Theoretical Studies On The Gas-phase Formation Mechanisms Of Halogenated Naphthalenes Via Halophenols Under High Temperatures

Halophenols(HPs)are the principal precursors for the formation of polyhalogenated naphthalenes(PHNs),which includes chlorophenols(CPs),bromophenols(BPs),fluorophenols(FPs)and bromochlorophenols(BCPs).Typically,the main sources of HPs are high-temperature processes such as typical combustion processes,accidental fires,and municipal waste incineration.Additionally,HPs have been widely used as feedstock and intermediates for many chemical products such as dyes,pulp,fungicides,wood preservatives,herbicides,and flame retardants.HPs have also been frequently detected in various environmental media,which can convert to halophenoxy radicals(HPRs)by losing H atom in O-H bond under high temperatures.Via isomerization and condensation,Bromochlorophenoxy radicals(BCPRs)can generate the more toxic polybromo-chlorinated napthalenes(PBCNs)and polybromo-chlorinated dibenzo-p-dioxin/furans(PBCDD/Fs).Hence,studies on the formation of BCPRs is the initial but most important step of PBCNs and PBCDD/Fs’ generation.PHNs,which are identified as the poisonously dangerous persistent organic contaminants(POPs),are widely distributed in the global environments.They can enter into the organisms through food chain and exerts strong damages to human being.PHNs are produced during the combustion of waste materials,metallurgical industry and many other high-temperature chemical processes.Before the 1970s,PCNs were mainly from the industry process such as paraffin wax,and banned production due to the serious environment pollution.In recent years,PHNs are mianly unintentionally generated together with dioxins as buproducts under high temperatures.PCNs have been hot research topic,which are considered as the typical PHNs.PCNs,which are also called dioxins-like componds,are supposed to have chemical toxicity,recalcitrant to biodegradation,bioaccumulation and can be transported over long distances.Exposure to higher chlorinated naphthalenes appears to be more toxic than lower chlorinated congeners.Among PCNs,1,2,3,4,5,6,7-heptachloronaphthalenes(1,2,3,4,5,6,7-HpCNs)have not only the higheset toxicity in environment,but the highest level under combustion conditions.Therefore,it is significant and inevitable to search for the formation mechanisms of the higher chlorinated congeners such as HpCN.In the mechanism of CPs as precursor,the dimerization of 2,3,4,5-terachlorophenoxy radical(2,3,4,5-TeCPR)as well as the cross-condensation of 2,3,4,5-TeCPR with pentachlorophenoxy radical(PCPR)are the important generation pathways of 1,2,3,4,5,6,7-HpCNs.1.Reaction Mechanism of BCPs with H Radical under high temperaturesIn this paper,with high-precision quantum chemical calculation methods,the series reactions of 96 bromochlorophenol(BCP)congeners with H was studied at MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p)level.This paper also calculated the rate constants of elementary reactions using canonical variational transition-state(CVT)theory along with small-curvature tunneling(SCT)contribution under the temperature of 600-1200 K.In addition,abstraction reactions of bromophenols(BPs)and chlorophenols(CPs)with H were compared with reactions of BCPs with H.The computational results indicate that the relative reactivity of the O-H bonds in the BCPs appeares to be assisted and dominated by the total numbers and patterns of halogenation,but it does not appear to be determined by the distribution of the bromine and chlorine atoms.In addition,halogen substitution at the ortho position increases the stability of the BCPs and reduces the reactivity of the O-H bonds.2.Formation mechanism of PCNs via the dimerization of 2,3,4,5-TeCPRs and cross-condensation of 2,3,4,5-TeCPR with PCPR under high temperaturesIn this paper,we focused on the possible reaction mechanism about the formation of higher chlorinated naphthalenes,especially for the more virulent 1,2,3,4,5,6,7-HpCN,from the radical/radical self-condensation of 2,3,4,5-TeCPRs as well as cross-dimerization of 2,3,4,5-TeCPR with PCPR.In this paper,the vibrational frequencies and single point energies of every intermidate and transtration state were obtained.Meanwhile,the rate constants of crucial elementary steps were calculated based on the canonical variational transition-state(CVT)theory with the small curvature tunneling(SCT)and the optimal generation routes of 1,2,3,4,5,6,7-HpCN and several other higher chlorinated naphthalenes were selected.Moreover,the results suggested that the last step ended with Cl atom elimination is much easier than H elimination process in the formation pathways.Additionally,the 1,2,3,4,5,6,7-HpCN and 1,2,3,4,5,6,8-HpCN are the main PCN products in the dimerization of 2,3,4,5-TeCP radicals,and the formation potential of 1,2,3,4,5,6,7-HpCN is lager than l,2,3,4,5,6,8-HpCN.The possible PCN products from the cross-condensation of PCPR with 1,2,3,4-TeCPR are 1,2,3,4,5,6,7-HpCN,1,2,3,4,5,6,8-HpCN and OCN,and the formation potential of the three products are similar.Another interesting finding of this work is that,with the increase of chlorination,the trend of PCNs formation will decrease and the H/Cl shift mechanism is harder to happen.3.The formation mechanisms of polybrominated naphthalene and polyfluorinated naphthalene from 2-bromophenol and 2-fluorophenolIn this study,the mechanism of gas-phase formation of polybrominated naphthalenes(PBNs)and polyfluorinated naphthalenes(PFNs)from 2-bromophenol and 2-fluorophenol as precursor were investigated theoretically,respectively.In addition,the rate constants of the typical elementary reactions were calculated.Dimerization of halogenated phenoxy radicals contains three C-C coupling modes,and the formation pathway of CH/CH is most energetically feasible.Moreover,the pathways ended with Cl elimination are favored over those ended with H elimination.Therefore the mono-substituted PCNs can form easier than the di-substituted one,resulting the mono-halogenated naphthalenes and naphthalenes.The study indicates that H can be directly abstracted by H,OH and Cl radicals.However,the H shift mechanism in the subsequent reactions of halogenated dihydrofulvene is infeasible.It is shown that the types of the halogen atoms affect the formation potential of final products by comparing three mechanisms.