Criegee Intermediate Reaction with Alcohol Is Enhanced by a Single Water Molecule

Abstract

The role of water in gas-phase reactions has gained considerable interest. Here we report a direct kinetic measurement of the reaction of syn-CH₃CHOO (a Criegee intermediate or carbonyl oxide) with methanol at various relative humidity (RH = 0-80%) under near-ambient conditions (298 K, 250-755 Torr). The data indicate that a single water molecule expedites the reaction by up to a factor of three. The rate coefficient of the corresponding reaction, syn-CH₃CHOO + CH₃OH + H₂O → products, has been determined to be (1.95 ± 0.11) × 10^-32 cm⁶ s^-1 at 298 K, with no observable pressure dependence for 250-755 Torr. Quantum chemistry calculation shows that the dominating pathway involves a hydrogen-bonded ring structure, in which methanol is donating a hydrogen atom to water, water is donating a hydrogen atom to the terminal oxygen atom of the Criegee intermediate, and, on the product side, H₂O is reformed and acts as a catalyst.