Mechanism and direct kinetics study on the homogeneous gas-phase formation of PBDD/Fs from 2-BP, 2,4-DBP, and 2,4,6-TBP as precursors

This study investigated the homogeneous gas-phase formation of polybrominated dibenzo-p-dioxin/dibenzofurans (PBDD/Fs) from 2-BP, 2,4-DBP, and 2,4,6-TBP as precursors. First, density functional theory (DFT) calculations were carried out for the formation mechanism. The geometries and frequencies of the stationary points were calculated at the MPWB1K/6-31+G(d,p) level, and the energetic parameters were further refined by the MPWB1K/6-311+G(3df,2p) method. Then, the formation mechanism of PBDD/Fs was compared and contrasted with the PCDD/F formation mechanism from 2-CP, 2,4-DCP, and 2,4,6-TCP as precursors. Finally, the rate constants of the crucial elementary reactions were evaluated by the canonical variational transition-state (CVT) theory with the small curvature tunneling (SCT) correction over a wide temperature range of 600-1200 K. Present results indicate that only BPs with bromine at the ortho position are capable of forming PBDDs. The study, together with works already published from our group, clearly shows an increased propensity for the dioxin formations from BPs over the analogous CPs. Multibromine substitutions suppress the PBDD/F formations.