Theoretical Study On Oxidation Mechanisms And Kinetics Of E,E-2,4-hexadienal Initiated By OH Radical、Cl Atom And NO₃ Radical

The aldehyde molecules such as acrolein,E,E-2,4-hexadienal,octenal,as oxygenated volatile organic compounds would be released into the atmosphere by Nature and man-made sources,which will not only cause harm to human health,but also lead to environmental pollution.Therefore,the study for the transformation and absorption of air pollutants is important to control and solve environmental pollution.E,E-2,4-hexadienal,as an unsaturated aldehyde,has certain toxicity and is also an intermediate product in the photochemical reactions.It can be released into the atmosphere through natural and man-made sources,resulting in troposphere pollution.It is found that E,E-2,4-hexadienal is easy to react with atmospheric oxidants such as（?）H,Cl,H（?）₂,N（?）₃,O3 in the atmosphere except photodegradation involved E,E-2,4-hexadienal.However,it is difficult to carry out the experimental study for these reactions.Therefore,quantum chemistry calculations were used to systematically study the reaction mechanisms of E,E-2,4-hexadienal with（?）H,N（?）₃ radicals and Cl atom.And the important intermediates（R（?）₂ and R（?））with a series of atmospheric oxidants（O2,NO,NO2,H（?）₂）reactions were also studied.Based on the reaction mechanisms,the transition state theory combined with Variflex program was used to calculate the total rate constants and branching rate constants for the carbon addition and hydrogen extraction channels.In this paper,the atmospheric oxidation of E,E-2,4-hexadienal with（?）H radical,Cl atom and N（?）₃ radical was studied.We first studied the microscopic mechanism and kinetic behavior of E,E-2,4-hexadienal and（?）H radical.（?）H radical,one of the most important oxidants in troposphere,can react with the volatile organic compounds（VOCs）so as to eliminate them.In this paper,we used density functional theory to study the reaction mechanism of E,E-2,4-hexadienal with（?）H radical and the oxidation process for the important secondary reactants in the atmosphere.The rate constants are also calculated for the carbon addition and hydrogen extraction reaction channels.Firstly,the reaction mechanisms of the E,E-2,4-hexadienal and（?）H radical and its stable intermediates with some atmospheric oxidants such as O2,NO,NO2 and H（?）₂were accurately studied at the level of BMC-CCSD//M06-2X/6-311G（d,p）.The results showed that there were six hydrogen abstraction and four carbon addition paths in the first stage of E,E-2,4-hexadienal and（?）H radical reaction.Due to the low energy barrier and the reaction exothermicity,all ten paths contribute to the entire reaction,and the paths that generate IM1（CH3CHOHCHCH=CHCHO）,IM4（CH3CH=CHCHCHOHCHO）,and P6（CH3CH=CHCH=CHCO+H2O）are more favorable.Secondly,IM1,the most favorable reaction species,can quickly combine with oxygen in atmosphere to generate R（?）₂（CH3CHOHCH（?）₂CHCHCHO）and R（?）（CH3CHOHCH（?）CHCHCHO）radicals.The same quantum chemical methods were used to study the reaction mechanism of R（?）₂ with NO,NO2,H（?）₂ and the self-reaction.It is found that R（?）is relatively easy to form in the above reactions.Therefore,we carried out a detailed study on the reaction mechanisms of R（?）with a series of atmospheric oxidants.The result showed that E-butenedial is relatively dominant product.The experiment also determined that E-butenedial was the important degradation product of the E,E-2,4-hexadienal.At the same time,we also found that some substances that were not detected in experimental researches,which is also supposed to the innovation of our research.It is found that these related reactions play a key role in the circulation of atmospheric free radicals,the production of ozone and the formation of SOA.Based on the reaction mechanisms of carbon addition and hydrogen abstraction,the total rate constant at 298 K and 700 Torr was calculated to be 1.05×10^-10 cm³molecule^-1s^-1,which was close to the experimental rate constant（6.78±0.47）×10^-11 cm³molecule^-1s^-1.At the same time,we also calculated the branching rate constants and their branching ratios at 200―3000 K and700 Torr.The atmospheric lifetime of E,E-2,4-hexadienal initiated by（?）H radical was calculated to be 2.6 hours at 298 K and 700 Torr.In coastal areas,the concentration of Cl atom is very high,reaching the highest point 1×10⁴ atoms cm^-3.Cl atom also plays an important role in atmospheric chemistry because the oxidation reaction initiated by Cl atom is faster than that of（?）H radical.Thus,Cl atom is of great significance for the cyclic conversion of oxygen-containing volatile organic compounds in tropospheric chemistry.Therefore,we studied the microscopic mechanism and kinetic behavior of E,E-2,4-hexadienal with Cl atom.In this paper,quantum chemistry methods were used to study the reaction mechanism and kinetic behavior of E,E-2,4-hexadienal and Cl atom in the atmosphere at the level of M06-2X/6-311G（d,p）.The structures of all important species were optimized.Based on the optimized structures,the BMC-CCSD combination method was used to calculate the single-point energy,summarize the energy and draw the potential energy surfaces in order to analysize the detailed mechanism.The branching and total rate constant of the carbon addition and hydrogen abstraction reaction channels were calculated using the variational transition state theory combined with the Variflex program.The results showed that the rate constant of E,E-2,4-hexadienal and Cl atom at 298 K and 700 Torr was 9.12×10^-10cm³molecule^-1s^-1,which was consistent with the experimental value（3.98±0.44）×10^-10 cm³molecule^-1s^-1,and the branching rate constant of each reaction at 700 Torr and 200―3000 K was also calculated.The atmospheric lifetime of E,E-2,4-hexadienal was calculated to be 2.6 days at 298 K and 700 Torr.N（?）₃ radical plays a dominant role in participating the atmospheric chemical reactions at night,therefore,we studied the microscopic mechanism and kinetic behavior of E,E-2,4-hexadienal and N（?）₃ radical.In this paper,the reaction mechanism of E,E-2,4-hexadienal and N（?）₃ radical,as well as the important intermediates formed from the initial reaction of E,E-2,4-hexadienal and N（?）₃ radical with atmospheric oxidants（O2,NO,NO2,H（?）₂.）and self-reactions of these intermediates were studied at the level of B3LYP/6-311++G（d,p）.The reaction energies and potential energy surfaces were obtained,and the mechanisms were also analyzed.Based on the mechanisms,the traditional transition state theory combined with the Variflex program was used to calculate the branching rate constants of the carbon addition and hydrogen abstraction reaction channels and total rate constants at298 K and 700 Torr.The total rate constant was 4.60×10^-13cm³molecule^-1s^-1,which was consistent with the experimental value（1.34±0.56）×10^-12 cm³molecule^-1s^-1.The branching rate constant and total rate constant under 200―3000 K and 700 Torr is also calculated.The changing trend of branching ratio was analyzed along with temperature 200―3000 K.The atmospheric lifetime was calculated to be 1.2 hours,which shows that E,E-2,4-hexadienal can be quickly absorbed by N（?）₃ radical at night.