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

Temperature-dependent rate constants for the reactions of CH2OO 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 similar to 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 k(Ac) = (4.8 +/- 0.4) x 10(-13) cm(3) s(-1), k(AcAc) = (8.0 +/- 0.7) x 10(-13) cm(3) s(-1), and k(BiAc) = (1.10 +/- 0.09) x 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 (Delta G degrees = +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.

Automated summary

Temperature-dependent rate constants for the reactions of CH2OO with acetone (Ac), biacetyl (BiAc), and acetylacetone (AcAc) have been measured. The reaction rates increase with decreasing temperature and follow linear Arrhenius plots. The calculations show that the reactions proceed via formation of an entrance channel complex followed by 1,3-dipolar cycloaddition. 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 weak temperature dependence observed for AcAc is likely due to competition between two reaction pathways.