Flash photolysis of water vapor in methane. Hydrogen and hydroxyl yields and rate constants for methyl reactions with hydrogen and hydroxyl

Abstract

Publication costs assisted by the U.S. Department of EnergyMethyl radicals, generated by reaction of CH4 with OH radicals from the flash photolysis of water vapor, decay by reaction with H, OH, and CH3. The concentration of CH3 ie followed by monitoring the absorption at 216 nm: use of the previously determined values of (3.1 f 0.6) X 1O1O MI1 s-l for k3(CH3+CH3) and t(CH3) = 9000 f 800 M-l cm-' at this wavelength fior a 0.6-nm bandwidth permits calculation of the amount of water dissociated per flash and the values of several rate constants. Measurements were made as a function of CH4 concentration by using Nz and Hz as diluents at atmospheric pressure. When numerical modeling was used to fit the CH3 data by the method of least squares, it was found that 0.9 X lo4 M OH and H concentrations are produced under typical operating conditions. The following absolute rate constants (296 K) were obtained: k,(lH+-(OH+-CH,-CH,+H,O) = (4.2 f 0.4) X lo6 M-I s-l. , a nd k,(OH+HZ-~H+HzO) = (5.1 f 1.1) X lo6 M-l s-l. The error limits were determined by eensitivity analysis. Information on water dissociation yields under these conditions permits the use of water vapor as a calibrated radical source for the study of other important systems.CH3+M-CH,) = (1.2 f 0.3) X 10l1 M-' s-l; k ,(OH+CH,+M-products) = (5.6 f 1.5) X 1O1O M-l s-l; k 2-

Thermal Rate Coefficients and Kinetic Isotope Effects for the Reaction OH + CH4 → H2O + CH3 on an ab Initio-Based Potential Energy Surface

“…This exothermic reaction (Δ H 0 = −13.5 kcal/mol) has a moderate barrier (Δ E ≈ 6–7 kcal/mol), − which features the transfer of a hydrogen atom. Because of its practical importance, the thermal rate coefficients of the title reaction have been measured with different techniques over a wide range (178–2025 K) of temperatures. − As shown in Figure , these experimental rate measurements, although generally consistent with each other, still have some discrepancies. It is not difficult to notice from the figure that the temperature dependence of the rate coefficient is non-Arrhenius, with a significant curvature at low temperatures.…”

Recrossing and Tunneling in the Kinetics Study of the OH + CH4 → H2O + CH3 Reaction

“…The OH + CH 4 hydrogen abstraction reaction is of great importance at low and high temperatures: at low temperatures it constitutes the major process for the removal of methane in atmospheric chemistry, in competition with the O­( 1 D) and Cl­( 2 P) reactions; and at high temperatures because of its importance in combustion. The thermal rate constants for the title reaction have been reported experimentally using a wide variety of techniques, − and the two most recent evaluations , recommend the following expressions, in cm 3 molecule –1 s –1 : and …”

New Pathway for the CH3 + OH → CH2 + H2O Reaction on a Triplet Surface

“…The reaction of CH 3 with OH is known to play a significant role in the combustion mechanisms of almost all hydrocarbons. Many theoretical − and experimental − studies of the reaction have, accordingly, been carried out. On the singlet surface, the reaction is believed to occur via formation and subsequent decomposition (or stabilization) of vibrationally excited methanol.…”