Theoretical study on the reactions of a series of polybromobenzenes with OH radicals: mechanism, kinetics, and QSAR

Polybromobenzenes are a kind of monocyclic aromatic flame retardants that are used as a substitute for polybrominated diphenyl ethers and hexabromocyclododecane. In this paper, the reaction mechanism and rate constants for the reaction of OH radicals with a series of polybromobenzenes such as hexabromobenzene (HBB), 1,2,4,5-tetrabromobenzenes (1,2,4,5-TeBB), pentabromobenzene (PEBB), pentabromoethylbenzene (PBEB), pentabromotoluene (PBT), and 2,4,5-tribromotoluene (2,4,5-TrBT) have been investigated by quantum chemical method. The reaction mechanism was obtained at the MPWB1K/6-311+g(3df,2p)//MPWB1K/6-31+g(d,p) level of theory and the rate constants were deduced over the temperature range of 200–370 K using canonical variational transition state (CVT) theory with the small curvature tunneling (SCT) method. The rate constants of OH radicals with HBB, 1,2,4,5-TeBB, PEBB, PBEB, PBT, and 2,4,5-TrBT are determined to be 5.72 × 10−13, 1.23 × 10−12, 8.78 × 10−13, 9.23 × 10−13, 6.46 × 10−13, and 1.69 × 10−12, respectively, at 298 K and 1 atm. The estimated atmospheric lifetimes of HBB (20.08 days), 1,2,4,5-TeBB (9.65 days), PEBB (13.5 days), PBEB (12.9 days), PBT (18.4 days), and 2,4,5-TrBT (7.0 days) determined by OH radicals indicate that polybromobenzenes have the potential for long-range transport. The genetic function approximation is used to study the quantitative structure–activity relationship. The coefficients indicate that the ELUMO has the highest correlation to logkOH.