Formation of Substituted Alkyls as Precursors of Peroxy Radicals with a Rapid H-Shift in the Atmosphere

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

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.

Predictive Combustion Kinetics of OH Radical Reactions with a C5 Unsaturated Alcohol: The Competitive H-Abstraction and OH-Addition Reactions of 2-Methyl-3-buten-2-ol

2-Methyl-3-buten-2-ol (MBO232) is a potential biofuel and renewable fuel additive. In a combustion environment, the consumption of MBO232 is mainly through the reaction with a OH radical, one of the most important oxidants. Here, we predict the intricate reactions of MBO232 and OH radicals under a broad range of combustion conditions, that is, 230-2500 K and 0.01-1000 atm. The potential energy surfaces of H-abstraction and OH-addition have been investigated at the CCSD(T)/CBS//M06-2X/def2-TZVP level, and the rate constants were calculated via Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) theory. The decomposition reactions of the critical intermediates from the OH-addition reactions have also been studied. Our results show that OH-addition reactions are dominant below 850 K, while H-abstraction reactions, especially the channel-abstracting H atoms from the methyl groups, are more competitive at higher temperatures. We found that it is necessary to discriminate H atoms attached to the same C atom, as their abstraction rates can differ by up to 1 order of magnitude. The calculated results show good agreement with the reported experimental data. We have provided the modified Arrhenius expressions for rate constants of the dominant channels. The kinetic data determined in this work are of much value for constructing the combustion models of MBO232 and understanding the combustion kinetics and mechanism of other unsaturated alcohols.