Kinetic modeling of α-hydrogen abstractions from unsaturated and saturated oxygenate compounds by carbon-centered radicals.

Abstract

Hydrogen abstractions are important elementary reactions in a variety of reacting media at high temperatures in which oxygenates and hydrocarbon radicals are present. Accurate kinetic data are obtained from CBS-QB3 ab initio (AI) calculations by using conventional transition-state theory within the high-pressure limit, including corrections for hindered rotation and tunneling. From the obtained results, a group-additive (GA) model is developed that allows the Arrhenius parameters and rate coefficients for abstraction of the α-hydrogen from a wide range of oxygenate compounds to be predicted at temperatures ranging from 300 to 1500 K. From a training set of 60 hydrogen abstractions from oxygenates by carbon-centered radicals, 15 GA values (ΔGAV°s) are obtained for both the forward and reverse reactions. Among them, four ΔGAV°s refer to primary contributions, and the remaining 11 ΔGAV°s refer to secondary ones. The accuracy of the model is further improved by introducing seven corrections for cross-resonance stabilization of the transition state from an additional set of 43 reactions. The determined ΔGAV°s are validated upon a test set of AI data for 17 reactions. The mean absolute deviation of the pre-exponential factors (log A) and activation energies (E(a)) for the forward reaction at 300 K are 0.238 log(m(3) mol(-1) s(-1)) and 1.5 kJ mol(-1), respectively, whereas the mean factor of deviation <ρ> between the GA-predicted and the AI-calculated rate coefficients is 1.6. In comparison with a compilation of 33 experimental rate coefficients, the <ρ> between the GA-predicted values and these experimental values is only 2.2. Hence, the constructed GA model can be reliably used in the prediction of the kinetics of α-hydrogen-abstraction reactions between a broad range of oxygenates and oxygenate radicals.