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

Abstract

• Primary ozonides persist long periods in the ozonolysis of guaiacol. • Criegee intermediates mainly transfer to dioxirane-type compounds. • The formation of secondary ozonides is negligible. • Rate constant positively depends on temperature but not sensitive to pressure. • Frontier molecular orbital property determines the selectivity and reactivity. 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 cm 3 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 (E L o -H ac ) between the lowest unoccupied molecular orbital of O 3 and the highest occupied molecular orbital of phenols. The substituents can reduce the value of E L o -H ac 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.