Pressure-dependent kinetics on benzoyl radical + O₂ and its implications for low temperature oxidation of benzaldehyde

Toluene is present in significant amounts of the aromatic compounds of real-fuel. Its fundamental combustion characteristics have attracted much attention. Benzaldehyde is a key intermediate in toluene oxidation and could have a potential impact in the construction of a toluene kinetic mechanism. Unfortunately, the understanding of benzaldehyde oxidation mechanism has not as yet reached consensus in the combustion community. Benzoyl as primary intermediate of benzaldehyde after H-atom abstraction, the recognition of its consumption pathway is to direct dissociation in both high- and low-temperature regimes, without considering the reaction channels of C₆H₅ĊO + ³O₂. In order to clarify the importance of this acyl radical addition to oxygen reaction in aldehydes oxidation particularly at low temperatures. The potential energy surface of C₆H₅ĊO + ³O₂ was evaluated at the DLPNO-CCSD(T)/cc-pVTZ//M06-2X/6-311++G(d,p) level of theory. RRKM/ME calculations were performed on the C₆H₅ĊO + ³O₂ reaction using thermochemical and kinetic parameters obtained from the ab-initio calculations, with micro-canonical variational transition state theory for treatment of barrierless kinetics. Impact of the newly calculated reaction channels on auto-ignition and oxidation of benzaldehyde was investigated computationally. Current results reveal that the benzaldehyde sub-mechanisms derived from different sources take on distinguished sensitivity for updating the C₆H₅ĊO + ³O₂ kinetics. Thermodynamic analysis was carried out to interpret the dependence of benzaldehyde model performance on C₆H₅ĊO + ³O₂ kinetics. Thermodynamic parameters of benzoyl radical was calculated by the compounded methods of G3, G4 and CBS-APNO due to the excessive effect on equilibrium of reaction for benzoyl consumption. Our results indicate that more accurate determination of thermochemistry of benzoyl is critical to improve the model capability of benzaldehyde in prediction of ignition delay time and identification of species evolution at lower temperature in particular.