Organic Acid Formation from the Atmospheric Oxidation of Gem Diols: Reaction Mechanism, Energetics, and Rates

Computational chemistry is used to investigate the gas phase reaction of several gem diols in the presence of OH radical and molecular oxygen (³O₂) as would occur in the Earth's troposphere. Four gem diols, represented generically as R-HC(OH)₂, with R being either -H, -CH₃, -HC(O), and -CH₃C(O) are investigated. We find that after the abstraction of the hydrogen atom from the C-H moiety of the diol by atmospheric OH, molecular oxygen quickly adds onto the resulting radicals leading to the formation of a geminal diol peroxy adduct (R-C(OO)(OH)₂), which is the key intermediate in the oxidation process. Unimolecular reaction of this R-C(OO)(OH)₂ radical adduct, occurs via a proton-coupled electron transfer (PCET) mechanism and leads to the formation of an organic acid and a HO₂ radical. Further, the barrier for the unimolecular reaction step decreases along the R substitution series: -H, -CH₃, -HC(O), -CH₃C(O); this trend most likely arises from increased internal hydrogen bonding along the series. The reaction where the R group is CH₃C(O), associated with methylglyoxal diol, has the lowest barrier with its transition state being ∼4.3 kcal/mol above the potential energy well of the corresponding CH₃C(O)-C(OO)(OH)₂ peroxy adduct. The rate constants for the four diol oxidation reactions were investigated using the MESMER master equation solver kinetics code over the temperature range between 200 and 300 K. The calculations suggest that once formed, gem diol radicals react rapidly with O₂ in the atmosphere to produce organic acids and HO₂ with an effective gas phase bimolecular rate constant of ∼1 × 10^-11 cm³/molecule s at 300 K.