Temperature‐dependent kinetics of the reactions of CH2OO with acetone, biacetyl, and acetylacetone

Abstract Temperature‐dependent rate constants for the reactions of CH 2 OO with acetone (Ac), biacetyl (BiAc), and acetylacetone (AcAc) have been measured over the range 275–335 K using a flash photolysis, transient absorption spectroscopy technique. The measurements were performed at a total pressure of ∼80 Torr in N 2 bath gas, which corresponds to the high‐pressure limit for these reactions. All three reactions show linear Arrhenius plots with negative temperature dependences. Rate constants increase in the order Ac < AcAc « BiAc across the temperature range; at 295 K the rate constants are k Ac = (4.8 ± 0.4) × 10 –13 cm 3 s –1 , k AcAc = (8.0 ± 0.7) × 10 –13 cm 3 s –1 , and k BiAc = (1.10 ± 0.09) × 10 –11 cm 3 s –1 . Sensitivity to temperature, characterized by the magnitude of the negative activation energy, increases in the order AcAc < BiAc < Ac ( E a / R values of –1830 ± 170 K, –1260 ± 170 K, and –460 ± 180 K, respectively). CBS‐QB3 calculations show that the Ac and BiAc reactions proceed via formation of an entrance channel complex followed by 1,3‐dipolar cycloaddition to form secondary ozonide products via a submerged transition state. For the BiAc reaction, the rate limiting step appears to be rearrangement of a long‐range van der Waals complex into the short‐range complex that subsequently leads directly to the cycloaddition transition state with a very low energy barrier. The calculations show that two reaction pathways are competitive for AcAc with nearly identical transition state free energies (Δ G ° = +10.1 kcal mol –1 at 298 K) found for cycloaddition at the C=O and at the C=C site of the dominant enolone tautomer. The weak temperature dependence observed is likely due to competition between these pathways.