Dr. Kaito Takahashi (Chemical Dynamics and Spectroscopy)
Latest Research Work
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 = − kobs[CH2OO]; kobs = 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 Ea1 = 2.6 ± 1.8 kcal mol−1, Ea2 = −10.8 ± 1.6 kcal mol−1, Ea3 = −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.
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