Theoretical Study For Gas-phase Formation And Transformation Mechanisms Of Several Aromatic Compounds

Aromatic compound is a general term for hydrocarbons that contain benzene rings in their molecules.They can be produced by animals and plants,coal,oil,and artificial synthesis.These compounds have structural stability,are difficult to decompose,and are quite toxic as well,which causes significant environmental pollution and poses serious health risks to humans.Aromatic compounds can undergo important reactions such as addition,substitution,and oxidation,which can convert them into aldehydes,ketones,quinones,and epoxides.These products are important intermediates and raw materials for organic synthesis,and many of them have been widely used in the production of pharmaceuticals,agrochemicals,dyes,fragrances,various additives,engineering plastics,and functional polymers.Therefore,it is of great importance to understand the formation and degradation mechanisms of aromatic compounds.However,due to the high toxicity of aromatic compounds and the limitations of experimental detection conditions,the mechanism of their transformation in the environment is not clear.In this paper,quantum chemical calculations and molecular simulations were used to study the gas-phase formation mechanism of polychlorinated naphthalenes(PCNs)as typical aromatic compounds in high temperature,the atmospheric oxidative degradation mechanism of carbazole,and the nucleation mechanism involving aromatic acid and sulfuric acid.Meanwhile,the reaction rate constants and nucleation kinetics of important radical processes were calculated.This study can provide an understanding of the environmental fate and transformation mechanisms of aromatic compounds,and provide theoretical guidance for the removal and management of aromatic compounds in the environment.1.The Homogeneous Gas-Phase Formation Mechanism of PCNs from CrossCondensation of Phenoxy Radical with 2-CPR and 3-CPRChlorophenols(CPs)and phenol(Ph)are abundant in thermal and combustion processes,which are key precursors for the formation of polychlorinated naphthalenes(PCNs).CPs and phenol can react with H or OH radicals to form chlorophenoxy radicals(CPRs)and phenoxy radical(PhR).The self-condensation of CPRs or cross-condensation of PhR with CPRs is the initial and most important step for PCN formation.In this work,detailed thermodynamics and kinetic calculations were carried out to investigate the PCN formation mechanisms from PhR with 2-CPR/3-CPR.Several energetically advantaged formation pathways were obtained.The rate constants of key elementary steps were calculated over 600～1200 K using canonical variational transition-state theory(CVT)with a small curvature tunneling contribution(SCT)method.The mechanisms were compared with the experimental observation and our previous works on the PCN formation from self-condensation of 2-CPRs/3-CPRs.The study shows that naphthalene(N)and 1-monochlorinated naphthalene(1-MCN)are the main PCN products from cross-condensation of PhR with 2-CPR,and N and 2-monochlorinated naphthalene(2-MCN)are the main PCN products from cross-condensation of PhR with 3-CPR.Pathways terminated with C1 elimination are preferred over those terminated with H elimination.PCN formation from cross-condensation of PhR with 3-CPR can occur much easier than that from crosscondensation of PhR with 2-CPR.This study along with the study of PCN formation from the self-condensation 2-CPRs/3-CPRs,can provide reasonable explanations for the experimental observations that the formation potential of N is larger than that of 1-MCN using 2-CP as the precursor,and an almost equal yield of 1-MCN and 2-MCN can be produced with 3-CP as the precursor.2.Theoretical perspectives on the gas-phase oxidation mechanism and kinetics of carbazole initiated by OH radical in the atmosphereCarbazole is one of the typical heterocyclic aromatic compounds(NSO-HETs)observed in the polluted urban atmosphere,which has become a serious environmental concern.In this study,we conducted a comprehensive study on the reaction mechanism of carbazole with OH radicals using the DFT method.Additionally,based on the structure optimization and energy calculation,rate constants and lifetime of the elementary reactions of carbazole were calculated at 298 K and 1 atm.There are four types of reaction:OH additions to "bend" C atoms,OH additions to "benzene ring" C atoms,H abstractions from C-H bonds,and the H abstraction from N-H bond.Among them,the OH addition to the C1 atom and the H abstraction from the N-H bond are predominant due to the lower potential barriers and stronger heat release.The hydroxycarba-zole,dialdehyde,carbazolequinone,hydroxy-carbazole-one,and hydroperoxylcarbazole-one are generated by OH addition reactions and their subsequent reactions while OH abstraction reactions produce carbazole-ol.The calculated overall rate constant of carbazole oxidation by OH radical is 6.52 × 10-12 cm3 molecule-1 s-1 and the atmospheric lifetime is 43.92 h,indicating a potential long-range transport in the atmosphere.The ranking of the lifetime of NSO-HETs determined by OH radicals is as follows:Pyrrole ≈ Indole<Dibenzofuran ≈Fluorene<Dibenzothiophene ≈ Carbazole.This work provides a theoretical investigation of the oxygenated mechanism of NSO-HETs in the atmosphere,which may help to clarify the potential health risk for determining the reaction pathways and environmental influence of carbazole.Due to the lack of efficient detection schemes for intermediate radical species,a full analysis of the atmospheric processes of the reaction mechanism of OH-initiated atmospheric oxidation of carbazole is limited to the laboratory studies,which hinders the further evaluation of the atmospheric effect of carbazole.3.Theoretical and computational evidence for the involvement of phthalic acid in nucleation mechanismPhthalic acid(PA)is a kind of common organic acid in the atmosphere,which comes from a wide range of sources.Moreover,aromatic acid can participate in the new particle formation and have an impact on atmospheric aerosols.In this paper,the interaction of phthalic acid with the common nucleation precursor sulfuric acid(SA)was investigated by combining quantum chemical calculations,molecular simulations,and molecular dynamics simulations.The nucleation rates and growth paths were calculated using ACDC nucleation on the basis of thermodynamic data obtained at the M06-2X/6-311+G(2df,2pd)level.In addition,molecular dynamics simulation(MD)usingAMBER force field(GAFF)were performed on a larger scale.The results showed that the aggregation speed of PA and SA molecules has improved with the increase of PA proportion.We propose that PA can actively participate in particle nucleation and may dominate the initial steps under high C(PA)/C(SA)ratios.Unexpectedly,the Gibbs Free Energy of pure PA is lower than that of pure dimethylamine(DMA)and even lower than pure sulfuric acid.Meanwhile,the kinetic simulation shows that pure PA has a stronger capacity for self-aggregation,suggesting that PA can initiate New Particle Formation(NPF)by itself or together with SA.The monomer evaporation is the main degradation way and the monomer collision is the main growth way in PA-S A system.The first step of(PA)m(SA)n(m=0-3,n=0-3)system growth is the formation of(PA)(SA)cluster.After the initial step,the main growth pathway is that SA and PA molecules added to(PA)(SA)cluster to form(PA)2(SA)2 cluster.The main clusters leaving the system are(PA)3(SA)4 and(PA)4(SA)4 clusters.This study provides a theoretical basis for the study of the nucleation of macromolecular systems.