Rate constants for the H abstraction from alkanes (R-H) by R′O•2 radicals:: A systematic study on the impact of R and R′

A possible source of chai n-branching in low temperature combustion is thermal decomposition of alkyl hydroperoxides (R'OOH). One way these species can be produced is via H atom abstraction reactions from alkanes (RH) by alkylperoxy radicals R'O-2(center dot). An earlier study focussing on the abstraction from ethane by HO2 center dot, CH3O2 center dot; and C2H5O2 center dot, revealed that these reactions have a noticeable impact on calculated ignition times of ethane/02 mixtures. Another outcome was that the abstraction rate constants for CH3O2 center dot and C2H5O2 center dot are virtually identical but smaller than that for HO2 center dot. The associated activation energies followed an Evans-Polanyi relationship while a common A-factor could be used to describe the kinetics of all three reactions within a factor of about 2-3. In this current study, we extend the investigation by (1) considering a set of alkanes (RH = CH4, C2H6, C3H8, C4H10) and (2) by including additional peroxy species (R'O'(2) with R'= H, CH3, C2H5, C3H7, C4H9, HC=O, and CH3C=O). We present rate constants for a total of 32 reactions and analyze the data with respect to systematic trends in the reactivity. The results reveal that the rate constants decrease in the order acylperoxy > HO2 center dot > alkylperoxy. The reactivity of different C-H bonds follows the bond strengths. Overall the heat of reaction is found to be the dominant but not the only rate constant controlling parameter. The accuracy of the calculations and implications of the results are discussed. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.