Exploring the Intricate Mechanism and Kinetics of the Reaction between C2-Criegee Intermediates (CH3CHOO) and Acetaldehyde: A Study Using Cavity Ring-Down Spectroscopy and Computational 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 k 3(T = 268 - 313 K) = (1.34 +/- 0.07) x 10-13 x 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) x 10-12 and (2.56 +/- 0.20) x 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.