Singlet (¹Δ_g) O₂ initiated gas phase oxidation as a potential tropospheric decay channel for ketene

The oxidation of CH₂CO by (¹Δ_g) O₂ has been investigated by means of high level quantum chemical and chemical kinetic calculations. The reaction was found to proceed through a four-membered cyclic transition state resulting from the addition of O₂ to the CC bond of ketene. The reaction energetics has been calculated employing post-CCSD(T) corrections. The energy of the transition state was found to be 33.6 kcal mol^-1 below that of the isolated reactants. The rate coefficient, calculated using master equations under tropospheric conditions, was found to be 5.1 × 10^-15 cm³ molecule^-1 s^-1 at 298 K and 1 bar. Atmospheric implications of the title reaction have been estimated by comparing the atmospheric lifetime of ketene for the title reaction against reactions with ˙OH, H₂O and NH₃. On a global scale, the lifetime for the title reaction was found to be almost 70 times that for the reaction with ˙OH. However, under special conditions, where the local concentration of singlet O₂ is significantly higher and/or the concentration of ˙OH is significantly lower, singlet O₂ initiated oxidation could become the most significant tropospheric loss mechanism of CH₂CO.