Quantum Chemistry Investigations On Environmental Photochemical Behavior Of Selective Organic Pollutants

Once discharged into the environment, organic compounds can undergo three main transformation processes, i.e., chemical transformation, photochemical transformation, and bio-transformation/bio-degradation. Photochemical transformation is a key process affecting the environmental behavior and fate of organic pollutants. However, experimental data for photochemical behavior of a large amount of organic compounds released into the environment are not available, and the mechanism underlying photochemical reaction is still being determined. Quantum chemistry calculations can be used to investigate molecular structures, elucidate the mechanisms of photochemical reactions, and predict the photochemical behavior of organic compounds at the microscopic level. In this thesis, the photochemical behavior and mechanism of five representative persistent/emerging organic pollutants were investigated on the basis of density functional theory (DFT)/time-dependent density functional theory (TDDFT). In addition, the pathways and products of photolysis of these organic pollutants were predicted. Main contents and results are as follows:(1) Conformations of35polychlorinated biphenyls (PCBs) in the first triplet excited state (T1) were investigated using TDDFT. The pathways and products of direct photolysis of PCBs were predicted by calculating the C-Cl bond dissociation energies, and the mechanisms of direct photolysis of PCB77were elucidated in DFT framwork. Results show that the conformations of PCBs in the T1state have three types:①Two benzene rings are in the same plane;②Two benzene rings tend to be coplanar;③Two benzene rings are deformed seriously and fold an angle. For the PCBs with two benzene rings in the same plane or tending to be coplanar in the T1state, the conjugation between the two benzene rings is enhanced, which results in a low photoreactivity of PCBs. For the PCBs with benzene rings deformed seriously and folding an angle, the conjugation between the two benzene rings is reduced, which leads to a high photoreactivity of PCBs. The predicted photolysis pathways are agreement with experimental findings. The dissociation of C-Cl bond is rate-determining step for the direct photolysis of PCB77.(2) Quantum chemical calculations studies were performed to investigate the underlying direct photolysis dehalogenation mechanisms of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs). The C-Cl/Br bond dissociation energies, C-Cl/Br bond vibrational frequencies, and electronic absorption spectra of PCDD/Fs and PBDD/Fs were calculated using DFT and TDDFT to investigate the effects of halogen substitution pattern on photodehalogenation reactivity of PCDD/Fs and PBDD/Fs. Results show that the photodehalogenation reactivity of PCDD/Fs and PBDD/Fs increases with an increase of halogenation degree. Halogen atoms attached to the benzene ring with more halogens substituted have higher photoreactivity. For PCDD/BDDs, the photoreactivity of halogens at2and3positions are higher than that of halogens at1and4positions. For PCDF/BDFs without halogen substitution at9position, the order of photoreactivity of halogens follows:2,3>1,4position. However, for PCDFs/BDFs with halogen substitutions at1and9positions, the order of photoreactivity of Cl/Br atoms follows:1>2>3>4position. In addition, the dissociation of C-Cl bond is rate-determining step for direct photolysis dechlorination of2,3,7,8-T4CDD/CDF as water becomes hydrogen donor.(3) Direct photolysis mechanisms of polychlorinated diphenyl ethers (PCDEs) were investigated by DFT. The direct photolysis of PCDEs has three potential reaction pathways including photodechlorination, C-O bond photodissociation, and PCDFs formation. Taking a representative PCDE (i.e., CDE8) for example, C-Cl bond dissociation is the rate-determining step for photodechlorination. The reaction activation energy of photodechlorination is the smallest among the three direct photolysis reaction pathways. The photoreactivity of PCDEs increases with an increase of chlorination degree.(4) Conformations of polybrominated diphenyl ethers (PBDEs) in the first singlet excited state (S1) and direct photolysis mechanisms were investigated using DFT and TDDFT. Each PBDE exhibits remarkably different geometries in the ground state and the S1state. The significant lengthening of C-Br bond in each PBDE is observed in the S1state. The positions of the lengthened C-Br bonds are agreement with the debromination positions of the main products obtained from direct photolysis experiments. The direct photolysis of PBDEs has three potential reaction pathways including photodebromination, C-O bond photodissociation, and PBDFs formation. Taking a representative PBDE (i.e., BDE100) for example, the reaction activation energy of photodebromination is the smallest among the three direct photolysis reaction pathways.(5) The mechanisms of triplet-sensitized photolysis of sulfadiazine (SDZ) with four DOM analogues, i.e., fluorenone (FL), thioxanthone (TX),2-acetonaphthone (2-AN), and4-benzoylbenzoic acid (CBBP), and three metal ions (i.e., Mg2+, Ca2+, and Zn2+) effects were investigated by means of DFT. Results indicate that the triplet-sensitized photolysis mechanism of SDZ0(neutral form) with FL, TX, and2-AN is hydrogen transfer, and with CBBP is electron transfer along with proton transfer. The triplet-sensitized photolysis mechanism of SDZ-(anionic form) with FL, TX, and CBBP is electron transfer along with proton transfer, and with2-AN is hydrogen transfer. The photolysis product of both SDZ0and SDZ-is a sulfur dioxide extrusion product (4-(2-iminopyrimidine-1(2H)-yl)aniline), but the formation routs of the products for SDZ0and SDZ-are different. In addition, the metal ions promote the triplet-sensitized photolysis of SDZ0, but inhibit the triplet-sensitized photolysis of SDZ-...