Modeling the influence of resonance stabilization on the kinetics of hydrogen abstractions

Abstract

Resonance stabilization of the transition state is one of the key factors in modeling the kinetics of hydrogen abstraction reactions between hydrocarbons. A group additive model is developed which allows the prediction of rate coefficients for bimolecular hydrogen abstraction reactions over a broad range of hydrocarbons and hydrocarbon radicals between 300 and 1300 K. Group additive values for 50 groups are determined from rate coefficients determined using the high level CBS-QB3 ab initio method, corrected for tunneling and the hindered internal rotation around the transitional bond. Resonance and hyperconjugative stabilization of the transition state is accounted for by introducing 4 corrections based on the structure of the reactive moiety of the transition state. The corrections, fitted to a set of 28 reactions, are temperature-independent and reduce the mean absolute deviation on E(a) to 0.7 kJ mol(-1) and to 0.05 for log A. Tunneling contributions are accounted for by using a fourth order polynomial in the activation energy. Final validation for 19 reactions yields a mean factor of deviation between group additive prediction and ab initio calculation of 2.4 at 300 K and 1.8 at 1000 K. In comparison with 6 experimental rate coefficients (600-719 K), the mean factor of deviation is less than 3.