Theoretical Study On The Atmospheric Degradation Mechanisms Of Typical Oxygenated Volatile Organic Compounds And Polyfluorinated Sulfonamides

Oxygenated volatile organic compounds (OVOCs) are very important trace con-stituents in the troposphere, and they are also significant intermediate products of many photooxidation reactions. They have extensive source and a wide variety of kinds. They can be transformed in the chemical reactions with hydroxyl radicals (OH), nitrate radical (NO3) and ozone (O3) or through direct photodegradation in the at-mosphere, and products are activated radicals and second pollutants. 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 environ-ment 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 investiga-tion of volatile organic compounds in atmosphere is of great significance for explor-ing the regional and global atmospheric chemistry, climate change and environmental effects.Perfluorocarboxylates (CF3(CF2)nCOO-,PFCAs) and perfluorosulfonates (CF3(CF2)nSO3-, PFSAs) have attracted much attentions in recent years as a result of their widespread detection, persistent nature and toxicity. They have been detected globally in the environment, wildlife, and humans, including those from remote re-gions such as the Arctic. The source of the perfluorinated acids in the Arctic is re-markable because under environmental conditions they exist mainly as anions due to the low volatility and high water solubility, and are not susceptible to long-range at-mospheric transport in the gas phase. One possible explanation is that precursor chemicals undergo atmospheric transport to remote locations and subsequent oxida-tion results in the production of PFCAs and PFSAs. Polyfluorinated sulfonamides have been suggested to be important precursors. So detailed mechanism and kinetic studies of atmospheric reactions for FSAs is prerequisite to understand their fate and contribution to PFCAs and PFSAs in remote environments. In this thesis, quantum chemical calculations have been carried out to study the degradation processes of typical OVOCs and FSAs, and CVT/SCT and multichannel RRKM-TST methods have been used to predict the branch and overall rate constants. As a result, some significant progresses have been made, which can be described as follows:1. Quantum Chemical Study on the Atmospheric Photooxidationof Methyl Vinyl Ether (MVE)The reactions of methyl vinyl ether (MVE, CH3OCH=CH2) with OH radicals have been studied using density functional theory (DFT/B3LYP) with the6-31G(d) basis set. The geometries and frequencies of all thestationary points and the minimum energy paths (MEPs) are calculated at the B3LYP/6-31G(d) level. The energetic in-formation along the MEPs is further refined at the MP2/6-311+G(d,p) level of theory. Three reaction pathways have been considered:one H abstraction and two OH addi-tions to>C=C<bonds. A complete description of the possible degradation mecha-nisms in the presence of O2and NOx has been calculated and discussed. The detailed profiles of the potential energy surfaces (PESs) for the reactions are also explained. The calculations show that the most energetically favorable isomer is that of OH addi-tionto the terminal carbon positions (C3atom). The main products of the OH-initiated atmospheric photooxidation of MVE are methyl formate, formaldehyde and glycolic acid methyl ester.2. Mechanism and Kinetic Studies for Atmospheric Oxidation of Methyl Propio-nate and a Series of Acrylate.(1) Mechanism and Kinetic Studies for OH Radical-initiated Atmospheric Oxidation of Methyl PropionateDFT molecular orbital theory calculations were carried out to investigate OH radical-initiated atmospheric oxidation degradation of methyl propionate. Geometry optimizations of the reactants as well as the intermediates, transition states and prod-ucts were performed at the B3LYP/6-31G(d,p) level. As the electron correlation and basis set effect, the single-point energies were computed by using various levels of theory, including second-order M(?)ller-Plesset perturbation theory (MP2) and the cou-pled-cluster theory with single and double excitations including perturbative correc-tions for the triple excitations (CCSD(T)). The detailed oxidation mechanism is pre-sented and discussed. The results indicate that the formation of3-oxo-methyl propio-nate (HC(O)CH2C(O)OCH3) is thermodynamically feasible and the isomerization of alkoxy radical IM17(CH3CH(O)C(O)OCH3) can occur readily under the general at-mospheric conditions. CVT/SCT theory was used to predict the rate constants. The overall rate constants were determined over the possible atmospheric temperature range of180-370K, k(T)(CH3CH2COOCH3+OH)=(1.35×10-12)exp(-174.19/T) cm3molecule-1s-1.(2)Mechanism and Kinetic Studies for OH and NO3radical initiated Atmospheric Oxidation of Methyl methacrylateDFT molecular orbital theory calculations were carried out to investigate OH and NO3radicals initiated atmospheric oxidation of methyl methacrylate (MMA). Geometry optimizations of the reactants as well as the intermediates, transition states and products were performed at the B3LYP/6-31G(d,p) level. As the electron correla-tion and basis set effect, the single-point energies were computed by using various levels of theory, including second-order M(?)ller-Plesset perturbation theory (MP2) and the coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). The detailed oxidation mechanism is presented and discussed. The reactions of MMA with OH and NO3radicals proceed via two possible mechanisms:addition-elimination and H abstraction. The H abstrac-tion is insignificant and can be ignored. The most energetically favorable reaction pathway is that of OH and NO3addition to the terminal carbon positions (C1atom). The branch and overall rate constants for the reaction of OH and NO3radicals with methyl methacrylate have been calculated using CVT/SCT and multichannel RRKM-TST methods. The overall rate constants were determined over the possible atmospheric temperature range of180-370K (cm3molecule-1s-1), k(T)(MMA+OH)=(1.83×10-12)exp(945.59/T), k(T)(MMA+NO3)=(6.75×10-16)exp(502.48/T). (3)Kinetic Studies for OH radical initiated Atmospheric Oxidation of a series of acry-late.To estimate the influence of H-substitution in the olefin on the reactivity toward the electrophilic attack of OH radicals, CVT/SCT theory was used to predict the rate constants of OH radical-initiated reaction with propylene, methyl acrylate and ethyl methacrylate at298K, which was compared with the rate constant of methyl methac-rylate with OH radicals at the same temperature. Results show that-C(O)OR func-tional group deactivates the double bond toward OH radicals attack. The electron-withdrawing mesomeric effect provided by the-C(O)OR group decreases the electron density in the p-bond that is attacked, whereas the electron-donating-R group in the-C(O)OR functional group can offset such decrease,but H-substitution by electron donor groups like-CH3or other alkyl groups in the α，β unsaturated ester increases quite significantly the reactivity of the compounds toward electrophilic attack of OH radicals.3. Mechanism and Kinetic Studies for OH Radical-initiated Atmospheric Oxida-tion of Polyfluorinated SulfonamidesPolyfluorinated sulfonamides (FSAs, F(CF2)nSO2NR1R2) are present in the at-mosphere and may serve as the source of perfluorocarboxylates (PFCAs) in remote locations, through long-range atmospheric transport and oxidation. DFT molecular orbital theory calculations were carried out to investigate OH radical-initiated atmos-pheric oxidation of a series of sulfonamides, F(CF2)nSO2NR1R2(n=4,6,8). Geometry optimizations of the reactants as well as the intermediates, transition states and prod-ucts were performed at the MPWB1K level with the6-31G+(d,p) basis set. Sin-gle-point energy calculations were carried out at the MPWB1K/6-311+G(3df,2p) lev-el of theory. The OH radicals initiated reaction mechanism is given and confirmed that the OH addition to the sulfone double bond producing perfluoroalkanesulfonic acid directly cannot occur in the general atmosphere. Use NEtFBSA as an example, detailed reaction mechanism is presented and discussed. Of these, the rate constants of some important elementary reaction are calculated using multichannel RRKM-TST methods to estimate the contributionto the burden of perfluorinated pollution in re-mote regions. CVT/SCT theory was used to predict the rate constants. The overall rate constants were determined over the possible atmospheric temperature range of180-370K,k(T)(NEtFBSA+OH)=(3.21×10-12)exp(-584.19/T),k(T)(NEtFHxSA+OH)=(3.21×10-12)exp(-543.24/T),k(T)(NEtFOSA+OH)=(2.17x10-12)exp(-504.96/7) cm3molecule-1s-1, indicating that the length of the F(CF2)n-group has no large effect on the reactivity of FSAs. Results show that the atmospheric lifetime of FSAs deter-mined by OH radicals will be20-40days, thus they may contribute to the burden of perfluorinated pollution in remote regions.