Mechanism for OH-Initiated Degradation of 2,3,7,8-Tetrachlorinated Dibenzo-p-Dioxins in the Presence of O₂ and NO/H₂O

Abstract

The atmospheric oxidation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TeCDD) is investigated theoretically by high-accuracy molecular orbital calculation. The study shows that the OH radical can easily be added to the C atom adjacent to the O atom in dioxin ring to form OH radical adduct. The 2,3,7,8-TeCDD-OH adduct can immediately react with O(2) to form the 2,3,7,8-TeCDD-OH-O(2) adduct which can react with NO or H(2)O to complete the decomposition process. The degradation mechanism varies with the addition position of O(2) and the O-abstraction by NO. The OH radical can be reproduced through the H-abstraction of H(2)O and initiate a new round of degradation. The direct dynamic calculation is performed, and the rate constants is calculated over a temperature range of 200-1200 K, using the canonical variational transition state theory with small-curvature tunneling effect. The four-parameter formula of rate constants with the temperature is fitted and the lifetimes of the reaction species in the troposphere are estimated according to the rate constants, which is helpful for the atmospheric model study on the formation and degradation of dioxin.

Theoretical Mechanistic and Kinetic Studies on Homogeneous Gas-Phase Formation of Polychlorinated Naphthalene from 2-Chlorophenol as Forerunner

“…The SSTEP, which is a variable keyword in Polyrate 9.7 program (University of Minnesota, Minneapolis, MN, USA) that specifies the step size along the mass-scaled MEP, were confirmed as 0.05. This method has been comprehensively used in our previous papers on the rate constants of PCDD/F formation and degradation [ 28 , 29 , 30 , 31 , 46 , 47 , 48 ].…”

Quantitative Structure-Activity Relationships Study on the Rate Constants of Polychlorinated Dibenzo-p-Dioxins with OH Radical

“…The MPWB1K method is a hybrid density functional theory model with excellent performance for thermochemistry and thermochemical kinetics [ 20 , 21 ]. This computational approach has been successfully used in TCDD and TCDF [ 22 , 23 ]. All open-shell species were treated with an unrestricted approach, while closed-shell species were described with a restricted method.…”

Homogeneous Gas-Phase Formation Mechanism of Emerging Organic Pollutants Polyfluorinated Dibenzo-p-dioxins and Dibenzofurans from 2-Fluorophenol

“…All geometrical parameters (reactants, transition states, intermediates and products) were fully optimized at the MPWB1K level of theory with a standard 6-31+G(d,p) basis set. The MPWB1K method has excellent performance in thermochemistry and thermochemical kinetics 21,22 , and this method has been successfully used in the studies of PCDD and PBDD formations 23,24 . The nature of various stationary points was determined by frequency calculations.…”