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 stabCH(2)-OO) 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 stabCH(2)OO with SO2 can occur via both the aforementioned scenarios whereas that of stabCI-CH3-OO and stabCIx-OO with SO2 is 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 SO2 as the most favorable pathway of 14 18 kcal mol(-1) activation energy and that the reaction of stabCI-OO and stabCH(2)OO with SO2 occurs mainly via the second reaction path The thermochemical analysis of the reaction between SO2 and stabilized Criegee intermediates indicates that the reaction of SO2 and stabilized Criegee intermediates formed from the exocyclic primary ozonide decomposition is the main pathway of the SO3 formation This is likely to explain the large (similar to 100%) difference in the production rate in the favor of the exocyclic compounds observed in recent experiments on the formation of H2SO4 from exocyclic and endocyclic compounds