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

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, ktot(T) = 1.27 × 10−22×T2.64× 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.