Probing the Reactivity of Singlet Oxygen with CyclicMonoterpenes

摘要

Monoterpenes represent a class of hydrocarbons consisting of two isoprene units. Like many other terpenes, monoterpenes emerge mainly from vegetation, indicating their significance in both atmospheric chemistry and pharmaceutical and food industries. The atmospheric recycling of monoterpenes constitutes a major source of secondary organic aerosols. Therefore, this contribution focuses on the mechanism and kinetics of atmospheric oxidation of five dominant monoterpenes (i.e., limonene, α-pinene, β-pinene, sabinene, and camphene) by singlet oxygen. The reactions are initiated via the ene-type addition of singlet oxygen (O2¹Δg) to the electron-rich double bond, progressing favorably through the concerted reaction mechanisms. The physical analyses of the frontier molecular orbitals agree well with the thermodynamic properties of the selected reagents, and the computed reaction rate parameters. The reactivity of monoterpenes with O2¹Δg follows the order of α-pinene > sabinene > limonene > β-pinene > camphene, i.e., α-pinene and campheneretainthe highest and lowest reactivity toward singlet oxygen, with rateexpressions of k(T) (M^–1 s^–1) = 1.13 × 10⁸ exp(−48(kJ)/RT(K)) and 6.93 × 10⁸ exp(−139(kJ)/RT(K)), respectively. The effect of solvent on the primaryreaction pathways triggers a slight reduction in energy, ranging between12 and 34 kJ/mol.