Quantum chemical and direct dynamic study on homogeneous gas-phase formation of PBDD/Fs from 2,4,5-tribromophenol and 3,4-dibromophenol.

2,4,5-Tribromophenol (2,4,5-TBP) and 3,4-dibromophenol (3,4-DBP) have the minimum number of Br atoms needed to form 2,3,7,8-PBDD/Fs, which are the most toxic among all 210 PBDD/F isomers. A mechanistic understanding of the formation of PBDD/Fs is a prerequisite for minimizing their emissions. In this paper, the homogeneous gas-phase formation of PBDD/Fs from 2,4,5-TBP and 3,4-DBP as precursors was investigated theoretically by using the density functional theory (DFT) method. The mathematical model to predict the formation of PBDD/Fs places a high demand on accurate kinetic parameters. So, the rate constants of key elementary steps involved in the formation of PBDD/Fs were calculated by using canonical variational transition-state (CVT) theory with small curvature tunneling (SCT) contribution over a wide temperature range of 600-1200K. The pre-exponential factors and the activation energies are also reported. This might be the first to investigate the formation of 2,3,7,8-PBDD/Fs. The present study shows that the formation of PBDDs dominates over the formation of PBDFs. The meta bromine facilitates the dimerization of bromophenoxy radicals (BPRs), whereas the para and ortho bromines suppress the dimerization of BPRs.