title: Unveiling self-sensitized photodegradation pathways by DFT calculations: A case of sunscreen p-aminobenzoic acid.
abstract: Self-sensitized photodegradation has been observed for diverse aquatic organic pollutants. However, photodegradation pathways have not been clarified in previous experimental studies. Here, we attempted to probe self-sensitized photodegradation pathways of organic pollutants employing both photolytic experiments and density functional theory calculations. By performing photolytic experiments, we found that singlet state oxygen ((1)O2) play an essential role in photodegradation of a sunscreen p-aminobenzoic acid (PABA). PABA can photogenerate (1)O2 and react fast with (1)O2. We hypothesized that PABA underwent (1)O2 induced self-sensitized photodegradation. By calculating transition states, intermediates and reaction barriers, we found that (1)O2 can oxidize PABA through electrophilic attacks on the benzene ring to abstract one H atom of the amino group following a 1,3-addition mechanism or to induce decarboxylation. Either pathway produces a hydroperoxide. O-O bond cleavage of the hydroperoxides occurring at ground states or the lowest triplet excited states can produce phenoxyl radical precursors of 4-amino-3-hydroxybenzoic acid and 4-aminophenol, which are photodegradation products detected in experiments. Thus, a viable (1)O2 self-sensitized photodegradation mechanism was unveiled for PABA.
authors.affiliation: Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Science, Shenyang 110016, China.
authors.forename: Siyu
authors.initials: S
authors.lastname: Zhang
authors.affiliation: Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China. Electronic address: jwchen@dlut.edu.cn.
authors.forename: Jingwen
authors.initials: J
authors.lastname: Chen
authors.affiliation: Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Science, Shenyang 110016, China.
authors.forename: Qing
authors.initials: Q
authors.lastname: Zhao
authors.affiliation: Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China.
authors.forename: Qing
authors.initials: Q
authors.lastname: Xie
authors.affiliation: Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China.
authors.forename: Xiaoxuan
authors.initials: X
authors.lastname: Wei
mesh_terms: D010129:4-Aminobenzoic Acid / Q000737:chemistry*; D062385:Hydroxybenzoates / Q000737:chemistry; D010782:Photolysis; D026082:Singlet Oxygen / Q000737:chemistry; D013473:Sunscreening Agents / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000737:chemistry; D062366:para-Aminobenzoates / Q000737:chemistry
chemical_list: C489259:4-amino-3-hydroxybenzoic acid; D062385:Hydroxybenzoates; D013473:Sunscreening Agents; D014874:Water Pollutants, Chemical; D062366:para-Aminobenzoates; D026082:Singlet Oxygen; D010129:4-Aminobenzoic Acid
keywords: Density functional theory; Self-sensitized photodegradation mechanism; Singlet state oxygen; p-aminobenzoic acid