- Reactions of the cumyloxyl and benzyloxyl radicals with strong hydrogen bond acceptors. Large enhancements in hydrogen abstraction reactivity determined by substrate/radical hydrogen bonding
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A kinetic study on hydrogen abstraction from strong hydrogen bond acceptors such as DMSO, HMPA, and tributylphosphine oxide (TBPO) by the cumyloxyl (CumO?) and benzyloxyl (BnO?) radicals was carried out in acetonitrile. The reactions
- Salamone, Michela,Dilabio, Gino A.,Bietti, Massimo
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p. 10479 - 10487
(2013/02/22)
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- Hydrogen atom abstraction reactions from tertiary amines by benzyloxyl and cumyloxyl radicals: Influence of structure on the rate-determining formation of a hydrogen-bonded prereaction complex
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A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO?) and benzyloxyl (BnO?) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (kH) were measured for the two radicals (kH(BnO ?)/kH(CumO?) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in kH(BnO?)/kH(CumO ?) ratios of 13 to 2027 times were observed. kH approaches the diffusion limit in the reactions between BnO? and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD2O?, BnO?- d2) and the 3,5-di-tert-butylbenzyloxyl radical.
- Salamone, Michela,Dilabio, Gino A.,Bietti, Massimo
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experimental part
p. 6264 - 6270
(2011/10/08)
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- Diffusion controlled hydrogen atom abstraction from tertiary amines by the benzyloxyl radical. the importance of C-H/N hydrogen bonding
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The rate constants for H-atom abstraction (kH) from 1,4-cyclohexadiene (CHD), triethylamine (TEA), triisobutylamine (TIBA), and DABCO by the cumyloxyl (CumO?) and benzyloxyl (BnO ?) radicals were measured. Comparable ksub
- Salamone, Michela,Anastasi, Gloria,Bietti, Massimo,Dilabio, Gino A.
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supporting information; experimental part
p. 260 - 263
(2011/03/22)
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- The role of structural effects on the reactions of alkoxyl radicals with trialkyl and triaryl phosphites. A time-resolved kinetic study
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(Figure presented) A time-resolved kinetic study on the reactions of alkoxyl radicals with trialkyl and triaryl phosphites ((RO)3P: R = Me, Et, i-Pr, t-Bu; (ArO)3P: Ar = C6H5, 2,4-(t-Bu)2C6H3) has been carried out. In the (RO)3P series, the alkoxyl radicals (cumyloxyl (CumO ·) and benzyloxyl (BnO·)) undergo addition to the phosphorus center with formation of intermediate tetraalkoxyphosphoranyl radicals (R′OP·(OR)3: R = Me, Et, i-Pr, t-Bu; R′ = Bn, Cum). The addition rate constants are influenced by steric effects, decreasing on going from R = Me to R = t-Bu and from BnO · to CumO·. Rate constants for β-scission of the phosphoranyl radicals R′OP·(OR) 3 have also been determined, increasing, for a given alkyl group R, in the order R′ = tert-butyl · reacts with triaryl phosphites (ArO) 3P to give phenoxyl radicals, with rate constants that are influenced to a limited extent by substitution of the aromatic rings. The radical scavenging ability of these substrates is briefly discussed.
- Bietti, Massimo,Calcagni, Alessandra,Salamone, Michela
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body text
p. 4514 - 4520
(2010/10/02)
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- Determination of the Enthalpies of Formation of C6H5CH2O·, p-CH3OC6H4CH2O·, and C6H5CHOH Radicals by Photoionization Mass Spectrometry
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The enthalpies of formation (kJ mol-1) of PhCH2O· (125), p-CH3OC6H4CH2O· (-37), and PhCHOH radicals (28) were determined from the energies of appearance of the corresponding fragment ions. The reliability of these data is confirmed by the coincidence of the experimental results with those calculated by the method of isodesmic reactions.
- Ponomarev,Takhistov,Orlov
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p. 1774 - 1776
(2007/10/03)
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- Kinetics of the cross reactions of CH3O2 and C2H5O2 radicals with selected peroxy radicals
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The kinetics of the reactions of selected peroxy radicals (RO2) with CH3O2 and with C2H5O2 have been investigated using two techniques: excimer-laser photolysis and conventional flash photolysis, both coupled with UV absorption spectrometry. Radicals were generated either by photolysis of molecular chlorine in the presence of suitable hydrocarbons or by photolysis of the appropriate alkyl chloride. All such cross-reaction kinetics were investigated at 760 Torr total pressure and room temperature except for the reaction of the allylperoxy radical with CH3O2, for which the rate constant was determined between 291 and 423 K, resulting in the following rate expression: k15 = (2.8 ± 0.7) × 10-13 exp[(515 ± 75)/T] cm3 molecule-1 s-1. Values of (2.0 ± 0.5) × 10-13, (1.5 ± 0.5) × 10-12, (9.0 ± 0.15) × 10-14, -12, (2.5 ± 0.5) × 10-12, and (8.2 ± 0.6) × 10-12 (units of cm3 molecule-1 s-1) have been obtained for the reactions of CH3O2 radicals with C2H5O2, neo-C5H11O2, c-C6H11O2, C6H5CH2O2, CH2ClO2, and CH3C(O)O2, respectively, and (1.0 ± 0.3) × 10-12, (5.6 ± 0.8) × 10-13, (4.0 ± 0.2) × 10-14, and (1.0 ± 0.3) × 10-11 (units of cm3 molecule-1 s-1) for the reactions of C2H5O2 with CH2=CHCH2O2, neo-C5H11O2, c-C6H11O2, and CH3C(O)O2 radicals, respectively. These rate constants were obtained by numerical simulations of the complete reaction mechanisms, which were deduced from the known mechanisms of the corresponding peroxy radical self-reactions. A systematic analysis of propagation of errors was carried out for each reaction to quantify the sensitivity of the cross-reaction rate constant to the parameters used in kinetic simulations. The rate constant for a given cross reaction is generally found to be between the rate constants for the self-reactions of RO2 and CH3O2 (or C2H5O2). However, when the RO2 self-reaction is fast, the cross reaction with CH3O2 (or C2H5O2) is also fast, with similar rate constants for both reactions, suggesting that these particular peroxy radical cross reactions can play a significant role in the chemistry of hydrocarbon oxidation processes in the troposphere and in low-temperature combustion. Relationships between cross-reaction and self-reaction rate constants are suggested.
- Villenave, Eric,Lesclaux, Robert
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p. 14372 - 14382
(2007/10/03)
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- A Kinetic and Mechanistic Study of the Self-Reaction and Reaction with H2O of the Benzylperoxy Radical
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The kinetics and mechanism of the reactions C6H5CH2O2 + C6H5CH2O2 -> 2C6H5CH2O + O2 (3a), C6H5CH2O2 + C6H5CH2O2 -> C6H5CHO + C6H5CH2OH + O2 (3b), and C6H5CH2O2 + HO2 -> C6H5CH2OOH + O2 (4) have been investigated using two complementary techniques: flash photolysis/UV absorption for kinetic measurements and continuous photolysis/FTIR spectroscopy for end-product analyses and branching ratio determinations.The reaction of chlorine atoms with toluene was found to yield benzyl radicals exclusively and was used to generate benzylperoxy radicals in excess oxygen.During this study, relative reaction rate constants of chlorine atoms with compounds related to those involved in the reaction mechanism have been measured at room temperature: k(Cl+toluene) = (6.1 +/- 0.2)E-11, k(Cl+benzaldehyde) = (9.6 +/- 0.4)E-11, k(Cl+benzyl chloride) = (9.7 +/- 0.6)E-12, k(Cl+benzyl alcohol) = (9.3 +/- 0.5)E-11, k(Cl+benzene) 3 molecule-1 s-1.The products identified following the self-reaction 3 were benzaldehyde, benzyl alcohol, and benzyl hydroperoxide.The latter is the product of the reaction of C6H5CH2O2 with HO2.The yield of products allowed us to determine the branching ratio α = k3a/k3 = 0.4.The UV absorption spectrum of the benzylperoxy radical was determined from 220 to 300 nm.It was similar to those of alkylperoxy radicals, with a maximum cross section at 245 nm of 6.8E-18 cm2 molecule-1.Kinetic data were obtained from the detailed simulation of experimental decay traces recorded at 250 nm over the temperature range 273-450 K.The resulting rate expression are k3 = (2.75 +/- 0.15)E-14 exp cm3 molecule-1 a-1 and k4 = (3.75 +/- 0.32)E-13 exp3 molecule-1 s-1 (errors = 1?).The UV absorption traces in the flash-photolysis kinetic study were well accounted for by the identified products in the FTIR study, thus providing good confidence in the results.However, about 20percent of the products have remained unidentified.Some uncertainties persist in the reaction mechanism leading us to assign a fairly large uncertainty of about 50percent to the rate constants k3 and k4 over the whole temperature range.This work shows that the aromatic substituent does not provide any specificity in the reactivity of peroxy radicals and confirms that large radicals tend to react faster with HO2 than generally assumed in current atmospheric models.
- Noziere, Barbara,Lesclaux, Robert,Hurley, Michael D.,Dearth, Mark A.,Wallington, Timothy J.
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p. 2864 - 2873
(2007/10/02)
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- Study of the kinetics and equilibrium of the benzyl-radical association reaction with molecular oxygen
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The forward rate constant, k1, and the equilibrium constant, Kp, for the association reaction of the benzyl radical with oxygen have been determined. The rate constant k1 was measured as a function of temperature (between 298 and 398 K) and pressure (at 20 and 760 torr of N2) by two different techniques, argon-lamp flash photolysis and excimer-laser flash photolysis, both of which employed UV absorption spectroscopy (at 253 nm and 305 nm, respectively) to monitor the benzyl radical concentration. Over the range of conditions studied, we find that the reaction is independent of pressure and is almost independent of temperature, which is in accord with two early studies of the reaction but in apparent disagreement with more recent work. For our results in 760 torr of N2 and for 298 1 = (7.6 ± 2.4) × 10-13 exp[(190 ± 160)K/T] cm3 molecule-1 s-1. With the flash-photolysis technique, we determined Kp over the temperature range 398-525 K. Experimental values were analyzed alone and combined with theoretically determined entropy values of the benzyl and benzylperoxy radicals to determine the enthalpy of reaction: ΔH°298 = (-91.4 ± 4) kJ mol-1. Previous work on the benzyl radical enthalpy of formation allows us to calculate ΔH°f 298 (Benzylperoxy) = (117 ± 6) kJ mol-1. In addition, we carried out an RRKM calculation of k1 using as constraints the thermodynamic information gained by the study of Kp. We find that all the studies of the association reaction are in good agreement once a fall-off effect is taken into account for the most recent work conducted at pressures near 1 torr of helium.
- Fenter,Noziere,Caralp,Lesclaux
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p. 171 - 189
(2007/10/03)
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