Kinetics of Hydroperoxymethyl Acetate with Carbonyl Oxide and OH Radicals: Implications for Atmospheric Chemistry

Hydroperoxymethyl acetate (HPMA, CH₃C(O)OCH₂OOH), formed from the reaction of carbonyl oxide (CH₂OO) with acetic acid (CH₃COOH), is a multifunctional organic peroxide of atmospheric relevance. Despite its potential importance, the kinetics of HPMA remain poorly characterized. In this work, we present a comprehensive theoretical study of its bimolecular reactions with CH₂OO and hydroxyl radicals (OH). For the electronic structure calculations, we benchmark against GMM(Q).FNO//DF-CCSD(T)-F12b/jun-cc-pVDZ, a high-level method that closely approaches CCSDT(Q)/CBS accuracy for the reaction with CH₂OO, and CCSD(T)-F12a/cc-pVTZ-F12//M08-HX/MG3S for the reaction with OH. We computed rate constants using conventional transition state theory at the benchmark electronic structure level, combined with multistructural canonical variational transition state theory with small-curvature tunneling using a well-benchmarked density functional. The results show cycloaddition dominates the CH₂OO + HPMA, while OH predominantly abstracts the -OOH hydrogen to yield RO₂. We find that for the CH₂OO + HPMA reaction, vibrational anharmonicity increases the hydroperoxide addition rate constant by factors of 4-40 at 190 to 350 K. In the HPMA + OH system, tunneling increases the rate constant by over 100-fold for hydrogen abstraction from the -OOH and -CH₃ groups at 190 K. Furthermore, our results indicate that under low-temperature or nighttime conditions, where OH concentrations are limited, the CH₂OO-initiated reaction becomes a dominant atmospheric sink for HPMA. The present work provides a valuable reference for kinetic evaluations of multifunctional organic peroxides in atmospheric oxidation.