A reinvestigation of the kinetics and the mechanism of the CH3C(O)O2 + HO2 reaction using both experimental and theoretical approaches

Abstract

The kinetics and the mechanism of the reaction CH₃C(O)O₂ + HO₂ were reinvestigated at room temperature using two complementary approaches: one experimental, using flash photolysis/UV absorption technique and one theoretical, with quantum chemistry calculations performed using the density functional theory (DFT) method with the three-parameter hybrid functional B3LYP associated with the 6-31G(d,p) basis set. According to a recent paper reported by Hasson et al., [J. Phys. Chem., 2004, 108, 5979–5989] this reaction may proceed by three different channels: CH₃C(O)O₂ + HO₂ → CH₃C(O)OOH + O₂ (1a); CH₃C(O)O₂ + HO₂ → CH₃C(O)OH + O₃ (1b); CH₃C(O)O₂ + HO₂ → CH₃C(O)O + OH + O₂ (1c). In experiments, CH₃C(O)O₂ and HO₂ radicals were generated using Cl-initiated oxidation of acetaldehyde and methanol, respectively, in the presence of oxygen. The addition of amounts of benzene in the system, forming hydroxycyclohexadienyl radicals in the presence of OH, allowed us to answer that channel (1c) is <10%. The rate constant k₁ of reaction (1) has been finally measured at (1.50 ± 0.08) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 298 K, after having considered the combination of all the possible values for the branching ratios k_1a/k_1,k_1b/k_1,k_1c/k₁ and has been compared to previous measurements. The branching ratio k_1b/k₁, determined by measuring ozone in situ, was found to be equal to (20 ± 1)%, a value consistent with the previous values reported in the literature. DFT calculations show that channel (1c) is also of minor importance: it was deduced unambiguously that the formation of CH₃C(O)OOH + O₂ (X ³Σ⁻_g) is the dominant product channel, followed by the second channel (1b) leading to CH₃C(O)OH and singlet O₃ and, much less importantly, channel (1c) which corresponds to OH formation. These conclusions give a reliable explanation of the experimental observations of this work. In conclusion, the present study demonstrates that the CH₃C(O)O₂ + HO₂ is still predominantly a radical chain termination reaction in the tropospheric ozone chain formation processes.