Mechanistic and kinetic study of the CH3CO+O2 reaction

Abstract

Potential-energy surface of the CH3CO + O2 reaction has been calculated by ab initio quantum chemistry methods. The geometries were optimized using the second-order Moller-Plesset theory (MP2) with the 6-311G(d,p) basis set and the coupled-cluster theory with single and double excitations (CCSD) with the correlation consistent polarized valence double zeta (cc-pVDZ) basis set. The relative energies were calculated using the Gaussian-3 second-order Moller-Plesset theory with the CCSD/cc-pVDZ geometries. Multireference self-consistent-field and MP2 methods were also employed using the 6-311G(d,p) and 6-311++G(3df,2p) basis sets. Both addition/elimination and direct abstraction mechanisms have been investigated. It was revealed that acetylperoxy radical [CH3C(O)OO] is the initial adduct and the formation of OH and alpha-lactone [CH2CO2(1A')] is the only energetically accessible decomposition channel. The other channels, e.g., abstraction, HO2 + CH2CO, O + CH3CO2, CO + CH3O2, and CO2 + CH3O, are negligible. Multichannel Rice-Ramsperger-Kassel-Marcus theory and transition state theory (E-resolved) were employed to calculate the overall and individual rate coefficients and the temperature and pressure dependences. Fairly good agreement between theory and experiments has been obtained without any adjustable parameters. It was concluded that at pressures below 3 Torr, OH and CH2CO2(1A') are the major nascent products of the oxidation of acetyl radicals, although CH2CO2(1A') might either undergo unimolecular decomposition to form the final products of CH2O + CO or react with OH and Cl to generate H2O and HCl. The acetylperoxy radicals formed by collisional stabilization are the major products at the elevated pressures. In atmosphere, the yield of acetylperoxy is nearly unity and the contribution of OH is only marginal.

OH yields from the CH3CO+O2 reaction using an internal standard

“…The CH 3 C(O)OH data confirm that at zero pressure the OH yield is approximately unity, as evidenced by the intercept in Figure 2 which is (1.07 ± 0.24). This result is supported by the theoretical calculations of Lee et al [11] and Hou et al [10] where at room temperature, OH is almost exclusively the bimolecular product; the yield of HO 2 +CH 2 CO, the next most facile bimolecular channel, was calculated by Hou et al [10] to be ≤ 1%.…”

“…one and a half less quenching of CH 3 CO(O 2 )* than the work from Tyndall et al [8,22] and ca. two and a half less quenching than the work from Hou et al [10]. The reasons for this discrepancy are not clear.…”

“…There is only one previous experimental study where OH yields were measured directly at different pressures of He; absolute, pressure dependent OH yields were assigned assuming unity OH yield at zero pressure [7] on the basis of the reaction mechanism and a strong collision model. However, the pressure dependence obtained does not agree with indirect measurements of channel R1b obtained by Tyndall et al [8] or with OH yields obtained by theoretical methods [10].…”

“…This is the reason, together with a larger quenching of the LIF OH signal by N 2 , why measurements were made over a smaller total pressure range. The results of this study together with those of Tyndall et al [8,22] and Hou et al [10] are plotted in Figure 3. The OH yields shown in Figure 3 for the work from Tyndall et al [8] have been recalculated from the original data using Fc =1.…”

“…However, experimental [3,[7][8][9] and theoretical [10,11] studies have shown that reaction (R1) has a dissociation channel that forms OH, especially at low pressure.…”