Flash photolysis study of the methylperoxy + methylperoxy reaction: rate constants and branching ratios from 248 to 573 K

Abstract

The reactions 2CH30, -+ 2CH30 + O2 (la), 2CH302 -CH30H + HCHO + 0, (lb), and 2CH302 -CH300CH3 + O2 (IC) have been studied at temperatures between 248 and 573 K. At temperatures above 373 K, the resulting decay traces were distorted away from pure second order at short wavelengths (around 210 nm), owing to the presence of the hydroperoxy radicals formed via the nonterminating pathway (la) and the subsequent rapid step CH30 + 0, -HCHO + H 0 2 (2). This distortion enabled the nonterminating/terminating branching ratio, 0, to be determined. Combining the present results with previously published work on the branching ratios gave In ( 3 = 3.80 -1470/T. Thus, although reaction 1 acts as a termination reaction under atmospheric conditions, it largely serves to convert CH302 into H 0 2 under combustion conditions.The temperature dependence of 0 enabled the real rate constant for the reaction, kl, to be. obtained over the entire experimental temperature range, giving k , = 1.3 X $E,R/K2 = 1712, and Absolute uncertainties, including contributions from both the experimental measurements and the dependence of kl on various analysis parameters, are estimated to be 22%, independent of temperature. No dependence of either the branching ratio or k , on the total pressure was found. The mechanism of the title reaction is discussed and the present results are compared with existing studies of alkylperoxy self-reactions. The implications for combustion and atmospheric modeling are also discussed. exp(365/n cm3 molecule-' s-I, with u2*/cm6 molecule-2 = 2.00 X molecule-' s-I = -5.61 X IntroductionThe self-reactions of peroxy radicals are of both practical and theoretical interest. Peroxy radicals are known to be key intermediates in both the combustion1 and atmospheric2 oxidation of hydrocarbons, and a detailed knowledge of the reactions between peroxy radicals would thus be an important contribution toward our understanding of these highly important and complicated processes. In general, such reactions display a negative temperature dependence and often several parallel reaction pathways, suggesting the intermediacy of an association complex, and a very complicated potential energy surface for the reaction. For ex-

Structure-reactivity relationships for the self- reactions of linear secondary alkylperoxy radicals: An experimental investigation

“…A conventional Ar flash-lamp photolysis technique, which has been detailed previously [18], was used to generate these peroxy radicals via the photolysis of molecular chlorine in the presence n-decane and excess oxygen, as shown below (where sec-decyl represent the ensemble of the four possible secondary radical centers respectively): …”

“…In this way, the following values were obtained at 298 K for the rate coefficients of the primary 1-pentylperoxy radical and the global secondary pentylperoxy radical self-reactions: To quantify the sensitivity of the rate coefficients towards the parameters used for analysis and the assumptions made in the chemical system, a sensitivity analysis of systematic errors and their propagation was carried out as described previously [7,18]. Briefly, artificial decay traces were generated using the kinetic scheme described above, with the same rate coefficients, initial radical concentrations, and absorption cross-sections used for analysis.…”

A new method for atmospheric detection of the CH<sub>3</sub>O<sub>2</sub> radical

“…As each HO 2 radical consumes one CH 3 O 2 species (Reacton R13) on the timescale of Reaction (R6), k obs is given by (Sander and Watson, 1981;Lightfoot et al, 1990)…”

“…The previous kinetic studies used time-resolved UV-absorption spectroscopy to detect CH 3 O 2 radical, typically at 250 nm, Watson, 1980, 1981;McAdam et al, 1987;Kurylo and Wallington, 1987;Jenkin et al, 1988;Simon et al, 1990;Lightfoot et al, 1990). UV-absorption spectroscopy is a relatively insensitive technique and hence the detection limits of CH 3 O 2 were quite high, for example approximately 4 × 10 12 molecule cm −3 Watson, 1980, 1981).…”

Combined experimental and theoretical investisgation of the reactivity of CH3O2 and C2H5O2 radicals

“…The UV spectra of different peroxy radicals are depicted in Figure 1.3. Due to the significance of CH3O2 radicals in the atmospheric degradation of methane, there has been a wealth of spectroscopic studies of the UV absorption spectrum of this radical from the last fifty years 7,[25][26][27][28][29][30][31][32][33] . Generally, two techniques have been employed: modulated and flash photolysis, both combined with UV absorption spectroscopy.…”