Quantum dynamics and kinetics of the abstraction reactions by H atoms of primary and secondary hydrogens in C3H8

The abstraction reaction of H atoms with propane molecules presents two concurrent channels. In this work we have determined the specific rate constants and the product branching ratios (BRs) between the two channels using quantum chemistry calculations and reduced dimensionality quantum dynamics. The potential energy surfaces were computed by treating explicitly the forming and breaking bonds during the reaction, and optimizing the geometries of all the remaining degrees of freedom. In this way, the dynamics of the reaction occurs on an effective reduced dimensionality hyper-surface accounting for the zero-point energy of the optimized degrees of freedom. Energies are calculated with the CCSD( T) method and the cc-pVTZ basis set, while frequencies are calculated with the MP2 method and the same basis set. The calculations give barrier heights of 0.46 (0.36) eV and the reactions are exothermic by 0.13 (0.25) eV for primary ( secondary) hydrogens in C3H8. At room temperature, quantum tunnelling and zero-point effects are found to contribute more than one order of magnitude to the rate constants, when compared to purely classical transition state theory ( TST) computations. The branching ratios show the importance of abstraction of secondary hydrogen in propane more significantly at a lower temperature in accord with experimental investigations.