The Atmospheric Oxidation Mechanism of Sabinene Initiated by the Hydroxyl Radicals.

The atmospheric oxidation mechanism of sabinene initiated by the OH radical has been studied using quantum chemistry calculations at the CBS-QB3 level and reaction kinetic calculations using transition state theory and unimolecular rate theory coupled with collisional energy transfer. The oxidation is initiated by OH radical additions to the CH2 = C< bond with a branching ratio of similar to(92-96)%, while all the hydrogen atom abstractions count for similar to(4-8)% of branching ratio, which was estimated by comparing the rate coefficients of the reactions of sabinene and sabinaketon with the OH radical. Addition of OH to the = C< carbon forms radical adduct Ra, while addition of OH to the terminal CH2 = carbon forms radical adduct Rb, which would break the three-membered ring promptly and almost completely to radical Re. RRKM-ME calculations obtained fractional yields of 0.40, 0.09, and 0.51 for radicals syn-Ra, anti-Ra, and Re, respectively, at 298 K and 760 Torr. In the atmosphere, the syn/anti-Ra radical would ultimately transform to sabinaketone in the presence of ppbv levels of NO, while in the transformation of the Re radical, both bimolecular reactions and unimolecular H-migrations could occur competitively for the peroxy radicals formed. The H-migrations in peroxy radicals result in the formation of unsaturated multifunctional compounds containing >C = O, -OH, and/or -OOH groups. Formation of sabinaketone from syn- and anti-Ra and formation of acetone from Re are predicted with yields of similar to 0.37 and similar to 0.38 in the presence of high NO, being larger than while in reasonable agreement with the experimental values of 0.19-0.23 and of 0.21-0.27, respectively.