Quantum Chemical Investigation On The Mechanism And Kinetics Of·OH Initiated Photo-degradation Of PBDEs In The Atmosphere

Semivolatile organic compounds (SOCs) which have lower vapor pressure and higher lipophilic can cause extensive environmental contamination and significant adverse effects on human health. The reaction with·OH which was formed by photochemical process in the troposphere strongly determines the environmental persistence and fate of SOCs. Understanding the mechanism and kinetics of this reaction is an indispensable component of risk assessment.Currently, several direct and indirect experimental methods have been developed to probe the mechanism and kinetics of the reactions of SOCs and·OH. However, almost all the experimental methods are time-consuming, costly and equipment dependent. They cannot meet the need of risk assessment and pollution prevention for newly synthesized compounds prior to large-scale production and commercialization. Consequently, kinds of theoretical methods must be developed to replace of conventional experiments.Quantitative structure activity relationship models are usually used to predict the physical and chemical properties and kinetics. Several QSAR models have been developed to predict the rate constants (kOH) for the reactions of organic chemicals with·OH. Generally, the QSAR models are attractive as they generate results with minimal computational cost and they are applicable for regulatory purposes. However, the utility of QSARs is constrained as they strongly rely on experimental databases, and are only valid for compounds within the applicability domain.Compared with QSAR models, quantum chemical methods have great advantage in predicting reaction mechanism and kinetics. The density functional theory (DFT) and statistical mechanics allow for accurate predictions of chemical reactivity without any prior knowledge on the substance, and they have been widely applied to identify reaction mechanisms and generate kinetic dataIn this study, we show that the density functional theory (DFT) can be successfully employed to probe the kinetics and mechanism of atmospheric photooxidation of polybrominated diphenyl ethers (PBDEs) by·OH, taking 4,4'-dibromodiphenyl ether (BDE-15) as a case. The predicted products (HO-PBDEs, brominated phenols and Br2) and overall rate constant (kOH) at 298 K are consistent with the experimental results. Two pathways leading to formation of HO-PBDEs are identified:Br substitution by·OH, and abstraction of H gem to·OH in BDE-OH adducts by O2. This study offers a cost-effective way for probing the atmospheric indirect photooxidation kinetics and mechanism of PBDEs.