Theoretical Study On The Oxidation Reaction Mechanism Of Linear CH₃OC_nH_2n+1（n=2~5） Induced By·OH In The Atmosphere

The study on the reaction mechanism of methyl ether oxidized to toxic organic matter has attracted great attention in recent years,but limited by the experimental conditions,and some chemical reactions can only measure the reaction rate constant at a small range of temperature and the rate constant of the same reaction at the same temperature.For the fast and difficult to control chemical reactions of toxic and volatile methyl ether,it is more difficult to detect or capture the reaction intermediates,and the true mechanism of such reactions cannot be understood.With the advantages of quantum chemical calculation methods,the reactants,transition states,intermediates and products of the oxidation reaction of methyl ether compounds are optimized at the atomic and molecular level,so as to better understand the various possible reactions of such reactions.It is of great theoretical significance to find a way to solve the environmental pollution caused by such substances.In this study,the oxidation reaction mechanism of CH₃OC_nH_2n+1（n=2～5）and·OH in the troposphere was studied by using density functional theory.The geometry of all reactants,intermediates,transition states,and products in the reaction system were optimized at the CCSD（T）/6-311++G（d,p）//M05-2X/6-311++G（d,p）level.Vibration frequency analysis was performed at all stable points at the same theoretical level.Intrinsic reaction coordinates（IRC）were used to verify the correct transition state connecting of the reactants and products.The reaction rate constants were calculated for all reaction paths using the conventional transition state theory（TST）combined with the S＆T tunnel correction.The study found that the reaction of CH₃OC_nH_2n+1（n=2～5）and·OH can be divided into initial reaction and subsequent reaction.The initial reactions are mainly that·OH abstracts H of CH₃OC_nH_2n+1（n=2～5）to generate C central radical and water,in which the reaction channel of·OH abstracting H neighbor to the oxygen(C_αand C_α′)to generate C_αcentral radical or C_α′central radical are easy to occur.At constant pressure and 298K,the total reaction rate constant of methyl ethyl ether,methyl propyl ether,methyl butyl ether,methyl pentyl ether with·OH are 0.9×10^-12cm~3·molecule^-1·s^-1,1.01×10^-12 cm~3·molecule^-1·s^-1,1.80×10^-12cm~3·molecule^-1·s^-1,2.59×10^-12 cm~3·molecule^-1·s^-1,respectively.With the growth of the carbon chain of CH₃OC_nH_2n+1（n=2～5）,the overall reaction rate constant increases,and that of·OH abstracting H on C_αslightly decreases,and that of·OH abstracting H on C_α′increases.The branching ratio of·OH abstracting H on C_αdecreases,and that of·OH abstracting H on C_α′is basically unchanged,and the branching ratio gap between all reaction paths decreases.In the temperature range of 200-400 K,the overall reaction rate constants for the reaction of CH₃OC_nH_2n+1（n=2～5）with·OH,the reaction rate constants for·OH abstracting H on C_α,and the reaction rate constants for·OH abstracting H on C_α′all increased with the increase of temperature.The subsequent reaction mechanism was explored by taking methyl ethyl ether and methyl amyl ether as examples.The initial reaction products of methyl ethyl ether are mainlyα-methyl ethyl ether radicals andα’-methyl ethyl ether radicals,and the initial reaction products of methyl amyl ether are mainlyα’-methyl pentyl ether radicals,α-methyl pentyl ether radicals,β-methyl pentyl ether radicals,γ-methyl pentyl ether radicals,andδ-methyl pentyl ether radicals,and all of them further undergo subsequent reactions respectively.Subsequent reactions are divided into bimolecular reactions and unimolecular reactions.Bimolecular reactions include four kinds of reactions:the initial reaction products react with HO₂·to generate·OH and alkoxy radicals,and those react with NO to generate NO₂ and alkoxy radicals,and those react with NO to generate HNO₂ and esters,and those react with NO₂ to generate HNO₃ and esters.Unimolecular reactions include the cleavage of C—H bonds at different positions of the radical to form aldehyde or epoxy groups.In the subsequent reactions of theα-methyl ethyl ether radicals andα’-methyl ethyl ether radicals,the favorable bimolecular reactions are the pathways with NO to generate NO₂ and alkoxy radicals,which generate methyl acetate and methyl formate,respectively.The favorable unimolecular reactions are C_α′—H bond cleavage and C_α—H bond cleavage,respectively,which generate acetaldehyde and formaldehyde.The theoretically predicted product is consistent with the experimentally determined product.In the subsequent reactions ofα’-methyl amyl ether radicals,α-methyl amyl ether radicals,β-methyl amyl ether radicals,γ-methyl amyl ether radicals,andδ-methyl amyl ether radicals,the favorable bimolecular reactions ofα’-methyl amyl ether radicals andα-methyl amyl ether radicals are the reactions with NO.The path to generate NO₂ and alkoxy radicals,respectively,which generate amyl formate,methyl formate,and butyraldehyde.The favorable bimolecular reactions ofβ-methyl amyl ether radicals,γ-methyl amyl ether radicals andδ-methyl amyl ether radicals are all the paths of reaction with NO₂.1-methoxy-2-pentanone,1-methoxy-3-pentanone and 1-methoxy-4-pentanone were respectively generated.The favorable unimolecular reactions are:C_α—H bond cleavage,C_α′—H bond cleavage,C_α—H bond cleavage,C_δ—H bond cleavage,C_γ—H bond cleavage,generating valeraldehyde,formaldehyde,1-methoxyl-1,2-pentane,5-methoxy-2,3-pentane.