High-accuracy first-principles-based rate coefficients for the reaction of OH and CH3OOH

Abstract

The ˙OH-initiated oxidation of methyl hydroperoxide, which plays an important role in the atmospheric chemistry of methane, was theoretically characterized using high-accuracy composite amHEAT-345(Q) coupled-cluster calculations followed by a two-dimensional E,J resolved master equation analysis. The reaction is found to proceed through two distinct hydrogen-bonded pre-reactive complexes leading to two product channels, in accord with the experimental observations: (i) ˙OH + CH₃OOH → CH₃OO˙ + H₂O with a yield of 0.8 ± 0.1, and (ii) ˙OH + CH₃OOH → HCHO + ˙OH + H₂O with a yield of 0.2 ± 0.1. The calculated reaction enthalpies are within 0.2 kcal mol⁻¹ of the benchmark ATcT values. Overall thermal rate coefficients obtained from first principles are found to be in the low-pressure limit at atmospheric pressure; the total rate coefficient can be expressed over the T = 200–450 K range as k(T) = 5.0 × 10⁻¹² × T^−0.152 × exp(287/T) cm³ s⁻¹, strongly supporting the experimental results of Vaghjiani and Ravishankara (J. Phys. Chem. 1989, 93, 1948), with which this expression agrees within ca. 15%. The current results show that (i) is the principal reaction channel and support the view that, due to its inherently fast transformations, CH₃OOH is an important redistribution species for HO_x˙ radicals in the Earth's atmosphere.