Mechanistic And Kinetic Study On Cl-initiated Oxidation Of Unsaturated Esters And Unsaturated Ketones

Both the unsaturated esters and unsaturated ketones are oxygenated volatile organic compounds.They mainly are released in the environment from anthropogenic emissions.They are widely used as monomers or copolymers for the production of polymers,such as coatings,resins,adhesives,films,synthetic fibers and other production processes.The carbon-carbon double bond directly connected to the carbonyl group that makes them very active.Once they released into the atmosphere,they will take the oxidative degradation reaction with Cl atoms,OH and N03 radicals and O₃ molecules,which can generate a series of secondary organic pollutants and result in the formation of secondary aerosol.In addition,these reactions can affect the air visibility and atmospheric oxidation capacity which have an influence on human health.Recently,peoples are paying more attention to the oxidation reaction of unsaturated volatile organic compounds initiated by Cl atoms.Therefore,it is of great significance to study the process of atmospheric oxidation of unsaturated esters and ketones initiated by Cl atoms to explore the regional and global atmospheric chemistry,climate change and their environmental effects.In this paper,the reaction of Cl atoms with two kinds of unsaturated esters?methyl acrylate,MA;ethyl acrylate,EA?and three unsaturated ketones?3-penten-2-one,P032;4-hexen-3-one,H043 and 5-hexen-2-one,HO52?were studied by quantum chemical method.Some conclusions are as follows:1.Mechanism and kinetics of unsaturated esters with Cl atoms1.1 Mechanism and kinetics of methyl acrylate with Cl atomsThe atmospheric oxidation mechanism and kinetics of Cl with methyl acrylate are investigated in this study.The mechanisms of MA with Cl atoms have been obtained in detail at the M06-2X/6-311++G?3df,2p?//M06-2X/6-31+G?d,p?level of theory.The reaction initiates with three kinds of reaction pathways:Cl-addition,H-abstraction and CH3-abstraction.In general,Cl atom attacking to terminal carbon?the pathway producing IM1?is more favorable.The reaction of CH3-abstraction is hard to occure due to the high energy barrier.Hydrogen abstraction from the-CH3 group also plays an important role.Besides,the kinetic results are determined by MESMER program on the basis of Rice-Ramsperger-Kassel-Marcus?RRKM?theory.The calculated results show that the addition reactions predominate the initial reaction under atmospheric condition and the calculated total rate constant is 1.66×10-10 cm3 molecule-1 s-1 at 298 K and 760 Torr.1.2 Mechanism and kinetics of ethyl acrylate with Cl atomsThe comprehensive and reasonable mechanisms of Cl-initiated oxidation of ethyl acrylate?EA?have been proposed by computing at the M06-2X/6-311++G?3df,2p?//M06-2X/6-31+G?d,p?level of theory.The primary reaction including eight channels:two Cl-additions and six H-abstractions.Compared all calculated results,the reactions of Cl-addition are easier to occur than that of H-abstraction.However,the hydrogen abstraction from the-CH2 and-CH3 groups can not be ignored.Based on the Rice-Ramsperger-Kassel-Marcus?RRKM?theory,the rate constants are determined employing MESMER program.The calculated total rate constant?at 298 K and 760 Torr?is 1.80×10-10 cm3 molecule-1 s-1 and shows negative dependence on temperature in the range of 198-648 K.The rate constants for Cl atoms of methyl acrylate?MA?,EA,methyl methacrylate?MMA?and allyl acetate?AAC?are kMMA?Cl?>kEA?Cl?-kMA?Cl?>kAAC[Cl]Atmospheric life time of EA is 154.3 h for Cl-initiated oxidation which is compared with that of other oxidants?OH radicals,O₃molecules and NO₃ radicals?reaction with EA.2.Mechanism and kinetics of unsaturated ketones with Cl atomsThe atmospheric oxidation degradation mechanisms of Cl-initiated oxidation of three unsaturated ketones?3-penten-2-one,P032;4-hexen-3-one,H043 and 5-hexen-2-one,H052?have been proposed by computing at the M06-2X/6-311++G?3df,2p?//M06-2X/6-31+G?d,p?level of theory,and the comprehensive mechanisms are obtained.Besides the two possible reactions?Cl-addition and H-abstraction reactions?mentioned above,we also studied the reaction of CH3-abstraction.For the first two types of reactions,the reactions of Cl atom addition to the carbon-carbon double bond are dominated.The reaction of Cl atoms addition to the terminal carbon atoms is favourable.However,the reactions of CH3-abstraction are hard to occur due to the high energy barriers.The rate constants of each elementary reaction were studied at the temperature range of 198-648 K and pressure range of 76-2000 Torr.The results show that the rate constants have negative dependence on temperature,however,they have little change with pressure.In addition,the atmospheric lifetimes of three unsaturated ketones for Cl-initiated oxidation were calculated.