中央研究院 原子與分子科學研究所

Yen-Ju Wu, Kaito Takahashi, and Jim Jr-Min Lin

J. Phys. Chem. A, 127, 39, 8059–8072 (2023).

The kinetics of the simplest Criegee intermediate (CH₂OO) reaction with water vapor was revisited. By improving signal-to-noise ratio and the precision of water concentration, we found that the kinetics of CH₂OO at 298 K 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 = (3.87 ± 0.71) × 10⁻¹⁶ cm³ s⁻¹, k₂ = (11.07 ± 0.62) × 10⁻³³ cm⁶ s⁻¹, k₃ = (1.39 ± 0.17) × 10⁻⁵⁰ cm⁹ s⁻¹ at 300 Torr with the respective Arrhenius activation energies of E_a1 = 2.6 ± 1.8 kcal mol⁻¹, E_a2 = −10.8 ± 1.6 kcal mol⁻¹, E_a3 = −18.0 ± 3.8 kcal mol⁻¹. The contribution of the k₃[water]³ term becomes insignificant at high temperatures around 345 K, but is not ignorable at room temperature 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 300 Torr and KIE(k₁) = k₁(H₂O)/k₁(D₂O) = 1.25 ± 0.32; similarly, KIE(k₂) = 2.09 ± 0.37 and KIE(k₃) = 1.03 ± 0.13. These mild KIE values are consistent with theoretical calculations based on transition state theory and high-level quantum chemistry methods. This confirms 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. Furthrmore, analysis based on dipole capture rates showed that the CH₂OO(H₂O) + (H₂O)₂ and CH₂OO(H₂O)₂ + H₂O pathways will dominate in this three water reaction.