A Theoretical Study On The Chemical Reaction Kinetics Of The Alkyl Peroxy Radicals And Aromatic Bicyclic Peroxy Radicals With HO₂ Reactions

Organic peroxy radicals(RO2)are the key intermediates in the atmospheric oxidation of volatile organic compounds(VOCs),which not only affect the atmospheric oxidation capacity,but also contribute to the formation of secondary pollutants,such as secondary organic aerosol(SOA),organic nitrates,hydroperoxides and ozone.There are various types and structures of RO2,and these are characteristic of instability,short lifespan,low concentrations,high reactivity,difficult to detect experimentally and high requirements for instruments.For a given RO2 radical,it may contain different isomers,and even conformers,resulting in a lack of atmospheric reaction mechanistic and kinetic data.However,quantum chemical methods can overcome some of the experimental difficulties,achieve accurate results for structures,energy,potential energy surfaces,etc.which have attracted more and more attention in the field of atmospheric chemistry.The reactions of RO2 with HO2 have been recognized important sinks of organic peroxy radicals in low NOx environments,and the studies of the detailed reaction mechanism and micro-dynamics of RO2 under atmospheric conditions are extremely significant for understanding the whole process of the atmospheric chemical reactions and pollutants causesIn this paper,the density functional theory,coupling cluster method and transition state theory have been used to study the atmospheric reactions of HO2 radical with Propyl Peroxy(C3H7O2),Butyl Peroxy(C4H9O2),Benzene Bicyclic Peroxy(BBP),M-xylene Bicyclic Peroxy(MXYL),and 1,3,5-trimethylbenzene Bicyclic Peroxy(1,3,5-BPR)radicals.The geometries of all stationary points have been fully optimized at the B3LYP/6-311G(d,p)level of theory.According to the CCSD(T)energy with high accuracy,we obtain the potential energy surfaces and analyze the micro dynamics.The results are as follows:(1)Through the computational study of small organic peroxy radical and HO2 reaction in the atmosphere,the theoretical method is verified by the application and applied to the larger RO2+HO2 reaction.Geometric optimization and frequency calculation were calculated at the B3LYP/6-311G(d,p)level of theory.Single point energy calculations were carried out at CCSD(T)/aug-cc-pVDZ level of theory and used to construct reaction potential energy surface.Finally,traditional transition state theory was carried out for rate constant calculations.The rate constants for the reactions CH3O2+HO2 and C2H5O2+HO2 are 3.41×10-12 and 9.77×10-12 cm3 molecule-1 s-1 at 298 K,respectively.The results are in good agreement with the values reported in the literature(3.5 × 10-12 and 7.8 × 10-12 cm3 molecule-1 s-1,respectively),which confirm the reliability of the theoretical method for the RO2+HO2 reaction.(2)The reactions of HO2 radical with propyl peroxy radical and butyl peroxy radical have been studied using the above theoretical method.The computational results show that hydroperoxides and 3O2 molecule can be obtained via hydrogen abstraction routes on the triplet surface,and various products can be formed via direct hydrogen ion and addition-elimination mechanisms on the singlet surface.In addition,the rate coefficients of these reactions have been studied at a temperature range of 258-378 K,which show a negative temperature dependence.In the studied temperature range,the total rate coefficients are determined by the triplet reaction while the singlet pathways can be negligible.From the reaction mechanism and kinetic results,the formation of hydroperoxides and 3O2 is the major reaction channel in the atmospheric environment.Comparing the rate constants of the reactions of HO2 with methyl peroxy radical,ethyl peroxy radical,n-propyl peroxy radical and n-butyl peroxy radical(3.41 × 10-12?9.77 × 10-12?6.27 × 10-12 and 7.69 × 10-12 cm3 molecule-1 s-1,respectively)at 298 K,which do not show certain rules with increasing alkyl chain length.For propyl peroxy radical and butyl peroxy radical with multiple isomers and conformations,we found that the slight changes in structure has an effect on the rate constants.For example,the rate constants of the reactions of HO2 with of n-propyl peroxy radical and iso-propyl peroxy radical are 6.27 × 10-12 and 3.25 × 10-2 cm3 molecule-1 s-1,respectively.(3)In addition to the reactions of alkyl peroxy radicals with HO2,we shall extend our research further into the reactions of HO2 with typical anthropogenic aromatic bicyclic peroxy radicals(Benzene bicyclic peroxy radical,m-xylene bicyclic peroxy radical,1,3,5-trimethylbenzene bicyclic peroxy radical).The result show that the reaction mechanism of aromatic bicyclic peroxy radicals with HO2 is similar to that of alkyl peroxy radicals.The major pathway is the triplet hydrogen abstraction process,which determines the overall rate constants.The overall rate coefficients of the BBP,MXYL,1,3,5-BPR and HO2 reactions are 3.77 × 10-12,1.86 × 10-11 and 6.88 × 10-13 cm3 molecule-1 s-1,respectively.It can be concluded that the reaction rate coefficients do not show certain rules with the increase of CH3 substituents in the benzene ring.