Mechanism Study On The Formation Of Fluorinated Dioxins And Nitro-Polycyclic Aromatic Hydrocarbons

Previous works have shown that chlorophenols (CPs) and bromophenol (BPs) are important precursors for formations of polychlorinated dibenzo-p-dioxin/dibenzofurans (PCDD/Fs) and PBDD/Fs. In this work, the formation of fluorophenoxy radicals (FPRs) from the complete series reactions of19fluorophenols (FPs) congeners with H and OH radicals was investigated theoretically by using the density functional theory (DFT) method and the direct dynamics method. The geometries and frequencies of the reactants, transition states, and products 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. The rate constants were evaluated by the canonical variational transition-state (CVT) theory with the small curvature tunneling (SCT) contribution over a wide temperature range of600-1200K. The present study indicates that the reactivity of the O-H bonds in FPs as well as the formation potential of halogenated phenoxy radicals from FPs is strongly related to the halogen atom substitution pattern. Substituted positions of halogen atoms have a significant impact on the strength and reactivity of H-O bond in halogen-phenols: halogen atoms substitutions at ortho position would decrease the strength of O-H bonds and further increase the reaction activity of O-H bonds. Weak intramolecular hydrogen bond exists in the syn conformers, resulting in the energies of syn forms are-3kcal mol"1lower than that of corresponding anti forms.The obtained results can be used for future estimates of polyfluomorinated dibenzo-p-dioxin/dibenzofurans (PFDD/Fs) emissions quantity based on the well estimated PCDD/F inventory.Gas-phase formation mechanism of PFDD/Fs from fluorophenols as precursor was studied; structural and energy information of short-lived intermediates during the reactions were described; rate constants, pre-exponential factors and activation energy of elementary reactions were obtained. Results show that strength of C-F bond plays a key role in the formation of PFDD/Fs, and formations of PFDDs and PFDFs are competitive. Theoretical calculations suggest that the main dioxin products from2-FP as precursor are1,6-DFDD,1,9-DFDD, and4,6-DFDF; the main dioxin products from2-FP and phenol as precursors are1-MFDD; the main dioxin products from2,4-DFP as precursors are1,3,6,8-TeFDD and1,3,7,9-TeFDD; Formation of PFDD/Fs from FPs is relatively more difficult than the formation of PCDD/Fs from CPs and PBDD/Fs from BPs.Nitro-PAHs are globally concerned air pollutants due to their high direct-acting mutagenicity and carcinogenicity. A mechanistic understanding of their formation is of crucial importance for successful prevention of their atmospheric pollution. Here, the formation of nitro-PAHs arising from the OH-initiated and NO3-initiated atmospheric reaction of PAHs was investigated by using quantum chemical calculation. Typical eight polycyclic aromatic hydrocarbons (PAHs) of naphthalene, anthracene, fluoranthene, fluorene, acenaphthene, acenaphthylene, phenanthrene and fluorene, coupled with benzene, were selected, and the formation mechanism of nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) arising from the OH/NO2and NO3/NO2atmospheric oxidation of PAHs were studied at molecular level. The influencing factors for gas-phase formation of nitro-PAHs from PAHs were discussed.In the atmosphere, it is widely assumed that OH or NO3radicals would attack the C atoms on the aromatic rings in the PAH molecules followed by addition of NO2to form the OH-PAH or NO3-PAH adducts at the ortho position, and the loss of water or nitric acid to form nitro-PAHs.The OH-PAH-NO2adduct will lose H2O via four-membered transition state to form nitro-PAH, and the NO3-PAH-NO2adduct will lose HNO3via six-membered transition state to form nitro-PAH. Calculations show that the direct loss of water from the OH-NO2-PAH adducts via the unimolecular decomposition is energetically unfavorable. This study finds for the first time that water plays an important catalytic effect on the loss of water from the OH-NO2-PAH adducts and promotes the formation of nitro-PAHs. In addition, the introduction of water unwraps new formation pathway through addition of NO2to the OH-PAH or NO3-PAH adduct at the para position. Steric configuration of PAH molecules has impact in the formation of corresponding nitro-PAHs. Once involved in chemical reactions, PAHs tend to retain their conjugated ring structure. For benzene and linear condensed naphthalene (bicyclic ring structure) and anthracene (tricyclic ring structure), the ranking of the exothermicity for the addition reaction of PAHs with OH or NO3is as follows: benzene<naphthalene<anthracene. The addition reaction with the molecular size increases its potential. More aromatic rings increase the potential of the addition reaction.The horn arranged phenanthrene has the same molecular size with its linear isomers anthracene. However, the addition reaction of phenanthrene with OH or NO3is less exothermic than the corresponding reaction of anthracene. In addition, the loss of water or nitric acid from the OH-NO2-phenanthrene or NO3-NO2-phenanthrene adducts has lower barrier compared to from the OH-NO2-anthracene or NO3-NO2-anthracene adducts.The individual and overall rate constants for the addition reactions of PAHs with OH and NO3radicals were deduced by using the RRKM theory on the basis of the electronic structure calculations.