Theoretical study on the mechanism and kinetics of the reaction of 2,2′,4,4′-tetrabrominated diphenyl ether (BDE-47) with OH radicals

The mechanism and kinetic properties of OH-initiated gas-phase reaction of 2,2′,4,4′-tetrabrominated diphenyl ether (BDE-47) have been studied at the MPWB1K/6-311 + G(3df,2p)//MPWB1K/6-31G(d) level of theory. Two types of reactions including hydroxyl addition and hydrogen abstraction have been considered. The calculation results indicate that addition reactions except for the bromo-substituted addition reaction have lower energy barriers than hydrogen abstraction reactions. Hydroxyl radical is most likely to be added to non-substituted C atoms (C(3), C(5), and C(6) atoms). Moreover, for all the reactions with OH radical, the bromo-substituted C atoms encountered the highest barrier compared to that of the rest C atoms. The rate constants and product branching ratios of each pathway have been deduced over a wide range of 200–1000 K using canonical variational transition state (CVT) theory with small curvature tunneling (SCT) contribution. This study can be regarded as an attempt to investigate the OH-initiated photochemical reaction mechanism of polybrominated diphenyl ethers (PBDEs).

Research highlights
► In this study, the reaction mechanism and the rate constants for the reaction of OH radicals with BDE-47 have been investigated deeply by using quantum chemical method. This method can accurately calculate the reaction energy barrier, the enthalpies of formation and the rate constants. By using these data, the detailed potential energy surface and completed reaction mechanism have been proposed.
► The results show that addition reaction is predominant than abstraction reaction. For addition reaction, Br atoms may block the OH radical adding to adjacent C atoms. The ortho-Br atom leads to higher barrier reaction of OH with ipso-C rather than with non-substituted C atoms. In addition, C(5) atom excel in reaction with OH radical when consider addition reaction and abstraction reaction separately.
► This work could be expected to be a supplement of experimental study and an attempt to theoretical investigation of the OH-initiated photochemical reaction mechanism of PBDEs.