Unraveling the Autoxidation Mechanisms of Limonene, α-Pinene, and β-Pinene: A Computational Study with Reactivity...

The hydrogen shift reactions of peroxy radicals derived from the ȮH-initiated oxidation of three atmospherically important monoterpenes, limonene, α-pinene, and β-pinene, have been studied. The Bell-Evans-Polanyi relationship (BEPR), Marcus cross relationship (MCR), and Robert-Steel relationship (RSR) are employed to study the factors that contribute to the kinetics of the H-shift reactions. Our results show distinct kinetic behaviors based on the size of the transition-state ring, the functional group present at the H atom abstraction site, and the type of carbon-centered radical formed. Except for the 1,5-H-shift reactions, the MCR successfully predicts the activation enthalpy with minimal mean absolute errors by dividing it into intrinsic and thermodynamic components. The RSR, which considers the bond dissociation energy, polarity effects, and structure factor while calculating the activation enthalpy, exhibits a good correlation (R² = 0.97) with the activation enthalpy calculated through electronic structure calculations. The present study elucidates the factors contributing to the kinetics of the H-shift reactions, aiding in the development of reactivity prediction models.