Molecular modeling of combustion kinetics. The abstraction of primary and secondary hydrogens by hydroxyl radical

We present a simulation of the combustion reaction OH + propane using variational transition-state theory, multidimensional semiclassical tunneling calculations, and a dual-level approach to direct dynamics as a way to interface dynamical theory with electronic structure theory. The propane reaction involves new features as compared to the simpler reactions that have been simulated previously; in particular three unique transition states are involved - two involving hydroxyl attack at the primary carbon and one involving attack at the secondary carbon. Optimizing the transition state with scaled electron correlation is found to have only a small effect on the geometry but gives improved barrier heights that are only 0.4-0.7 kcal above our best estimates. Combining the three transition state structures with five different isotopic substitution patterns that have been considered experimentally leads to 22 unique reaction processes, for all of which we calculate the reaction rate by dual-level direct dynamics with an empirically scaled barrier height. The results confirm the assumptions used by experimentalists that primary and secondary site reaction rate constants are almost the same in different isotopic environments. The calculations show that the experimentally measured kinetic isotope effects are dominated by tunneling effects.