Kinetics of the Strongly Correlated CH₃O + O₂ Reaction: The Importance of Quadruple Excitations in Atmospheric and Combustion Chemistry

Kinetics measurements on radical–radical reactions are often unavailable experimentally, and obtaining quantitative rate constants for such reactions by theoretical methods is challenging because the transition states and the reactants are often strongly correlated. Treating strongly correlated systems by coupled cluster theory limited to single, double, and triple connected excitations is often inadequate. We therefore use a new method, called GMM(P), for extrapolation to the complete configuration interaction limit to go beyond triple excitations and in particular to approximate the CCSDTQ(P)/CBS limit. Here, we present this method and use it to investigate the CH₃O + O₂ reaction. The contribution of connected quadruple excitations to the barrier height energy is found to be −3.13 kcal/mol, and adding a quasiperturbative calculation of the effect of connected pentuple excitations brings the post-connected-triples contributions to −3.44 kcal/mol, which corresponds to Boltzmann factors that increase calculated rate constants by factors of 1.0 × 10³, 3.3 × 10², and 18 at 250, 298, and 600 K, respectively. We present rate constants for temperatures from 250 to 2000 K, and we find that the Arrhenius activation energy increases from 0.58 to 9.68 kcal/mol over this range. We also find reasonably good accuracy for the barrier height with the MN15-L exchange–correlation functional, and we calculate rate constants by a combination of GMM(P) and MN15-L electronic structure calculations and conventional and variational transition state theory, in particular canonical variational theory with small-curvature tunneling. The present findings have broad implications for obtaining quantitative rate constants for complex reaction systems in atmospheric and combustion chemistry.