DFT Insight into the Kinetics and Mechanism of OH-Initiated Atmospheric Oxidation of Catechol: Addition of 3?g- O2 and Isomerization of Peroxy Radicals

Atmospheric oxidation mechanism of predominant catechol-OH adduct [C6H6O2 OH]* (AD2) by oxygen molecule in its triplet electronic ground state and isomerization processes of catechol peroxy radicals [C6H6O2 OH]* O2 into bicyclic peroxy radicals have been studied at the M06-2X/aug-cc-pVTZ level in conjugation with the RRKM theory. The first step begins via the complexation of [C6H6O2 OH]* with oxygen molecule and formation of two pre-reactive van der Waals complexes in syn and anti modes. Syn mode addition of O2 is more favorable over the anti mode due to the formation of intramolecular hydrogen bond. Kinetically and thermodynamically, addition of O2 at the C5 position in syn mode, namely peroxy radical AD25OO-syn, is the most efficient process with bimolecular RRKM rate coefficient 2.80x10 16 cm3 molecule 1 s 1 at ambient conditions. Isomerization (cyclization) of peroxy radical AD2-5OOsyn into bicyclic peroxy radicals via the formation of OO bridge are strongly endothermic and nonspontaneous processes with high activation energies. Ring closure into the bicyclic peroxy radical AD2-5,6OO-syn is the most favorable process with unimolecular RRKM rate coefficient 1.41x10 8 s 1 at ambient conditions.