Kinetics of the Simplest Criegee Intermediate Reaction with Water Vapor: Revisit and Isotope Effect

The kinetics of the simplest Criegee intermediate (CH₂OO) reaction with water vapor was revisited. By improving the signal-to-noise ratio and the precision of water concentration, we found that the kinetics of CH₂OO involves not only two water molecules but also one and three water molecules. Our experimental results suggest that the decay of CH₂OO can be described as d[CH₂OO]/dt = -k_obs[CH₂OO]; k_obs = k₀ + k₁[water] + k₂[water]² + k₃[water]³; k₁ = (4.22 ± 0.48) × 10^-16 cm³ s^-1, k₂ = (10.66 ± 0.83) × 10^-33 cm⁶ s^-1, k₃ = (1.48 ± 0.17) × 10^-50 cm⁹ s^-1 at 298 K and 300 Torr with the respective Arrhenius activation energies of E_a1 = 1.8 ± 1.1 kcal mol^-1, E_a2 = -11.1 ± 2.1 kcal mol^-1, E_a3 = -17.4 ± 3.9 kcal mol^-1. The contribution of the k₃[water]³ term becomes less significant at higher temperatures around 345 K, but it is not ignorable at 298 K and lower temperatures. By quantifying the concentrations of H₂O and D₂O with a Coriolis-type direct mass flow sensor, the kinetic isotope effect (KIE) was investigated at 298 K and 300 Torr and KIE(k₁) = k₁(H₂O)/k₁(D₂O) = 1.30 ± 0.32; similarly, KIE(k₂) = 2.25 ± 0.44 and KIE(k₃) = 0.99 ± 0.13. These mild KIE values are consistent with theoretical calculations based on the variational transition state theory, confirming that the title reaction has a broad and low barrier, and the reaction coordinate involves not only the motion of a hydrogen atom but also that of an oxygen atom. Comparing the results recorded under 300 Torr (N₂ buffer gas) with those under 600 Torr, a weak pressure effect of k₃ was found. From quantum chemistry calculations, we found that the CH₂OO + 3H₂O reaction is dominated by the reaction pathways involving a ring structure consisting of two water molecules, which facilitate the hydrogen atom transfer, while the third water molecule is hydrogen-bonded outside the ring. Furthermore, analysis based on dipole capture rates showed that the CH₂OO(H₂O) + (H₂O)₂ and CH₂OO(H₂O)₂ + H₂O pathways will dominate in the three water reaction.