Temperature dependence of the reaction of anti-CH₃CHOO with water vapor

The kinetics of the reaction of anti-CH₃CHOO with water vapor were investigated using transient UV absorption spectroscopy at temperatures from 288 to 328 K and 500 Torr. We found that both the water monomer and the water dimer react with anti-CH₃CHOO. The rate coefficients of the reaction of the water monomer and dimer with anti-CH₃CHOO at 298 K were determined to be (1.31 ± 0.26) × 10^-14 cm³ s^-1 and (4.40 ± 0.29) × 10^-11 cm³ s^-1, respectively. Furthermore, for the water dimer reaction, we observed very large negative temperature dependence with an activation energy of -12.17 ± 0.66 kcal mol^-1. On the other hand, the monomer reaction showed minimal temperature dependence with nearly zero activation energy. At atmospherically relevant humidity, in opposition to previous experiments conducted for CH₂OO in which water dimer reaction predominates at room temperature, for anti-CH₃CHOO, the water monomer reaction can contribute significantly (∼30% of the reaction with water vapor at relative humidity RH = 40% and 298 K). These results show that substitution of an alkyl group can greatly affect the reaction of Criegee intermediates with water vapor, especially changing the contributions of water monomer and dimer reactions.