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

The kinetics of the simplest Criegee intermediate (CH 2 OO) 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 CH 2 OO involves not only two water molecules but also one and three water molecules. Our experimental results suggest that the decay of CH 2 OO can be described as d[CH 2 OO]/d t = - k obs [CH 2 OO]; k obs = k 0 + k 1 [water] + k 2 [water] 2 + k 3 [water] 3 ; k 1 = (4.22 ± 0.48) × 10 -16 cm 3 s -1 , k 2 = (10.66 ± 0.83) × 10 -33 cm 6 s -1 , k 3 = (1.48 ± 0.17) × 10 -50 cm 9 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 k 3 [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 H 2 O and D 2 O with a Coriolis-type direct mass flow sensor, the kinetic isotope effect (KIE) was investigated at 298 K and 300 Torr and KIE( k 1 ) = k 1 (H 2 O)/ k 1 (D 2 O) = 1.30 ± 0.32; similarly, KIE( k 2 ) = 2.25 ± 0.44 and KIE( k 3 ) = 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 (N 2 buffer gas) with those under 600 Torr, a weak pressure effect of k 3 was found. From quantum chemistry calculations, we found that the CH 2 OO + 3H 2 O 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 CH 2 OO(H 2 O) + (H 2 O) 2 and CH 2 OO(H 2 O) 2 + H 2 O pathways will dominate in the three water reaction.