Temperature and isotope effects in the reaction of CH₃CHOO with methanol

Carbonyl oxides, also known as Criegee intermediates, are generated from ozonolysis of unsaturated hydrocarbons in the atmosphere. Alcohols are often used as a scavenger of the Criegee intermediates in laboratory studies. In this work, the reaction kinetics of CH₃CHOO with methanol vapor was investigated at various temperatures, pressures, and isotopic substitutions using time-resolved UV absorption spectroscopy. The observed rate coefficients of the reaction of anti-CH₃CHOO with methanol show a linear dependence on [CH₃OH]. The bimolecular rate coefficient was determined to be k_1Ha = (4.8 ± 0.5) × 10^-12 cm³ s^-1 at 298 K and 250 Torr with a negative activation energy E_a = -2.8 ± 0.3 kcal mol^-1 for T = 288-315 K [k(T) = A exp(-E_a/RT)]. For the reaction of syn-CH₃CHOO with methanol vapor, the observed rate coefficients show a quadratic dependence on [CH₃OH], indicating that two methanol molecules participate in the reaction. The termolecular rate coefficient was determined to be k_2Hs = (8.0 ± 1.0) × 10^-32 cm⁶ s^-1 at 298 K and 250 Torr with a strong negative temperature dependence (E_a = -13.2 ± 0.3 kcal mol^-1) at 273-323 K. No significant pressure effect was observed at 250-760 Torr. A kinetic isotope effect, k_2Hs/k_2Ds = 2.5, was observed by changing CH₃OH to CH₃OD. Quantum chemistry and transition state theory calculations suggest that the observed isotope effect is mainly attributed to the changes of the vibrational zero-point energies and partition functions while tunneling plays a very minor role. The reaction of syn-CH₃CHOO with one CH₃OH molecule was not observed in the studied concentration range.