| Enols,known as the less stable isomers of carbonyl compounds,are important intermediates in the hydrocarbon oxidation process.They are likely to react with the common oxidation species in the atmosphere,such as OH,NO3,Cl radicals and O3,and have recently been implicated as efficient precursors for the formation of atmospheric organic acids.Reaction with OH radical is one of the key removal processes for enols in the atmosphere.Considering the current limitation of experimental condition and the lacking theoretical studies,in this work,we theoretically investigate the mechanisms of OH-initiated oxidation reactions of vinyl alcohol and propenols in the atmosphere using density functional theory(DFT)and transition state theory(TST).The content of this research includes four sections.The first section introduces the production pathways of vinyl alcohol and propenols in the atmosphere and their impacts on the atmospheric environment.In addition,we established the significance and necessity of the current research through summarizing and analyzing recent experimental and theoretical researches.In the second section,we intruduced the fundamental principles of quantum chemistry and the methods used in this work.The third and fourth sections are the presentations of results,that is,the mechanisms of OH-initiated oxidation reactions of vinyl alcohol and propenols in the atmosphere.The main conclusions as follows:(1)The atmospheric oxidation mechanism of vinyl alcohol initiated by OH radical have been theoretically reinvestigated at the CCSD(T)//M06-2X/aug-cc-pVTZ level of theory.The results indicate that a conformer of VA has a significant influence on the regioselectivity of the addition reactions between VA and OH radical.The calculations clearly show that the β-C-addition channel(-97%)contributes the most to the syn-VA+OH reaction,while the anti-VA+OH reaction mainly happens through theα-C-addition pathway(～65%).Upon derivation,it is found that～84%of the total initial reaction takes place through the β-C-addition channel and～16%of the total initial reaction happens by the a-C-addition channel,which are different from the branching ratio of α:β=66%:33%from the prediction of reference(Environ.Sci.Technol.2014,48,6694-6701)using the metastable reactant anti-VA.The predicted main products of the VA+OH+O2 reaction are glycolaldehyde and HO2 radical,whereas FA is just a byproduct,which is also different from an earlier theoretical prediction as one of the main oxidation products.The kinetic data indicate that both rate coefficients of the two initial reactions should exhibit a slightly negative temperature dependence.At tropospheric OH radical concentration,the lifetime of VA toward reaction with OH radical is calculated to be 19.4 and 16.1 h for syn-VA and anti-VA,respectively,and the average lifetime is estimated to be 18.8 h.(2)The mechanisms of OH-initiated oxidation reactions of propenols in the presence of O2 have been theoretically investigated at the CCSD(T)//BH&HLYP/6-311++G(d,p)level of theory.The calculations clearly indicate that OH-addition channels contribute maximum to the total reaction,both for 1-and 2-propenol,while H-abstraction channels can be neglected at the temperature range of 220-520 K.The deduced Arrhenius expressions are k1-propenol=1.43 × 10-12 exp[(743.7 K)/T]and k2-propenol=2.86 ×10-12 exp[(310.5 K)/T]cm3·molecule-1·s-1.At tropospheric OH concentration,the lifetime of 1-and 2-propenol toward reaction with OH is calculated to be 16.7 and 36.1 h,respectively.In the atmosphere,the most likely oxidation products for 1-propenol are CH3CH(OH)CHO and HO2 radical;while for 2-propenol,the primary products are CH3COCH2OH and HO2 radical.Moreover,it is found that the OH-initiated oxidation of propenols involves an oxygen-exchange mechanism,that is,all of the oxygen atoms of the regenerated OH and HO2 radicals are from O2 molecules,which provides theoretical reference for further laboratory investigations. |
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