism and on the decay kinetics of the solute phenol radical cations was analysed. The results were further substantiated using a quantum chemical approach.
- Ganapathi,Hermann,Naumov,Brede
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p. 4947 - 4955
(2007/10/03)
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- Hydrogen atom abstraction kinetics from intramolecularly hydrogen bonded ubiquinol-0 and other (poly)methoxy phenols
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The effect of methoxy substitution on the abstraction of the phenolic hydrogen atom involved in intramolecular hydrogen bonding by tert-butoxyl and cumyloxyl radicals has been investigated by laser flash photolysis. Also transition state calculations for
- De Heer, Martine I.,Mulder, Peter,Korth, Hans-Gert,Ingold, Keith U.,Lusztyk, Janusz
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p. 2355 - 2360
(2007/10/03)
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- Reactivity of substituted phenols toward alkyl radicals
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The rate constants for the reaction of primary alkyl radicals with substituted phenolic compounds have been measured in benzene or toluene at room temperature by using the radical clock technique. With three representative phenols, containing in the ortho positions substituents of different size, the kinetics of the hydrogen transfer to alkyl radicals was studied at different temperatures to obtain the corresponding Arrhenius parameters. The kinetic solvent effect on the reaction with α-tocopherol was also investigated in six different solvents behaving as hydrogen bond acceptors, while the reaction with 2,4,6-trimethylphenol and 2,6-di-tert-butylphenol was studied in toluene and γ-valerolactone. For some phenols, the effect of self-aggregation on the kinetic parameters was also studied.
- Franchi, Paola,Lucarini, Marco,Pedulli, Gian Franco,Valgimigli, Luca,Lunelli, Bruno
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p. 507 - 514
(2007/10/03)
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- Effect of Solvation on the Bond Dissociation Energies of Phenolic Antioxidants
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The effect of solvent on the bond dissociation energies (BDEs) of the oxygen-hydrogen bond in substituted phenolic antioxidants has been investigated by means of an EPR technique.On changing the solvent from benzene to tert-butanol the BDE's were found to increase by ca. 2.2 kcal/mol for phenols without ortho substituents, by ca. 1 kcal/mol for 2,6-dimethyl substituted phenols while in 2,6-di-tert-butyl phenols they seem to be substantially unaffected.This behaviour has been interpreted by admitting that the BDE increase observed in tert-butanol is essentially due to the solvation of the hydroxylic hydrogen which stabilises the phenol, leaving the energy of the phenoxyl radical unaltered.Thus, solvation effects are expected to be large with unhindered phenols and relatively unimportant in phenols containing bulky substituents in the proximity of the OH group.
- Pedrielli, Pamela,Pedulli, Gian Franco
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p. 509 - 512
(2007/10/03)
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- Reduction potentials of flavonoid and model phenoxyl radicals. Which ring in flavonoids is responsible for antioxidant activity?
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Model phenoxyl and more complex flavonoid radicals were generated by azide radical induced one-electron oxidation in aqueous solutions. Spectral, acid-base and redox properties of the radicals were investigated by the pulse radiolysis technique. The physicochemical characteristics of the flavonoid radicals closely match those of the ring with the lower reduction potential. In flavonoids which have a 3,5-dihydroxyanisole (catechins), or a 2,4-dihydroxyacetophenone (hesperidin, rutin, quercetin)-like A ring and a catechol- or 2-methoxyphenol-like B ring, the antioxidant active moiety is clearly the B ring [reduction potential difference between the model phenoxyls is ΔE(A-B ring models) > 0.1 V]. In galangin, where the B ring is unsubstituted phenyl, the antioxidant active moiety is the A ring. Even though the A ring is not a good electron donor, E7, > 0.8/NHE V, it can still scavenge alkyl peroxyl radicals, E7, = 1.06 V, and the Superoxide radical, E7 > 1.06 V. Quercetin is the best electron donor of all investigated flavonoids (measured E10.8 = 0.09 V, and calculated E7 = 0.33 V). The favourable electron-donating properties originate from the electron donating O-3 hydroxy group in the C ring, which is conjugated to the catechol (B ring) radical through the 2,3-double bond. The conjugation of the A and B rings is apparently minimal, amounting to less than 2.5% of the substituent effect in either direction. Thus, neglecting the acid-base equilibria of the A ring, and using those of the B ring and the measured values of the reduction potentials at pH 3,7 and 13.5, the pH dependence of the reduction potentials of the flavonoid radicals can be calculated. In neutral and slightly alkaline media (pH 7-9), all investigated flavonoids are inferior electron donors to ascorbate. Quercetin, E7 = 0.33 V, and gallocatechins, E7 = 0.43 V, can reduce vitamin E radicals (assuming the same reduction potential as Trolox C radicals, E7 = 0.48 V). Since all investigated flavonoid radicals have reduction potentials lower than E7 = 1.06 V of alkyl peroxyl radicals, the parent flavonoids qualify as chain-breaking antioxidants in any oxidation process mediated by these radicals.
- Jovanovic, Slobodan V.,Steenken, Steen,Hara, Yukihiko,Simic, Michael G.
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p. 2497 - 2504
(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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- Negative activation energies and compensation effects for the reactions of diarylanimyl radicals with phenols
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The temperature dependence of the rate constants of the reaction of 4,4'-disubstituted diphenylaminyl radicals (ArAr'N.) with phenols has been studied by a laser pulse photolysis technique.The linear relationships between activation energies E1 (negative in the majority of cases) and logarithms of pre-exponential factors logA1 have been established.These results coincide with the two-parametric correlations of logk1 with ?+ constants of substituents in phenol and aminyl radicals under isothermal conditions.The reaction rate constants decrease considerably when toluene is used instead of n-decane. - Key words: aminyl radicals, phenols, negative temperature coefficient, isoparametric correlations.
- Varlamov, V. T.,Denisov, N. N.,Nadtochenko, V. A.
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p. 2282 - 2286
(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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- A Mechanistic Study of the Oxidation of Phenols in Aqueous Solution by Oxoiron(IV) Tetra(N-methylpyridyl)porphyrins. A Model for Horseradish Peroxidase Compound II?
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The reaction of oxoiron(IV) tetra(2-N-methylpyridyl)porphyrin (OFeIVT2MPyP), generated from iron(III) tetra(2-N-methylpyridyl)porphyrin and tert-butyl hydroperoxide, with 3-cyanophenol in aqueous solution (pH 7.7) shows first-order dependence o
- Colclough, Nicola,Smith, John R. Lindsay
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p. 1139 - 1150
(2007/10/02)
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- Formation and Reactivity of Phenylperoxyl Radicals in Aqueous Solutions
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The reaction of phenyl radicals with oxygen, to produce phenylperoxyl radicals, and the reactions of several phenylperoxyl radicals with a number of organic compounds in aqueous solutions have been studied by pulse radiolysis.Phenyl radicals were produced by reduction of aryl halides with hydrated electrons.The rate constant for the reaction of 4-carboxyphenyl with O2 was determined from the rate of buildup of the peroxyl radical absorption at 520 nm as a function of and found to be 1.6E9 L mol-1 s-1.Phenyl radicals react with 2-PrOH by H abstraction; a rate constants of 4E6 L mol-1 s-1 was determined for 4-carboxyphenyl by competition with the reaction of this radical with O2.Phenylperoxyl radicals react with 4-methoxyphenolate ions, trolox C (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), ascorbate ions, chloropromazine, and ABTS by one-electron oxidation.The rate constants for such reactions, determined from the rate of formation of the one-electron oxidation product as a function of substrate concentration, were found to be near E8-E9 L mol-1 s-1.The reaction with neutral phenols, however, was much slower and could not be observed under pulse radiolysis conditions.Phenylperoxyl radicals are found to be much more reactive than methylperoxyl and more reactive than most substituted methylperoxyls, except for the halogen-substituted radicals.Electron-withdrawing substituents at the 4-position of phenylperoxyl increase the rate constant and electron-donating groups decrease the rate constant for oxidation by this radical, in accordance with the Hammett substituent constants.
- Alfassi, Z. B.,Marguet, S.,Neta, P.
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p. 8019 - 8023
(2007/10/02)
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- Electron-transfer reactions of alkyl peroxy radicals
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One-electron-transfer reactions of alkyl peroxy radicals were studied by pulse radiolysis of aqueous solutions. At pH 13, the methyl peroxy radical was found to rapidly, k = 1 × 105-4.9 × 107 s-1, and quantitatively oxidize various organic substrates with E13 = 0.13-0.76 V vs NHE. On the other hand, this radical was unreactive with compounds with E13 ≥ 0.85 V. Consequently, E13 of the methyl peroxy radical is higher than 0.76 V and lower than 0.85 V, which means that E7 is in the range 1.02-1.11 V. At pH 8, the rate constants of the oxidation of four ferrocene derivatives by the alkyl peroxy radicals ranged from 7.1 × 104 M-1 s-1 for ferrocenedicarboxylate (E8 = 0.66 V) to 2.3 × 106 M-1 s-1 for (hydroxymethyl)ferrocene (E8 = 0.42 V). These rate constants were used to evaluate the reduction potential and self-exchange rate of alkyl peroxy radicals in neutral media from the Marcus equation. The calculated E7 = 1.05 V is in excellent agreement with the estimated E7 = 1.02-1.11 V and with one of the perviously published values E7 = 1.0 V, but the value is in excellent agreement higher than the other E7 ~ 0.6 V. It is suggested that the high reorganization energy, λ = 72 kcal mol-1 redox couple originates from the requirement for solvent reorganization due to the solvation of hydroperoxide anion in the transition state. In support of this are the activation parameters of the reaction of the methyl peroxy radical with uric acid. The activation entropy is 9 eu lower at pH 7.3 than it is at pH 13.2, whereas the activation enthalpies are unchanged. The importance of entropy control was verified in the reactions of cyclohexyl peroxy radicals with α- and δ-tocopherol in aerated cyclohexane (ΔH+ ≈ 0 kcal/mol, and ΔS+ = -25 and -26 eu). The implications of these findings on the inactivation of alkyl peroxy radicals in general are discussed.
- Jovanovic, Slobodan V.,Jankovic, Ivana,Josimovic, Ljubica
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p. 9018 - 9021
(2007/10/02)
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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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- Bromine Atom Complexes with Bromoalkanes. Their Formation in the Pulse Radiolysis of Di-, Tri-, and Tetrabromomethane and Their Reactivity with Organic Reductants
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Bromine atoms were produced in the pulse radiolysis of neat dibromomethane (DBM) and bromoform and of cyclohexane solutions containing DBM, bromoform, carbon tetrabromide, or ethyl bromide.The Br atoms form complexes with dimethyl sulfoxide (λmax 425 nm), with aromatic compounds, and with oxygen-containing compounds.In the absence of other complexing agents, since Br atoms do not abstract H from the solvents rapidly, they form complexes with their parent bromo compounds.The absorption maxima of these complexes are at 365 nm for C2H5Br*Br, 390 nm for CH2Br2*Br, 425 nm for CHBr3*Br, and 480 nm for CBr4*Br.The stability of RBr*Br appears to increase with the number of Br atoms in the molecule.These complexes act as oxidants towards p-methoxyphenol, 1,3,5-trimethoxybenzene, triphenylamine, and N,N,N',N'-tetramethyl-p-phenylenediamine.The cate constants for the oxidations were about 1E10 M-1 s-1 with CH2Br2*Br but only of the order of 1E8 - 1E9 M-1 s-1 with CBr4*Br.The initial products of the oxidation are the ion pairs between the radical cation of the organic substrate and the Br- ion.In the case of p-methoxyphenol the initial ion pair releases HBr under neutral or basic conditions to form the neutral p-methoxyphenoxyl radical.The Br atom complexes are also capable of abstracting H from weak C-H bonds.The benzylic and allylic hydrogens in hexamethylbenzene and cyclohexene are abstracted with rate constants near 1E9 by CH2Br2*Br and near 1E7 M-1 s-1 by CBr4*Br.The behavior of Br atoms is compared with that of Cl and I atoms.
- Shoute, Lian C. T.,Neta, P.
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p. 2447 - 2453
(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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- The one-electron reduction potential of 4-substituted phenoxyl radicals in water
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By means of pulse radiolysis the one-electron reduction potentials of twelve 4-substituted phenoxy radicals have been determined. The main reference used was the ClO2./ClO2- couple. By combining the redox potentials of phenoxyl radicals with the aqueous acidities of phenols the bond strength of the phenolic O-H bond was calculated. These values were found to be in good agreement with O-H bond dissociation enthalpies measured in the gas phase.
- Lind,Shen,Eriksen,Merényi
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p. 479 - 482
(2007/10/02)
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- Reactivities of Chlorine Atoms and Peroxyl Radicals Formed in the Radiolysis of Dichloromethane
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Radiolysis of dichloromethane (DCM) leads to formation of primary oxidizing radicals and carbon-centered radicals.The latter react with oxygen to yield peroxyl radicals.The yields and chemical behavior of these intermediates were studied by pulse radiolysis of DCM solutions containing various solutes: phenols, anilines, dimethoxybenzene, hexamethylbenzene,cyclohexene, dimethyl sulfoxide, and zinc tetratolylporphyrin.At low concentrations, some of these solutes were found to be oxidized by two peroxyl radicals, CH2ClO2* and CHCl2O2*, with different rate constants.At higher concentrations, the solutes react also with the primary radicals: Cl atoms and the radical cations CH2Cl2(+*), with diffusion-controlled rate constants.The rates of these reactions were determined by competition kinetics because of the very short lifetimes of the species.Cl atoms were found to have a half-life of about 5 ns in DCM, reacting predominantly with the solvent by hydrogen abstraction.The radical cations decay within a fraction of a nanosecond.The total yield of these primary radicals was determined to be G = 3.6 and appears to be divided about equally between Cl and the radical cations.The total yield of oxidation, by the primary and the peroxyl radicals, was found to be G = 7.5.Cl atoms were found to be very reactive in electron transfer as well as addition and hydrogen abstraction reactions.
- Alfassi, Z. B.,Mosseri, S.,Neta, P.
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p. 1380 - 1385
(2007/10/02)
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- KINETIC STUDY OF THE REACTION OF BENZOYL PEROXIDE WITH PHENOLIC COMPOUNDS. A TECHNIQUE FOR THE EVALUATION OF PHENOLIC O-H BOND DISSOCIATION ENERGIES
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From kinetic data concerning the reaction of benzoyl peroxide (POB) with several phenolic antioxidants (ArOH): POB + ArOH -> C6H5CO2. + C6H5CO2H + ArO., it is shown that differences in the phenolic O-H bond dissociation energies may be obtained.In the particular case of α-tocopherol (α-TH), a stabilization energy defined with respect to phenol: D(C6H5O-H) - D(α-T-H) = 10 kcal.mol-1 has been estimated in heptanol as solvent.It would correspond for α-TH to an O-H bond dissociation energy around 78 kcal.mol-1.
- Rousseau-Richard, Claire,Richard, Claude,Martin, Rene
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p. 2057 - 2066
(2007/10/02)
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- Solvent Effects in the Reactions of Peroxyl Radicals with Organic Reductants. Evidence for Proton-Transfer-Mediated Electron Transfer
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Absolute rate constants for the reaction of substituted methylperoxyl radicals with ascorbate, urate, trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) and TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) have been determined by pulse radiolysis in different solvents.In water-alcohol or water-dioxane solutions, the rate constants for trihalomethylperoxyl radicals generally increase with increasing water content.The rate constant for the reaction of CCl3O2* radicals with trolox was measured in water, MeOH, i-PrOH, t-BuOH, ethylene glycol, diethyl ether, dioxane, acetone, acetonitrile, formamide, dimethylformamide, pyridie, and CCl4.The rate constants were found to correlate well with a two-parameter equation that includes the dielectric constant of the solvent and the coordinate covalency parameter, a measure of the proton-transfer basicity of the solvent.Kinetic isotope effects in H2O/D2O of about 2 and the activation entropies of about -10 eu for reduction of RO2* by the organic reductants indicate that electron transfer to the peroxyl radical is concerted with the transfer of proton from the solvent to the incipient hydroperoxide anion.
- Neta, P.,Huie, R. E.,Maruthamuthu, P.,Steenken, S.
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p. 7654 - 7659
(2007/10/02)
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- Reaction of Azide Radicals with Aromatic Compounds. Azide as a Selective Oxidant
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In basic aqueous solution the N3. radical is found to oxidize aromatic systems such as aniline and phenoxide ions and their deivatives at rate constants of (3-5) X 109 M-1s-1.In contrast to the reactions of OH radicals, wh
- Alfassi, Zeev B.,Schuler, Robert H.
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p. 3359 - 3363
(2007/10/02)
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- Chemical Behavior of SO3- and SO5- Radicals in Aqueous Solutions
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The chemistry of the radicals SO3- and SO5- has been investigated by using pulse radiolysis with kinetic spectrophotometry.Rate constants for the oxidation by SO3- of a variety of organic compounds were measured and equilibrium constants determined for the reactions of SO3- with chlorpromazine and phenol.SO3- was found to be a mild oxidant with a redox potential of E(SO3-/SO32-) = 0.63 V (vs.NHE) at pH>7 and E(SO3-/HSO3-) = 0.84 V at pH 3.6.The reaction of SO3- with O2 was shown to produce SO5-.The oxidation of several compounds by SO5- was found to occur more rapidly than their oxidation by SO3-.E(SO5-/HSO3-) was estimated to be approximately 1.1 V at pH 7.
- Huie, Robert E.,Neta, P.
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p. 5665 - 5669
(2007/10/02)
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- One-Electron Oxidation in Irradiated Carbon Tetrachloride Solutions of ZnTPP, TMPD, and Phenols
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One-electron oxidation of phenol, p-methoxyphenol, N,N,N',N'-tetramethyl-p-phenylenediamine, chlorpromazine, and zinc tetraphenylporphyrin was studied by pulse radiolysis in carbon tetrachloride solutions.Phenols form phenoxyl radicals and the other compounds form cation radicals with yields strongly dependent on solute concentration.The highest yield in deoxygenated solutions approached G = 4.In the presence of oxygen an additional oxidation step is observed owing to CCl3O2. radicals and the overall oxidation yield approached G = 8.ZnTPP was found to be oxidized to the cation radical without any side effects, unlike oxidation in 1,2-dichloroethane which was accompanied by demetallation owing to HCl production.
- Grodkowski, J.,Neta, P.
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p. 1205 - 1209
(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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