A Theoretical Study on Reductive Debromination of Polybrominated Diphenyl Ethers

Ji-Wei Hu,Xian-Fei Huang,Xiong-Hui Wei,Jin Luo,Yuan Zhuang

doi:10.3390/ijms13079332

Abstract

Recent progress has been made in the reductive debromination of polybrominated diphenyl ethers (PBDEs) by nanoscale zero-valent iron (nZVI). To better understand the mechanism of this reaction, seven selected BDE congeners and their anions were investigated at the density functional theory (DFT) level using four different methods, including B3LYP/6-31G(d), B3LYP/6-31+G(d), B3LYP/6-31G(d,p) and B3LYP/6-311G(d,p). The cleaved C–Br bonds observed in the equilibrium structures of anionic PBDEs were adopted as the probe of the susceptible debromination position of PBDEs in the presence of nZVI, and the proposed major reaction pathways based on our calculations can satisfactorily conform to the reported experimental results. The debromination preference is theoretically evaluated as meta-Br > ortho-Br > para-Br. In addition, both the calculated frontier orbital energies and adiabatic electronic affinities were found to be highly related to their experimental reductive debromination rate constants. The highest linear regression coefficient was observed in the case using the energy of lowest unoccupied molecular orbital as the molecular descriptor obtained from B3LYP/6-31G(d) (R2 = 0.961, n = 7) or B3LYP/6-31G(d,p) (R2 = 0.961, n = 7). The results clearly showed the evidence of an electron transfer mechanism associated with this reductive debromination reaction.

Highlights

Polybrominated diphenyl ethers (PBDEs) have been widely used as additive flame retardants in polymers in many consumer products
Arulmozhiraja and Morita performed a study on the anionic dibenzofurans (PCDFs), in which the chlorine atoms at a specific substituted position were bent as the C–Cl bond stretches, and the reductive dechlorination could take place through the π*–σ* orbital mixing [14]
Previous researchers have found that the reaction rates for the debromination of BDE congeners by zerovalent iron correlated well with its lowest unoccupied molecular energy (ELUMO) and electron affinity [13,16,17], and it is generally believed that this reaction probably occurs by an electron transfer mechanism

Summary

Introduction

Polybrominated diphenyl ethers (PBDEs) have been widely used as additive flame retardants in polymers in many consumer products. Zhao et al investigated the possible dominant reductive dechlorination products of 1,2,3,7,8-PeCDD via a radical anion calculation, which were found to be in agreement with reported experimental results by the use of electron capture negative ion mass spectrometry (ECNI-MS) [15]. In both of the above cases, the significantly elongated carbon-halogen bonds at specific position were adopted as the probe to elucidate regioselectivity of reductive dehalogenation. Frontier orbital energies and adiabatic electronic affinities of the selected BDE congeners were investigated, and employed for correlation study with their reaction rate constant for the debromination by nZVI

Results and Discussion

Experimental Section

Conclusions