Theoretical Studies On The Nighttime Atmospheric Degradation Mechanism And Kinetics Of 4-hydroxy-3-hexanone And Its Isomer

Oxygenated volatile organic compounds(OVOCs)in the atmospheric environment can not only enhance the production of urban photochemical smog but also cause large amounts of secondary pollutants.Hence they can cause many environmental issues and have negative effects on human health.As a typical of OVOCs,the oxidation behavior of hydroxyketones has been reported during the daytime.In the nightly atmosphere,the degradation reaction of OVOCs initiated by NO3 radicals is the main pathway.However,the nocturnal degradation and transformation mechanisms and risk assessments of OVOCs have not been revealed.which has practical significance to study the nighttime transformation of hydroxyketones.Therefore,this work systematically studied the oxidation mechanism and kinetics of 4-hydroxy-3-hexanone(CH3CH2COCH(OH)CH2CH3,4H3H)and its isomer 4-hydroxy-4-methyl-2-pentanone(CH3COCH2C(OH)(CH3)2,4H4M2P)initiated by NO3 radicals.The goal is to evaluate their impacts on the atmospheric pollution at night and provide theoretical support for further experimental investigations.For the detailed mechanism and kinetic properties of possible H-abstraction reaction pathways,the density functional theory of BH&HLYP method with 6-311++G(d,p)basis set is adopted to optimize the geometrical structures and calculate the frequencies.And the high-level energy correction is enforced by the method of CCSD(T).Based on the canonical variational transition state theory coupled with small curvature tunneling correction,the rate constants and branching ratios of possible pathways are calculated in the temperature range of 260-330 K.The atmospheric lifetimes of 4H3H and 4H4M2P in nighttime are evaluated.The subsequent degradation processes of dominant product for the reaction of 4H3H with NO3 are studied.The results are shown as follows:1.Among the possible paths of H-abstraction reaction,the paths of NO3 radical extracts the-CH-group in 4H3H and-CH2-group in 4H4M2P,with the lowest activation energy barriers,are the main path of the corresponding reactions.All the paths are exothermic and spontaneous.The active sites for H-abstraction are related to the bond dissociation energy(BDE)of C-H bonds in 4H3H and 4H4M2P molecules.It is found that the dominant path coincided with a relatively low bond dissociation energy site.The analysis of Mulliken charge and molecular electrostatic potential map further confirms that the H-atom at the-CH-site of 4H3H is easily to be abstracted.2.By calculating the rate constants and branching ratios,the process for H-abstraction of-CH-site plays a decisive role for the reaction of 4H3H with NO3 radical.The contribution at-CH2-sites will increases,while the contribution at the-CH3 site can be ignored as the temperature increases.The total rate constant is in good agreement with the experimental value at 298 K.And the total rate constant shows negative temperature dependence.The H-abstraction path at the-CH2-site in 4H4M2P has the largest branching ratio,and the total rate constant reveals positive temperature dependence.Through comparative analysis,the reaction of 4H3H with NO3 has relatively faster rate constant than 4H4M2P.3.The dominant product of the reaction 4H3H with NO3 radical is alkyl radical(CH3CH2C(O)C·(OH)CH2CH3).Its subsequent degradation firstly forms peroxy radicals by combining with O2.The peroxy radical undergoes H-migration and reacts with HO2·radical or NO radical.The reaction with NO radical plays a leading role in the subsequent conversion processes.And propionic acid,NO2 and ozone are the main final products.The atmospheric lifetimes of 4H3H and 4H4M2P are 19 days and 40 days at 298 K,proving that 4H3H and 4H4M2P have moderate impacts on the nighttime environment.