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.