title: Theoretical study on the formation mechanism of pre-intermediates for PXDD/Fs from 2-Bromophenol and 2-Chlorophenol precursors via radical/molecule reactions.
abstract: This study investigates reaction pathways for the formation of pre-PXDD/F intermediates via a radical/molecule mechanism. Thermodynamic and kinetic parameters for the combination reactions of 2-bromophenol (2-BP) and 2-chlorophenol (2-CP) precursors with key radical species including the phenoxy radicals, the phenyl radicals and the phenoxyl diradicals were calculated in detail. The couplings of phenoxy radicals with 2-B(C)P tend to produce pre-PXDD intermediates of halogenated o-phenoxyphenol. The combinations of phenyl and phenoxyl diradicals with 2-B(C)P produce two types of structures, i.e., dihydroxybiphenyl and o-phenoxyphenyl, which exclusively act as prestructures of PXDFs. These condensation reactions, especially those involving the phenyl C atom sites in phenyl and phenoxyl diradicals, are proven to be both thermodynamically and kinetically favorable and are nearly comparable with the corresponding steps involved in the radical/radical reactions. Most importantly, reactions of phenyl and phenoxyl diradicals with halogenated phenols solely lead to the formation of PXDFs, which to some extent provides a plausible explanation for the high PXDF-to-PXDD ratios in the real environment. Therefore, our study reveals the pivotal role of the radical/molecule mechanism in homogeneous gas-phase PXDD/F formation, especially in PXDF formation. The present results fill in a knowledge gap that has hitherto existed regarding dioxin formation and improve our understanding of PXDD/F formation characteristics in the environment.
authors.affiliation: State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China.
authors.forename: Wenxiao
authors.initials: W
authors.lastname: Pan
authors.affiliation: State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China; College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100190, PR China.
authors.forename: Jianjie
authors.initials: J
authors.lastname: Fu
authors.affiliation: State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China; College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100190, PR China. Electronic address: aqzhang@rcees.ac.cn.
authors.forename: Aiqian
authors.initials: A
authors.lastname: Zhang
mesh_terms: D001572:Benzofurans / Q000737:chemistry; D002733:Chlorophenols / Q000737:chemistry*; D000072318:Dibenzofurans / Q000737:chemistry*; D004147:Dioxins / Q000737:chemistry*; D054879:Halogenation; D007700:Kinetics; D008956:Models, Chemical; D008962:Models, Theoretical; D010636:Phenols / Q000737:chemistry*; D000072317:Polychlorinated Dibenzodioxins; D013816:Thermodynamics
chemical_list: D001572:Benzofurans; D002733:Chlorophenols; D000072318:Dibenzofurans; D004147:Dioxins; D010636:Phenols; D000072317:Polychlorinated Dibenzodioxins; C042329:phenoxy radical; C032038:2-bromophenol; C030683:2-chlorophenol
keywords: DFT study; Formation mechanism; PXDD/Fs; Radical/molecule reactions