Exploring the IntricateMechanism and Kinetics ofthe Reaction between C2-Criegee Intermediates (CH₃CHOO)and Acetaldehyde: A Study Using Cavity Ring-Down Spectroscopy andComputational Methods

Temperature-dependent kinetics for the reaction of C2-Criegee intermediates (CH₃CHOO) with acetaldehyde (CH₃CHO) was studied at 268–313 K and 50 Torr using cavity ring-down spectroscopy with single-wavelength (360 nm) probing. The measured rate coefficients are expected to have contributions from both the anti- and syn-conformers of CH₃CHOO. Negative T dependence was observed for the title reaction, and the corresponding Arrhenius equation is <i>k</i>₃(<i>T</i> = 268 – 313 K) = (1.34 ± 0.07) × 10^–13 × exp{(1.71 ± 0.03) kcal mol^–1/RT}. The room temperature rate coefficients measured at 50 and 100 Torr are (2.43 ± 0.17) × 10^–12 and (2.56 ± 0.20) × 10^–12 cm³ molecule^–1 s^–1, respectively. Theoretical calculations were performed at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p) level of theory to obtain the high-pressure limit rate coefficients for the reaction of anti- and syn-CH₃CHOO with CH₃CHO. The high-pressure limit rate coefficient for syn-CH₃CHOO is approximately 3 orders of magnitude smaller than that of the anti-conformer, the latter being closely aligned with the experimental value. The rate coefficients for anti-CH₃CHOO + CH₃CHO at 50 Torr using the master equation solver (MESMER) are in agreement with the experimental values in the studied temperature range. MESMER also predicted CH₃COOH to be the major product for both anti- and syn-CH₃CHOO reactions by comparing the rate coefficients for the product formation pathways. A dramatic dependence of the pressure on stabilization of the SOZs was also observed for both conformers at different pressures.