Influence of water on the homogeneous gas-phase formation mechanism of polyhalogenated dioxins/furans from chlorinated/brominated phenols as precursors

• Influences of water on gas-phase formations of PHDD/Fs were revealed for the first time by lowering barriers.
• Water plays a catalytic effect on the formation of HPRs and the H-shift step.
• New lower-energy pathways were found in the H abstraction reactions of HPs.
• The energy profile in the presence/absence of water was constructed and compared.

Water is of great chemical importance due to its ability to form hydrogen bond. Polyhalogenated dibenzo-p-dioxin/benzofurans (PHDD/Fs) are notorious due to their persistence, bioaccumulation and extremely high toxicity. Water is ubiquitous, and a deep knowledge of its influence on the formation mechanism of PHDD/Fs is necessary. This work investigated the influence of water on the homogeneous gas-phase formation of PHDD/Fs from halogenated phenols (HPs) as precursors by using quantum chemical calculations with the aid of the MPWB1K theoretical approach in connection with the 6-31+G(d,p) and 6-311+G(3df,2p) basis sets. The schematic energy profile in the presence of water was constructed and compared with the situation without water. This study reveals for the first time that the introduction of water promotes the formation of halogenated phenoxy radicals (HPRs) from the H abstraction reactions of HPs with atomic H and OH radicals by lowering the reaction energy barriers and opening new low-energy pathways. Another intriguing finding of this work is that the inclusion of a water molecule produces a catalytic effect on the H-shift step involved in the formation of PHDFs and thus their formation potential is enhanced.

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