Pressure-Dependent Kinetics of <i>o-</i> Xylene Peroxy Radical Chemistry

In this work, we provided an in-depth understanding of the low-temperature oxidation of <i>o</i>-xylene by focusing on its peroxy radical chemistry. High-precision theoretical calculations were performed to investigate the kinetics of reactions involving <i>o</i>-methylbenzylperoxy radical (RO₂•) and <i>o</i>-hydroperoxymethyl-benzyl radical (•QOOH), including their formation by the reaction of <i>o</i>-xylyl radical with O₂, as well as the consumption to form the cyclic oxide and 2-methylbenzaldehyde. These peroxy radicals are characterized by both aromatic and aliphatic properties, and the <i>C</i>₀² diagnostic calculations revealed the non-negligible multireference characters in these reaction systems, prompting us to calculate their reaction kinetics based on highly accurate PESs obtained by CASPT2 and WMS methods. The multistructural variational transition state theory incorporating the multistructural torsional anharmonicity based on a coupled torsional potential and the small-curvature tunneling approximation was employed to obtain the high-pressure-limit (HPL) rate constants of elementary reaction steps. The tunneling results show that both hydrogen and heavy-atom tunneling play a crucial role in the low-temperature chemical oxidation of <i>o</i>-xylene. In particular, at a room temperature of 298.15 K, hydrogen tunneling increases the rate constant of related reactions by about 3 orders of magnitude, and heavy-atom tunneling doubles it; even at a combustion temperature of 600 K, they can also enhance the reactions by 3-5 times and 17%, respectively. This observation highlights the subtle but decisive role of hydrogen and heavy-atom tunneling in driving the low-temperature reactivity of <i>o</i>-xylene. Lastly, we investigated the pressure dependence of three dissociation pathways of the •QOOH radical using the SS-QRRK method. The calculated branching fractions under varying temperatures and pressures provide a theoretical basis for optimizing combustion processes and controlling product distributions.