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 tu nneling 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 uni que transition states are involved-two involving hydroxyl attack at th e primary carbon and one involving attack at the secondary carbon. Opt imizing the transition state with scaled electron correlation is found to have only a small effect on the geometry but gives improved barrie r heights that are only 0.4-0.7 kcal above our best estimates. Combini ng the three transition state structures with five different isotopic substitution patterns that have been considered experimentally leads t o 22 unique reaction processes, for all of which we calculate the reac tion rate by dual-level direct dynamics with an empirically scaled bar rier height. The results confirm the assumptions used by experimentali sts that primary and secondary site reaction rate constants are almost the same in different isotopic environments. The calculations show th at the experimentally measured kinetic isotope effects are dominated b y tunneling effects.