Theoretical investigations on direct photolysis mechanisms of polychlorinated diphenyl ethers.

Polychlorinated diphenyl ethers (PCDEs) are a focus of current environmental concern as a group of ubiquitous potential persistent organic pollutants. There are still significant gaps in our knowledge concerning the photolysis mechanisms of PCDEs. In this study, the direct photolysis mechanisms of PCDEs were investigated by density functional theory. The direct photolysis of PCDEs has three potential reaction pathways including photodechlorination, C-O bond photodissociation, and PCDFs formation. Taking a representative PCDE (i.e., CDE8) for example, we found that C-Cl bond dissociation is the rate-determining step for the photodechlorination. Chlorobenzene is predicted to be photoproduct of CDE8 through the photodissociation of the C-O bond. Furthermore, the calculated mean bond dissociation energies of both C-Cl and C-O bonds of 20 PCDEs decrease with the increased degree of chlorination. It is also found that the photoactivity of PCDEs increases with an increase of chlorination degree by evaluating the average charge of Cl atoms and mean bond dissociation energies of C-Cl and C-O bonds from reaction thermodynamics. Our findings provided a new insight into the mechanisms of direct photolysis of PCDEs, which may be useful in the future in utilizing quantum chemistry calculation in investigating the behavior and fate of organic pollutants in the environment.