Reaction of Stabilized Criegee Intermediates from Ozonolysis of Limonene with Sulfur Dioxide: Ab Initio and DFT Study.

The mechanism of the reaction of the sulfur dioxide (SO2) with four stabilized Criegee intermediates (stabCI−CH3−OO, stabCI−OO, stabCIx−OO, and stabCH2OO) produced via the ozonolysis of limonene have been investigated using ab initio and DFT (density functional theory) methods. It has been shown that the intermediate adduct formed by the initiation of these reactions may be followed by two different reaction pathways such as H migration reaction to form carboxylic acids and rearrangement of oxygen to produce the sulfur trioxide (SO3) from the terminal oxygen of the COO group and SO2. We found that the reaction of stabCI−OO and stabCH2OO with SO2can occur via both the aforementioned scenarios, whereas that of stabCI−CH3−OO and stabCIx−OO with SO2is limited to the second pathway only due to the absence of migrating H atoms. It has been shown that at the CCSD(T)/6-31G(d) + CF level of theory the activation energies of six reaction pathways are in the range of 14.18−22.59 kcal mol−1, with the reaction between stabCIx−OO and SO2as the most favorable pathway of 14.18 kcal mol−1activation energy and that the reaction of stabCI−OO and stabCH2OO with SO2occurs mainly via the second reaction path. The thermochemical analysis of the reaction between SO2and stabilized Criegee intermediates indicates that the reaction of SO2and stabilized Criegee intermediates formed from the exocyclic primary ozonide decomposition is the main pathway of the SO3formation. This is likely to explain the large (∼100%) difference in the production rate in the favor of the exocyclic compounds observed in recent experiments on the formation of H2SO4from exocyclic and endocyclic compounds. [ABSTRACT FROM AUTHOR]