Theoretical Study On The Excited States Of C_nF_2n+1COOH （n=1-9）and The Photodegradation Mechanism Of CF₃COOH

In recent years, the sunlight or artificial light radiation has become an effective means to degradate the organic pollutants, and the mechanism of their degradation is closely related with the excited states. Nowdays, the time-dependent density functional theory(TDDFT) has become the most effective method to study the excited state properties of medium and even large moleculars after the development and improvement over nearly thirty years. On the one hand, a variety of photochemical behaviors of organic pollutants can be predicted and probed by the study of excited states, which has the inspiration for the development of pollutant photochemical treatments; On the other hand, to figure out all kinds of reactions of the active intermediates(such as free radicals, etc) that have produced during the reaction process may contribute to analyze the photochemical conversion of pollutants. The research work is divided into three parts:Trifluoroacetic acid(TFA, the lowest PFCAs homolog) comes mainly from the atmospheric degradation of Freon substitutes, and based on its photolysis experiments, its possible degradation pathways in the triplet state have been discussed by density functional theory and the microscopic mechanism of each reaction pathway has been elucidated, which is expected to provide certain theoretical basis for the development of effective photodegradation methods; CnF2n+1COOH(n=1-9) has been taken as the research object and their low occupied excited states have been calculated by TDDFT method and DFT method separately, and then their photolysis reactions have been explored; Finally, based on the importance of HOCO radicals generated by the photolysis of trifluoroacetic acid, their mutual reaction has been calculated and discussed in detail, which makes up for the lack of the research on the HOCO radical reaction and has a important theoretical and practical significance for combustion process and atmospheric chemistry. The main results are as follows:1.At B3LYP/6-311++G(3df,3pd) level, a detailed study of the trifluoroacetic acid’s photodegradation reaction has been carried out in the triplet state. The results are as follows:(1)Because of the coexistence of the triplet trifluoroacetic acid and its isomer, the C-C bond of two of them can dissociate simultaneously and trans-HOCO and cis-HOCO can be generated respectively.(2)From the potential energy curves of the ground and excited states of trifluoroacetic acid along C-O bond, the C-O bond of trifluoroacetic acid can dissociate in the triplet state with an exit channel barrier, which is in accord with the fact that the OH radical from the C-O bond cleavage has high translation energy. When the excitation wavelength is shorter,the resulting CF3CO decomposes into CF3+CO immediately.(3)The C-O bond cleaves generating CF3CO2+H after the isomerization of the triplet trifluoroacetic acid, and CF3CO2 is extremely unstable and decomposes into CF3+CO2 immediately, which shows that trifluoroacetic acid producing CF3+CO2+H eventually is a continuous three-body dissociation process.(4)The triplet CF2CO2 is generated during the elimination of HF after the isomerization of the triplet trifluoroacetic acid. From CF2CO2 to the product CF2+CO2 is a concerted process, that is, along with the bending vibration, CF2CO2 begins to deviate from the plane structure and then the C-C stretching vibration gradually gains energy eventually causing the C-C bond to break. The triplet CF2 can react with HF generating CF3H, which may be the cause that HF has not detected in the photodegradation of trifluoroacetic acid.2.The geometry optimization and frequency analysis of the singlet excited states and the triplet excited state have been performed at B3LYP/6-311++G(d,p) level for CnF2n+1COOH(n=1-9). CnF2n+1COOH(n=1-9) is excited to the S1 state, followed by intersystem crossing from S1 to T1. On the T1 potential energy surface, the α-C-C bond first break. For C5F11COOH,C6F13COOH and C7F15COOH, the γ-C-C bond will dissociate directly if they can be excited to the second excited state.3. On the basis of density functional theory,the reaction paths for HOCO and HOCO have been deeply studied. The reactants, intermediates, transition states, and products have been optimized at the B3LYP/6-311++G(d, p) level. IRC(intrinsic reaction coordinate) calculations have also been processed. The energies with zero point energy correction have been calculated at the CCSD(T)/cc-pVQZ level. Combining the experiments, the main channels of the reaction of HOCO with HOCO have been confirmed, which shows that the hydrogen abstraction from the cis-HOCO radical can be performed quickly by the self-interaction of HOCO radicals in the absence of other radicals or atoms, which indirectly indicates for the first time that the ground-state cis-HOCO radical is not decomposed into CO2+H by the tunneling effect. Moreover, the study shows that the self-interaction of HOCO radicals is very prone to not only hydrogen abstraction reaction but also hydroxyl abstraction reaction, but the probability of hydroxyl abstraction reaction is so low that the products are difficult to be detected.