Abstract:

Hydrogen abstraction from alkanes by hydroperoxyl radical is an important reaction class in the combustion of hydrocarbon fuel, particularly at low and intermediate temperature regimes. In this study, kinetic parameters for this reaction class are calculated using the isodesmic reaction method based on conservation of geometric structures for the reaction center of the transition states. The geometries for all the reactants, transition states, and products are optimized at the HF/6-31+G (d) level. Hydrogen abstraction from ethane by hydroperoxyl radical is chosen as the reference reaction; other reactions are target reactions. The isodesmic reaction method is used to correct the approximate energy barriers and rate constants of the target reactions at the HF/6-31+G (d) level. To validate the reliability of the isodesmic reaction method, the energy barriers calculated by the isodesmic reaction method and at a high level of CCSD (T)/CBS for alkanes containing less than five carbon atoms are compared. The maximum absolute difference of energy barriers between the isodesmic reaction method and CCSD (T)/CBS method is 5.58 kJ·mol^-1. Therefore, after correction, using the isodesmic reaction method, the low-level HF method can reproduce the high-level CCSD (T)/CBS calculated energy barriers. Thus, we have solved the problem of accurately calculating energy barriers for large molecular systems in this reaction class. The present work provides accurate kinetic parameters for hydrogen abstraction from alkanes by hydroperoxyl radical, which are important for combustion modeling at low and intermediate temperature regimes.

Key words: Reaction class isodesmic reaction method, Energy barrier of reaction, Rate constant, Hydrogen abstraction of alkanes, Combustion at low and intermediate temperature regimes