Theoretical Study On The Mechanism Of Atmospheric Degradation And Transformation For Severai Volatile Organic Compounds

Volatile organic compounds (VOCs) such as unsaturated alcohols, organosulfur compounds, alkenes and cycloalkenes are emitted into the atmosphere from biogenic and anthropogenic sources, which are expected to play an important role on tropospheric chemistry. These VOCs together with their oxidation products are known to exert a profound influence on the formation and growth of secondary organic aerosol, and the formation of acid rain in urban and regional areas as well as in the global areas. Moreover, they may significantly contribute to the formation and growth of CCN (Cloud Condensation Nuclei), which may have a significant influence on the Earth’s radiation budget and possibly in climate regulation.It is necessary to know the atmospheric transformations in order to better evaluate the atmospheric and environmental impact of these VOCs. Reactions with the main atmospheric oxidants such as hydroxyl radicals (OH), nitrate radicals (NO3), ozone (O3), and chlorine atoms (Cl) are considered as the predominant removal processes for the majority of the organics compounds. Kinetics and mechanisms of the gas-phase reactions of the main atmospheric oxidants with volatile organic compounds have received much attention in the past two decade. Compared with the experimental studies on VOCs, their theoretical investigations are relatively laggard. Therefore, detailed theoretical studies on the mechanisms and kinetics of those reactions in the atmospheric conditions are profound significance to further reveal their impact on atmospheric environment and also to control atmospheric pollutions.Using ab initio and density function theory (DFT) chemistry methods, detailed theoretical studies have been done for the atmospheric degradation and transformation of several typical volatile organic compounds, including2-methyl-3-buten-2-ol (MBO232,(CH3)2C(OH)CH=CH2), divinyl sulfoxide (DVSO, CH2=CHS(O)CH=CH2) and cyclohexene. Important information of potential energy surfaces of these VOCs reactions with active radicals (such as OH radicals and Cl atoms) are obtained from the theoretical investigations. Then, possible reaction channels, reaction mechanisms and major products have been discussed. The rate constants and the temperature dependence are also predicted for the reaction of cyclohexene with OH radicals. The calculations in the present thesis may be helpful for further experimental studies of this kind of reactions. The main research contents and innovative production in this thesis are summarized as follows: 1. The reacion mechanism of2-methyl-3-buten-2-ol (MBO232) with Cl atoms in the presence O2has been investigated. The thermodynamic and kinetic properties have been calculated for the reaction of MBO232+Cl+O2. The geometries of various species involved in the entrance, isomerization, and decomposition pathways have been optimized at the MP2(full)/6-311G(d,p) level of theory. The potential energy surfaces have been constructed at the CCSD(T)/6-311+G(d,p) level of theory. The calculations show that the most feasible channels are to barrierlessly form the nascent adducts (CH3)2C(OH)CHCH2C1(IM1) and (CH3)2C(OH)CHClCH2(IM2) in the entrance pathways of MBO232+Cl. The direct H-abstraction pathways as well as a complex series of isomerization and decomposition pathways of IMl and IM2are kinetically much less competitive. The newly formed adducts IM1and IM2can then react with O2in the atmosphere, followed by the formation of two alkyl peroxy radicals (CH3)2C(OH)CH(OO·)CH2Cl and (CH3)2C(OH)CHClCH2(OO·), respectively. Then, the alkyl peroxy radicals can further react with CH3O2in the atmosphere to give rise to two alkoxy radicals (CH3)2C(OH)CH(O·)CH2Cl and (CH3)2C(OH)CHClCH2O·, respectively.We found that the further transformation mechanisms of two alkoxy radicals are much different in the atmosphere. The most favorable pathway of alkoxy radical (CH3)2C(OH)CH(O·)CH2Cl is the formation of CH2ClCHO+(CH3)2C(OH) by C-C bond rupture, followed by reaction with O2to give CH2ClCHO and CH3C(O)CH3. However, another alkoxy radical (CH3)2C(OH)CHClCH2O·isomerizes preferentially to form isomer (CH3)2C(O·)CHClCH2OH, then undergo C-C bond scission to produce CH3C(O)CH3and HOCH2CHCl.The theoretical results indicate that the products CH2ClCHO and CH3C(O)CH3are major and predominant on the potential energy surface, which is in good agreement with the experimental finding. The other observed products HCHO, HC(O)Cl and HOCH2CHO can then be formed from secondary reactions of HOCH2CHC1.2. The reaction of OH radicals with divinyl sulfoxide (DVSO) in the presence of O2/NO has been studied theoretically. The potential energy surfaces for the OH+DVSO+O2/NO have been constructed. The geometric parameters of reactants, intermediates, transition states and products have been optimized at the BH&HLYP/6-311++g(d,p) level of theory. The relative energies of all stationary points have been calculated at the CCSD(T)/6-311+G(d,p) level of theory. There are large number of possible product channels covering the H-abstraction and the addition-elimination reaction pathways on the potential energy surface. The calculations illustrate that the addition-elimination mechanism dominates the OH+divinyl sulfoxide reaction. The addition reactions between OH radicals and divinyl sulfoxide begin with the barrierless formation of a reactant complex (RC) in the entrance channel, and subsequently the CH2(OH)CHS(O)CH=CH2(IM1) and the CH2CH(OH)S(O)CH=CH2(IM2) are formed by OH radicals’electrophilic additions to the double bond. It is found that the formation of IM1is kinetically more favored than the formation of IM2. Under atmospheric conditions, IM1can further combine with O2/NO to form CH2(OH)C(OONO)HS(O)CH=CH2, then CH2(OH)C(OONO)HS(O)CH=CH2dissociates to alkoxy radical CH2(OH)C(O·)HS(O)CH=CH2. The optimal channel from alkoxy radical CH2(OH)C(O·)HS(O)CH=CH2is the formation of C(O)HS(O)CH=CH2+·CH2OH through C-C cleavage. The resulting radical·CH2OH then reacts with O2to yield HCHO. The results show that the formation of HCHO+C(O)HS(O)CH=CH2is dominant, which is consistent with the recent experimental findings.3. The products and detailed mechanism of the reaction of cyclohexene with OH radicals in the presence of O2/NO have been studied at the CCSD(T)/6-311+G(d,p)//M06-2X/6-311++G(d,p) levels of theory. The calculations indicate that the optimal pathway is the formation of adduct IM2((?)) by initial addition of OH radicals to the unsaturated carbon atom of cyclohexene. Under atmospheric conditions, i.e., in the presence of O2and NO, the nascent activated IM2, will successively react with O2and NO to yield peroxy nitrite ((?)). Then, the peroxy nitrite can directly decompose to alkoxy radicals ((?)--) by NO2elimination. The ultimate fate of alkoxy radicals in the atmosphere is to generate the dicarbonyls1,6-hexanedial((C(O)H(CH2)4C(O)H) via first ring opening reaction and further reaction with O2. The calculated results are consistent with the available experimental observations. Furthermore, the additional product2-hydroxy-cyclohexanone is also predicted as secondary product. The rate constants at the temperature range298-498K for the reaction of OH radicals with cyclohexene have been calculated using the conventional transition state theory with Wigner’s tunneling correction. The theoretical rate constant at the temperature298K matches well with the experimental value. The rate constants calculated at the temperature range298-498K can be represented by the following expression k=1.71×10-12×exp(905.7/T) cm3molecule-1s-1. Clearly, the reaction of OH radicals with cyclohexene shows a negative temperature dependence of the rate constants.