Kinetic and mechanistic insight into the OH[rad]–initiated atmospheric oxidation of 2,3,7,8-tetrachlorodibenzo-p-dioxin via OH[rad]–addition and hydrogen abstraction pathways: A theoretical investigation

Abstract

The 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic polychlorinated dibenzo-p-dioxin. The OH–initiated oxidation of TCDD has been studied using the density functional, canonical transition state, and canonical Rice-Ramsperger-Kassel-Marcus theories. The kinetic data were corrected for quantum tunneling by the Wigner and Eckart models. All OH addition and hydrogen atom abstraction channels were thermodynamically exergonic. The kinetic and thermodynamic data analysis at the reliable level MPWB1K/MG3S//M06-2X/MG3S indicate that the addition of OH to the carbon atom adjacent to the oxygen atom in dioxin ring leads to the formation of predominant adduct. The calculated bimolecular rate constant for the formation of predominant adduct was ~5.97–6.75 × 10−13 cm3 molecule−1 s−1, its branching ratio was ~0.955, and the overall rate constant for the OH–initiated oxidation of TCDD was ~6.25–7.08 × 10−13 cm3 molecule−1 s−1. The atmospheric lifetime of TCDD determined by OH was ~8.17–9.26 days indicating the TCDD can be categorized as medium lifetime organic pollutant.