Density functional study of the phenylethyl + O2 reaction: Kinetic analysis for the low‐temperature autoignition of ethylbenzenes

Quantum chemical calculations at the CBS-QB3 level of theory have been carried out to investigate the potential energy surfaces for the reactions of α- and β-phenylethyl radicals with molecular oxygen. For the α-phenylethyl + O2 reaction, all of the transition states for the isomerization reactions of α-phenylethylperoxy radicals were positioned above the total energy of the reactants of α-phenylethyl + O2. For the β-phenylethyl + O2 reaction, on the other hand, most of the transition states were positioned below the total energy of the reactants of β-phenylethyl + O2. The RRKM rate constant analysis revealed that the backward reaction forming α-phenylethyl + O2 was dominant in the α-phenylethyl radicals + O2 reaction system at the temperature range between 300 and 1500 K, whereas the reaction pathway forming cyclic O2 structures (5b) was dominant in the β-phenylethyl radicals + O2 reaction system at the same temperature range. In the reactions of both α- and β-phenylethyl radicals with molecular oxygen, the HO2 elimination reaction channels became more and more important when the temperature increased up to around 1000 K. Further decomposition channels of the cyclic O2 structures (5b) were investigated using the density functional B3LYP theories and found that all of these decomposition reactions could proceed without any large activation barriers. The transition state structures forming such cyclic O2 structures in the phenylpropyl + O2 have also been calculated, and it was found that these cyclic O2 structures were one of the major products on the high-temperature reactions of β- and γ-phenylpropyl radicals with molecular oxygen. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011