- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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