DFT Insight into the Kinetics and Mechanism of the OH.-Initiated Atmospheric Oxidation of Catechol: OH. Addition and Hydrogen Abstraction Pathways

Gas phase oxidation of catechol with hydroxyl radical is expected as dominant atmospheric removal process. The mechanism and kinetics of OH.-initiated atmospheric oxidation of catechol was investigated theoretically by employing of M06-2X/aug-cc-pVTZ level of theory at 300 K and 760 Torr by considering of OH. addition and H-atom abstraction reactions. Oxidation of catechol begins with reversible formation of pre-reactive molecular complex and its conversion to products in unimolecular manner. Fall-off pressure expression indicated that canonical transition state theory breaks down to estimate rate constant at 760 Torr. RRKM unimolecular rates were corrected for basis set superposition error and quantum tunneling effects. RRKM bimolecular rates for OH. addition and H-atom abstraction pathways at 300 K and 760 Torr were about 10(-12) cm(3) molecule(-1) s(-1). The RRKM bimolecular rate for oxidation of catechol triggered by OH. at 300 K and 760 Torr was 9.45x10(-12) cm(3) molecule(-1) s(-1) and its temperature dependence over 200-400 K can be expressed by the lnk=(2929.8/T)+(5.82x10(-16)), indicating that reaction rate is negatively dependent on the temperature. H-atom abstraction from the hydroxy group at the C-2 position and addition of OH. onto the C-2 atom are the most favorable processes. Evolution of branching ratios demonstrate that the OH.-initiated oxidation of catechol is not essentially selective process.

Automated summary

The gas phase oxidation of catechol with hydroxyl radical is the main atmospheric removal process. Using theoretical modeling, the mechanism and kinetics of OH.-initiated atmospheric oxidation of catechol were investigated, showing that the reaction rates are negatively dependent on temperature. The most favorable processes are H-atom abstraction from the hydroxy group at the C-2 position and addition of OH. onto the C-2 atom.