Theoretical Study On The Atmospheric Reaction Mechanism Of Volatile Organic Compounds

Volatile organic compounds(VOCs) are very important trace constituents of the troposphere.They have huge emissions and a wide range of structural diversities. They can be transformed by the chemical reactions with hydroxyl radical(OH), nitrate radical(NO₃) radical and ozone(O₃),and products are typically multifunctional low-volatile oxidation products containing aldehyde,ketone, organic acid,ester and alcohol moieties.Most of products in the atmosphere can contribute to the formation of secondary organic aerosol(SOA) through nucleation, hydration or absorption since their polarity and water-solubility are enhanced. Because of the small size,long average lifetime and adverse environment effects of SOA,they not only degrade air visibility,change global climate,but also are a severe threaten to human body.Therefore,the oxidation process investigation of volatile organic compounds in atmosphere is of great significance for exploring the regional and global atmospheric chemistry,climate change and environmental effects.In this thesis,a series of quantum chemical calculations based on density functional theory have been carried out to study several different atmospheric oxidation reaction systems(sabinene andβ-caryophyllene with ozone,n-octane and 1-pentanol with OH radical).As a result,some significant progresses have been made,which can be described as follows:(1) Atmospheric reaction of sabinene with ozoneThe reaction mechanism of sabinene with ozone has been investigated systematically for the first time.The calculation results indicate that the main gas-phase products are sabina ketone,formaldehyde,and formic acid;The formation mechanisms of 3-oxo-sabina ketone provided by experiment are unreasonable and the more favorable pathway is found for the first time in this paper;The structure of product(molecular weight is 154) is just like 3-hydroxy sabina ketone but not 1-hydroxy sabina ketone;the compound(molecular weight is 170) can be found in gas phase and their structures are mostly due to 2-isopropyl-2-(2-oxoethyl)cyclopropane carboxylic acid and(2-formyl-1-isopropyl-cyclopropyl)acetic acid;2-formyl-1-isopropylcyclopropanecarboxylic acid and 1-isopropylcyclopropane-1,2- dicarboxylic acid do exist in gas phase.On the basis of density functional theory data,the kinetics calculations have been deduced using canonical variational transition state theory(CVT) with small-curvature tunneling correction method(SCT) over the temperature range of 200-400 K.Changes of geometries,generalized normal-mode vibrational frequencies,and potential energies along the reaction paths are discussed and compared.Studies show:the quantum tunneling effect are significant for the O₃ addition reaction with sabinene;the calculated rate constants exhibit typical non-Arrhenius behavior;the calculated CVT/SCT rate constants are in excellent agreement with the experimental values;The relationship of rate-temperature can be described as follows:k(T)=(2.49×10^-25)T^3.37exp(-195.49/T) cm³ molecule^-1 s^-1;For the two decompositions of IM2,the quantum tunneling effect can be neglected.(2) Atmospheric reaction ofβ-caryophyllene with ozoneThe O₃-initiated atmospheric oxidation reaction ofβ-caryophyllene is studied at the B3LYP/6-31G(d) level.The results show that 4-[3,3-dimethyl-2-(3-oxobutyl)-cyclobutyl]pent-4-enoic acid,4-[3,3-dimethyl-2-(3-oxobutyl)cyclobutyl]pent-4-enal, and formaldehyde are the primary gas-phase products;4-[2-(4-hydroxy-3-oxobutyl) -3,3-dimethylcyclobutyl]pent-4-enal and 7,11,11-trimethyl-2-oxabicyclo[8.2.0]dod-ec-6-en-3-one do not exist in the gas phase;The formation mechanisms of 4-[3,3-dimethyl-2-(3-oxobutyl)cyclobutyl]pent-4-enoic acid provided by experiment are investigated and the pathway from IM6 is feasible under general atmospheric conditions.The structure of product(molecular weight is 154) is just like 1-hydroxyl-β-caryophylla ketone but not 3-hydroxyl-β-caryophylla ketone.(3) Atmospheric reaction of n-octane with OH radicalsQuantum chemiscal calculations are carried out to study the OH-initiated atmospheric oxidation reaction of n-octane at B3LYP/6-31G(d) level.Several major conclusions can be drawn as follows:The main first generation products are 1-propylpentyl nitrate,4-octanone,and 7-hydroxy-4-octanone;2-hydroxy-3-oxo-5- methyl-2-propyldihydrofuran and 1-methyl-3-oxopropyl butyrate are the primary second generation products;the reaction of 4-octoxy intermediate with O₂ occurs easily in atmosphere,which is more favorable than the isomerization and decomposition reactions;for the formation of 2-methyl-5-propyl-2,3-dihydrofuran from 5-methyl-2-propyl-2-hydroxytetrahydrofuran,the participation of H₂O molecule results in the formation of a six-membered ring,which reduces the barrier of OH transfer.The behavior of H₂O indicates that it prefers an activator to a reactant.(4) Atmospheric reaction of 1-pentanol with OH radicalsThe reaction mechanism of 1-pentanol with OH radical is studied at the B3LYP/6-31G(d) level,which leads to the following valuable conclusions:the decomposition reaction ofβ-hydroxyalkoxy with the rupture of C1-C2 bond has a low potential barrier of 0.37 kcal/mol,which is much lower than that with the cleavage of C2-C3 bond(8.54 kcal/mol);3-hydroxy-propanal is the gaseous product of the reactions ofγ-hydroxyalkoxy radical;The energy barrier of the isomerization ofδ-hydroxyalkoxy is just 1.79 kcal/mol,which means the process is favorable under atmospheric conditions;Comparing the energy barriers of R(21) and R(26) one can see that the isomerization involving H-atom abstraction from -CH₂- groups dominates over that from -CH₃ groups throughout the troposphere.