- 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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- 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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- 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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- Synthesis and inhibitory activity of alkyl(hydroxyaryl)amines
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The reaction of ω-(4-hydroxyaryl)haloalkanes with various nitrogen-containing agents afforded primary, secondary, and tertiary amino derivatives of 2,6-dialkylphenols. For the compounds synthesized, the reaction rate constants with peroxide radicals were
- Dyubchenko,Nikulina,Terakh,Prosenko,Grigor'ev
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p. 1149 - 1155
(2008/09/17)
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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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- 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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- 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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- 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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- Photoinduced hydrogen abstraction from phenols by aromatic ketones. A new mechanism for hydrogen abstraction by carbonyl n,π* and π,π,* triplets
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Nanosecond laser flash photolysis studies have been carried out of the kinetics of inter- and intramolecular phenolic hydrogen abstraction by alkoxyacetophenone, 5-alkoxyindanone, and 4-alkoxybenzophenone triplets in acetonitrile and benzene solution. Information on the geometric requirements for abstraction by carbonyl and π,π,* triplets is derived from the results for a series.of ketones which contain a para-phenolic moiety attached via a para-oxyethyl linkage. For all of these compounds, the deuterium kinetic isotope effect on the triplet lifetime in acetonitrile solution indicates that triplet decay is determined by the rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding phenoxyl-hemipinacol biradical. The biradicals have also been detected, and are about an order of magnitude longer-lived than the triplet in each case. For three of the compounds, the rates of the intramolecular process follow the same trend as that observed in the rates of bimolecular quenching of the parent methoxy-substituted ketones by p-cresol. Deviation from this trend is observed for the alkoxyindanone derivative, where an in-plane approach of the phenolic hydrogen to the carbonyl n-orbital is not possible. The trends in the rate constants for bimolecular quenching of a series of substituted benzophenones by p-cresol indicate that for n,π,* triplet abstractions, the quenching mechanism is different for electron donor-substituted and electron acceptor-substituted ketones: For π,π,* triplets and donor-substituted (n,π,* triplet) benzophenones, abstraction is proposed to occur by a mechanism involving the intermediacy.of a hydrogen-bonded exciplex, which yields the corresponding radicals by sequential electron- and proton-transfer. The rate constant for quenching by this mechanism thus depends mainly on the basicity of the ketone triplet state and the oxidation potential of the phenol.
- Leigh, William J.,Lathioor, Edward C.,St. Pierre, Michael J.
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p. 12339 - 12348
(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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- Laser flash photolysis determination of absolute rate constants for reactions of bromine atoms in solution
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The photodecomposition of vicinal dibromides at 266 nm produces bromine atoms with a quantum yield of ~2.0. This results from an efficient primary photocleavage of a C-Br bond, followed by rapid elimination of a second bromine atom from radicals of the ty
- Scaiano,Barra,Krzywinski,Sinta,Calabrese
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p. 8340 - 8344
(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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- 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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- Photophysical Properties of Methylated Phenols in Nonpolar Solvents
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The photophysical properties of phenol and a series of mono-, di-, and trimethyl-substituted phenols excited in their first singlet state in hydrocarbon solvents have been investigated by measuring fluorescence lifetimes and quantum yields and lowest trip
- Grabner, G.,Koehler, G.,Marconi, G.,Monti, S.,Venuti, E.
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p. 3609 - 3613
(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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- Electron Spin Resonance Studies. Part 71. Side-chain Oxidation Pathways in the Reactions of .OH and SO4-. with Some Phenyl-substituted Carboxylic Acids, their Anions, and Some related Compounds
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A series of arene radical-cations has been generated in situ by the reactions of methylbenzene, phenylethanoic acid, and some derivatives and cyclic analogues with both SO4-. and .OH (the latter in acid solution).The results are interpreted in terms of a variety of subsequent rapid reactions including hydration, deprotonation (to give benzylic radicals), and fragmentation (decarboxylation: for a series of radical-zwitterions +.Ar(CH2)nCO2- (n=1-3) decarboxylation (k >/= 1E9 dm3 mol-1 s-1) appears to proceed via direct intramolecular elecron-transfer, though in some cases formation of a discrete ?-bonded intermediate cannot be ruled out.
- Gilbert, Bruce C.,Scarratt, Cathryn J.,Thomas, C. Barry,Young, John
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p. 371 - 380
(2007/10/02)
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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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- 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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- 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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