Quantum chemical study on the ozonolysis mechanism of guaiacol and the structure-reactivity relationship of phenols with hydroxyl, methoxy, and methyl substituents

Abstract Quantum chemical calculations with the M06-2X method were performed to uncover the ozonolysis mechanism of guaiacol and several hydroxylated, methoxylated, and methylated phenols to improve the understanding of the ozone-induced attenuation of phenolic compounds. The detailed kinetic properties were obtained by the transition state theory (TST) and the Rice−Ramsperger−Kassel−Marcus (RRKM) theory. Results confirm that the primary ozonides originated from guaiacol occupy considerably long retention time than the short-lived primary ozonides formed from vinyl-type compounds in the air. The TST rate constant of guaiacol ozonolysis is calculated with a value of 8.81 × 10-20 cm3 molecule-1 s-1 at room temperature. The RRKM results prove that the rate constant is positively dependent on temperature but shows no relation with pressure. The reactivity of substituted phenols exhibits a strong correlation with the ozone affinity of carbon atoms and the orbital energy difference (ELo-Hac) between the lowest unoccupied molecular orbital of O3 and the highest occupied molecular orbital of phenols. The substituents can reduce the value of ELo-Hac and improve the phenols' reactivity, especially when they locate at the ortho- and para-position. The substituents also improve the ozone affinity of carbon atoms due to the electron-withdrawing effect. The methyl group delivers a moderate impact on the reactivity compared to hydroxyl and methoxy groups.