Theoretical study of the gas-phase ozonolysis of β-pinene (C10H16)

Abstract

The O₃-initiated oxidation of β-pinene, a monoterpene emitted in forested areas, was theoretically characterized using DFT, CBS-QB3 and CASPT2 quantum chemical calculations combined with statistical kinetic RRKM/master equation analysis and transition state theory. The first-principles based rate coefficient of the initial O₃ attack on the exocyclic double bond shows a slight positive temperature dependence, k_tot(T) = 1.27 × 10⁻²²×T^2.64× exp(−714 K/T) cm³ molecule⁻¹ s⁻¹, and is in close agreement with experiment. The resulting primary ozonides are found to give rise to two distinct, non-interconvertible conformers of the predominant Criegee intermediate (CI-1 and CI-2), with subsequent chemistries that are shown to be very different; this crucial aspect of β-pinene ozonolysis was not taken into account in earlier studies. One of the conformers CI-2—carrying nearly half the total reaction flux—cannot undergo the usual “hydroperoxide channel”, thus rationalizing why the OH yield from β-pinene is barely half of that from α-pinene. The predicted first-generation product distribution for atmospheric conditions is consistent with the available experimental data on the overall products. Our final results predict 5% of nopinone formation, 28% OH radicals with 2-oxo-alkyl radical coproducts, 37% of stabilized Criegee intermediates (SCI), 17% lactones, 10% CO₂ formed after an intersystem crossing, and 3% of a newly proposed biradical formed from prompt ring opening in the CI. In atmospheric conditions, additional OH production is expected from the stabilized CI-1 conformer via the thermal unimolecular “hydroperoxide channel”, whereas the stabilized CI-2 can react with H₂O and its dimer, to produce additional nopinone. The expected subsequent chemistries of the large coproduct radicals formed from reactions of the two CI are also addressed in extenso.