- Temperature-Dependent Rate Constants and Product Branching Ratios for the Gas-Phase Reaction Between CH3O2 and ClO
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Kinetic and product branching data have been measured for the reaction of CH3O2 radicals with ClO in the temperature range 225-355 K using the discharge-flow/mass spectrometry technique.The pressure-independent overall reaction rate constant is described by k(12)(225-355 K) = (3.25 +/- 0.50) * 10-12 exp((-114 +/- 38)/T) cm3 molecule-1 s-1.Two reaction channels were identified, leading to CH3O + ClOO (12a) and CH3OCl + O2 (12b), respectively.The branching ratios, α12a = k12a/k12 and α12b = k12b/k12, are also independent of pressure and are described by α12a = (1.51 +/- 0.56) exp((-218 +/- 93)/T) and α12b = (0.080 +/- 0.059) exp((-377 +/- 178)/T).These expressions yield roughly equal rate constants of k12a and k12b of ca. 1 * 10-12 cm3 molecule-1 s-1 at the low temperatures prevalent in the polar winter and early springtime stratosphere.We thus identify CH3OCl as a potentially important species in ozone hole chemistry.
- Helleis, Frank,Crowley, John N.,Moortgat, Geert K.
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- Temperature-dependent study of the CH3C(O)O2 + NO reaction
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The kinetics of the reaction between acetylperoxy radicals and nitric oxide have been examined both by transient IR absorption and by time-resolved UV spectroscopy. The former technique enables measurements of NO loss and NO2 formation, whereas the latter provides data on the decay of acetylperoxy radicals, the secondary formation of methylperoxy radicals, and their conversion into methylnitrite. The two methods give consistent rate constants which are fit by the expression k1 = (2.1-0.8+1.4) × 10-12e(570±140)/T cm3 s-1. The room temperature value of k1 = (1.4 ± 0.2) × 10-11 cm3 s-1 is somewhat smaller than the currently recommended value, which is based on indirect determinations of k1. Measurements of the CH3O2 and NO2 yields indicate that the reaction proceeds exclusively to the products CH3C(O)O and NO2. The negative temperature dependence suggests that the reaction proceeds via an intermediate adduct that rearranges and dissociates into the products.
- Maricq, M. Matti,Szente, Joseph J.
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p. 12380 - 12385
(2007/10/03)
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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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- Kinetics of Reactions of CH3O2 and HOCH2CH2O2 Radicals Produced by the Photolysis of Iodomethane and 2-Iodoethanol
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The molecular modulation technique coupled with UV absorption spectroscopy has been used to investigate the UV spectra and kinetics of reactions of the methylperoxy radical (CH3O2) and the 2-hydroxyethylperoxy radical (HOCH2CH2O2), generated by the 254-nm photolysis of the organic iodides CH3I and HOCH2CH2I: RI + hν (λ = 254 nm) -> R + I (7) and R + O2 + M -> RO2 + M (8).Measurements of the UV spectra of both RO2 radicals were complicated by the production of additional transient species absorbing strongly at wavelengths above 240 nm.These are believed to be CH3OOIand HOCH2CH2OOI formed as intermediates in the RO2-chaperoned recombination of iodine atoms.Both CH3O2 and HOCH2CH2O2 were found to obey second-order kinetic behaviour owing to removal by a series of reactions initiated by the self-reactions: CH3O2 + CH3O2 -> products (9) and HOCH2CH2O2 + HOCH2CH2O2 -> products (10).The parameter k9obs/? (where k9obs is the observed second-order rate coefficient) had a value of (1.01 +/- 0.09)E5 cm s-1 at 230 nm, independent of pressure in the range 10.8-760 Torr, at 298 K.Additional measurements made over the temperature range 268-350 K indicated that this parameter displays a weak negative temperature dependence.E/R was found to have a value of -220 +/- 72 K at 760 Torr, and a value of -92 +/- 53 K at 10.8 Torr.The parameter k10obs/? had a value of (6.8 +/- 0.4)E5 cm s-1 at 230 nm (p = 760 Torr, T = 298 K).Assuming the photolysis of HOCH2CH2I leads exclusively to the production of HOCH2CH2O2, the following values of ?(230 nm) and k10obs were concluded: ?(230 nm) = (2.35 +/- 0.25)E-18 cm2 molecule-1 and k10obs = (1.60 +/- 0.17)E-12 cm3 molecule-1 s-1.At lower pressures, HO2 was also generated in the HOCH2CH2I system in significant quantities, enabling investigation of reaction 11 at 10 Torr and 298 K: HOCH2CH2O2 + HO2 -> products (11).A value of k11 = (4.8 +/- 1.5)E-12 cm3 molecule-1 s-1 was concluded from these measurements, based on the above value of ?(230 nm).
- Jenkin, Michael E.,Cox, Richard A.
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p. 3229 - 3237
(2007/10/02)
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- Reactivities of Acylperoxy Radicals in the Photoreaction of α-Diketones and Oxygen
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The photoepoxidation of olefins with α-diketones and oxygen has been studied mechanistically focusing on the reactivities of intermediate radicals.One mole of α-diketone resulted in the formation of 2 mol of epoxide together with 2 equiv of C-C cleavage of olefins.The photoepoxidation proceeds via acylperoxy radicals RCO3* and the C-C cleavage of olefins is caused by acyloxy radical RCO2*.The addition of RCO3* to olefins was found to be ca.105-fold faster than that of acylperoxy radical ROO*.The relative reactivities of olefins suggest that acylperoxy radicals behave as a strongly electrophilic radical.That is, ρ values of -1 (vs.?+) obtained in the photoepoxidation of substituted styrenes are of the same magnitude as those in the epoxidation with molecular peracids.Although the relative reactivities of olefins toward the photoepoxidation roughly parallel those for the peracid epoxidations, the additivity of methyl substituent is not always operative.This is explained by a steric retardation by too many substituents on the carbon attacked by RCO3* in addition to the relative stabilities of resulting adduct radicals between olefins and RCO3*.Since acylperoxy radicals are not reactive towards sulfides, sulfoxides, or pyridine, a selective epoxidation of double bonds is possible.The relative reactivities of olefins toward benzoyloxy and methylperoxy redicals revealed a much less electrophilic nature of these oxy radicals, the ρ values for styrenes being -0.1 to -0.2.
- Sawaki, Yasuhiko,Ogata, Yoshiro
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p. 3344 - 3349
(2007/10/02)
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- A Fourier Transform Infrared Study of the Kinetics and Mechanism for the Reaction HO + CH3OOH
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The analysis of products in the photolysis of mixtures containing C2H5ONO, NO, and CH3OOH in ppm concentrations in 700 torr of O2-N2 diluent by FT IR is consistent with a value of (k1a/k1b) = 0.77 (+/-20percent) for the competitive H-abstraction channels HO + CH3OOH -> CH2OOH + H2O (1a) and -> CH3OO + H2O (1b).An absolute value of (k1a + k1b) = 1.0 * 10-11 cm3 molecule-1 s-1 was derived from the decay rates of CH3OOH relative to those of C2H4 and CH3CHO.
- Niki, H.,Maker, P. D.,Savage,C. M.,Breitenbach, L. P.
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p. 2190 - 2193
(2007/10/02)
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