Theoretical Study On The Formation Mechanism Of Sulfur-substituted Dioxins And The Nucleation Mechanism Of Atmospheric Typical Organic Compounds

Polychlorinated thianthrenes(PCTAs),polychlorinated phenoxathiins(PCPTs),polychlorinated dibenzothiophenes(PCDTs),are sulfur analogues of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans(PCDD/DFs).Their toxicity,persistence,lipophilic and physicochemical properties are similar.PCTA/PT/DT/DD/DFs can be formed by two main formation pathways,precursors synthesis and de novo synthesis in the high-temperature gas phase.Chlorothiophenols(CTPs)and chlorophenols(CPs)are key precursors to form PCTA/PT/DTs.which can form chloro(thio)phenoxy radical,sulfydryl/hydroxyl-substituted phenyl radical and(thio)phenoxyl diradicals via abstraction reactions by H,OH,Cl,or O(3P)radicals.High-temperature gas phase mechanism includes radial/radical coupling and radial/molecule coupling in the formation of PCTA/PT/DTs from CTPs/CPs.The available radical/radical formation mechanism failed to explain the higher concentration of PCDTs than that of PCTAs under the pyrolysis or combustion conditions.So it is important to study radical/molecule formation mechanism to form PCTA/DTs.New particles formation is a key source of particulate matters and cloud condensation nuclei(CNN)in the atmosphere influencing regional ambient air quality,climate and human health.The formation of new particles covers two phases,including a nucleation phase leading to a metastable critical nucleus(approximately 1?3 nm in diameter)and a growth phase where the critical cluster increases easily in size.Some mechanisms have been put forward to explain atmospheric nucleation events,including binary H2SO4/H2O,ternary H2SO4/H2O/NH3 and ternary H2SO4/H2O/amine,ternary H2SO4/H2O/extremely low volatile organic compounds(ELVOCs),iodine oxide,ion-mediated nucleation,and nucleation enhanced by organic compounds.Organic compounds can promote the nucleation of aerosols.It is not clear which one or which kind of organic compounds are involved in the nucleation process because there are many kinds of low volatile organic compounds in the atmosphere.The current consensus is that the role of organic acids and amines should not be neglected.It is confirmed that some monocarboxylic acid,dicarboxylic acids,aromatic acids,amines,amino acids and other substances can play a role in stabilizing the cluster,and participate in the nucleation of aerosols and the generation of new particles.It is difficult for amides to promote nucleation with sulfuric acid and participate in the nucleation of new particle formation.Organic peroxides are not likely to participate in the initial step of new particle formation and maybe contribute to the growth of particles.Due to the limitation of observation instruments,it is difficult for researchers to study the process of the formation of critical nucleus at the microscopic scale.The combination of artificial bee colony algorithm and quantum chemical computation can help researchers search for the lowest energy configuration of clusters and obtain the geometric configuration,thermodynamics and dynamics data of molecuar clusters.Researchers can also calculate the nucleation rate of precursors using the atmospheric cluster dynamics code(ACDC).Therefore,it is of great significance to to study the role of amides and organic peroxides in the nucleation of aerosols at the molecular level.1.Quantum chemical and kinetic study on radical/molecule formation mechanism of pre-intermediates for PCTA/PT/DT/DFs from 2-chlorothiophenol and 2-chlorophenol precursors.In this study,we present a systematic theoretical investigation on the initial pathways of PCTA/PT/DT formation from the condensation reactions of 2-CP/2-CTP with chlorinated phenoxy radical 2-chlorophenoxy(CPR1)and chlorinated thiophenoxy radical 2-chlorothiophenoxy(CTPR1),phenyl radicals 2-hydroxylphenyl(PR2)and 2-sulfydrylphenyi(TPR2),chlorinated phenyl radicals 2-hydroxyl-3-chloro-phenyl(CPR2)and 2-sulfydryl-3-chloro-phenyl(CTPR2),phenoxyl diradical(PDR)and thiophenoxyl diradical(TPDR).and chlorinated phenoxyl diradical(CPDR)and chlorinated thiophenoxyl diradical(CTPDR)at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p)level using the DFT method The rate constants of the formation of the pre-PCTA/PT/DT/DF intermediates from cross-condensation reactions of 2-C(T)P molecules with related radicals were calculated by using conventional transition state theory(TST)method in the temperature range of 600?1 200 K.To be used more effectively,the rate constants were fitted,and Arrhenius form ulas are given.We found that:(1)The S/C coupling modes are preferable over the O/C coupling modes.The S/C coupling modes can form pre-PCTAs and pre-PCPTs intermediates in the coupling of chloro(thio)phenoxy radical with 2-C(T)P and form pre-PCDTs intermediates in the coupling of(thio)phenoxyl diradical with 2-C(T)P.(2)For the self-coupling of 2-CTP and corresponding sulfur-substituted radicals,the pre-PCTA intermediates can only be produced from the coupling of chlorothiophenoxy radical with 2-CTP,and the pre-PCDT intermediates can be formed from the coupling of sulfydryl-substituted phenyl radical with 2-CTP and the coupling of thiophenoxyl diradical with 2-CTP.The pre-PCDT intermediates formation pathways are more favorable than the pre-PCTA intermediates formation pathways,which,to some extent,can give reasonable explanation for the high PCDT-to-PCTA ratio under the pyrolysis or combustion conditions.(3)The S(O)/?-CH couplings are energetically more favorable than the S(O)/?-CCl couplings to form the(thio)ether-(thio)enol intermediates,and the C-/CH coupling can take place much easier than the C·/CCl coupling to form the(thio)keto-(thio)enol intermediates.(4)In the case of chloro(thio)phenoxy radical with 2-C(T)P,the S/C coupling to form pre-PCTA/PTs intermediates are more likely to occur than the C/C coupling to form pre-PCDT/DFs intermediates.However,in the case of(thio)phenoxyl diradical with 2-C(T)P,the C/C coupling are much easier than the S/C coupling,both of which can contribute to the formation of pre-PCDT/DFs intermediates.2.Theoretical Study on the Nucleation Mechanism of Amides.In this study,We investigated the formation of binary clusters of sulfuric acid(SA),acetic acid(HAC),propanoic acid(PAC),oxalic acid(OA)and malonic acid(MOA)with formamide(FA),acetamide(AA),N-methylformamide(MF),propanamide(PA),N-methylacetamide(MA)and N,N-dimethylformamide(DMF)using theoretical calculation.The study is mainly divided into two steps.Firstly,we used the ABC algorithm to obtain the global minimum;Secondly,we calculated the bonding energy,enthalpies,entropies,Gibbs free energy at the PW91PW91/6-311++G(3df,3pd)level using the DFT method.And the properties of hydrogen bonds in binary clusters were analyzed based on configuration,frequency,and topology parameters.We found that(1)The interaction between acid and amide leads to the formation of stable hydrogen bond complexes.The stabilization effect of amides is similar to that of organic amines but stronger than that of ammonia.Among the six amides,trans-MF has the strongest stabilization effect and DMF is the weakest.Compared with the interaction between acid and amide,the nucleation capacity of sulfuric acid is greater than that of organic acid.(2)By comparing the hydrogen bond length,vibration frequency and topological parameters,the strength of the four types of hydrogen bonds are arranged in the following order:SO-H…O>CO-H…>N-H…O>C-H…O.(3)It is difficult for amides to promote nucleation with sulfuric acid and participate in the nucleation of new particle formation.(4)Because the atmospheric concentration of amide is abundant,strong hydrogen bond interaction between acid and amide provides the driving force for the acid-amide complexes formation.It is possible to reduce the nucleation barrier and promote the formation and growth of new particles by adding hydrophilic species(such as SA.H2O,NH3,etc.)to the hydrophilic side of clusters which can change the hydrophobicity of organic acids.3.Theoretical Study on the Nucleation Mechanism of organic peroxides.In this study,We investigated the structure and thermodynamic properties of binary clusters of peroxyformic acid(PFA)peroxyacetic acid(PAA),methyl hydroperoxide(MHP),ethyl hydroperoxide(EHP),hydroxymethyl hydroperoxide(HMHP),1-hydroxyethyl hydroperoxide(1-HEHP).2-hydroxyethyl hydroperoxide(2-HEHP)with H2SO4,H2O,NH3,methylamine(MAE),dimethylamine(DMA),trimethylamine(TMA)at the PW91PW91/6-311++G(3df,3pd)level using the DFT method.We also studyed that the hydration effects of 0-6 water molecules on the nucleation of PAA-SA.PAA-NH3 and PAA-DMA clusters.We found that(1)Organic peroxides form molecular complexes with nucleation precursors through hydrogen bond.HMHP has the strongest nucleation ability,while the nucleation ability of PFA.PAA.MHP,EHP.1-HEHP and 2-HEHP are weakern Among nucleation precursors,sulfuric acid has the strongest nucleation ability and H2O is weakest.(2)Organic peroxides can not participate in the initial step of new particle formation and only contribute to the growth of atmospheric particles.(3)Hydration play a important role in aerosol nucleation.For(PAA)(SA)(W)n,the maximum Gibbs free energy change of the reaction is reached when PAA is hydrated with 6 molecules of water and sulfuric acid is anhydrated.For(PAA)(NH3)(W)n,the maximum Gibbs free energy change of reaction is reached when both PAA and NH3 are all hydrated.For(PAA)(DMA)(W)n,hydration can't promote aerosol nucleation visibly.