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Download file# Kinetics
of the Strongly Correlated CH_{3}O + O_{2} Reaction:
The Importance of Quadruple Excitations
in Atmospheric and Combustion Chemistry

journal contribution

posted on 2018-12-13, 00:00 authored by Bo Long, Junwei Lucas Bao, Donald G. TruhlarKinetics
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 CH

_{3}O + O_{2}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 × 10^{3}, 3.3 × 10^{2}, 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.## History

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O 2 Reactionrate constantsfactorcanonical variational theorymethodvariational transition state theorybarrier height energy15-LCCSDTQcontributionkcalCH 3 Oconfiguration interaction limitMNCorrelated CH 3 OGMMcalculationArrhenius activation energy increasesO 2 reactionexcitationCombustion Chemistry Kinetics measurements