Mechanistic Insight into the Reactivities of Aqueous-Phase Singlet Oxygen with Organic Compounds

Abstract

Singlet oxygen (1O2) is a selective reactive oxygen species that plays a key role for the fate of various organic compounds in the aquatic environment under sunlight irradiation, engineered water oxidation systems, atmospheric water droplets, and biomedical systems. While the initial rate-determining charge-transfer reaction mechanisms and kinetics of 1O2 have been studied extensively, no comprehensive studies have been performed to elucidate the reaction mechanisms with organic compounds that have various functional groups. In this study, we use density functional theory calculations to determine elementary reaction mechanisms with a wide variety of organic compounds. The theoretically calculated aqueous-phase free energies of activation of single electron transfer and 1O2 addition reactions are compared to the experimentally determined rate constants in the literature to determine linear free-energy relationships. The theoretically calculated free energies of activation for the groups of phenolates and phenols show excellent correlations with the Hammett constants that accept electron densities by through-resonance. The dominant elementary reaction mechanism is discussed for each group of compounds. As a practical implication, we demonstrate the fate of environmentally relevant organic compounds induced by photochemically produced intermediate species at different pH and evaluate the impact of predicting rate constants to the half-life.

Toward Selective Oxidation of Contaminants in Aqueous Systems

“…The widely applied chemical oxidants include hypochlorous acid (HOCl), chlorine dioxide (ClO 2 ), permanganate (MnO 4 – ), , ozone (O 3 ), , and hydroxyl radical (HO • ) produced in AOPs . In recent decades, SO 4 •– , , ferrate (FeO 4 2– ), singlet oxygen ( 1 O 2 ), , carbonate radical (CO 3 •– ), , and chlorine radical (Cl 2 •– ) have been proved to play a primary role in pollutant transformation under specified conditions. The reactivity is highly related to the reduction potential (Supporting Information (SI) Table S1).…”

“…The reactivity is highly related to the reduction potential (Supporting Information (SI) Table S1). Each oxidant shows preferred reaction pathways and kinetics toward different compounds, and most of them have been critically summarized in many delicate reviews. − ,,,− This section briefly introduces the reactivity of common oxidants and discusses their selectivity by comparing the kinetics of substrate degradation to that of the oxidant scavenging from the water matrices.…”

“…For example, O 3 adds to olefins or aromatic ring via Criegee mechanism, , featuring the formation of a five-member ring followed by cleavage at different sites. However, 1 O 2 may add to the CC bond through 1,2-addition, ene reaction, or 1,4-cycloaddition analogous to Diels–Alder reaction. ,, Moreover, oxidation of the heteroatoms (e.g., S, N, and P) by both O 3 and 1 O 2 normally generate oxygen transfer products. ,,,, …”

“…Besides the specific oxidation pathways, the selectivity of an oxidant is also reflected by the substrate dependent reactivity. So far, there are a few hundred rate constants with CO 3 •– , , Cl 2 •– , ,− O 3 , , SO 4 •– , ,− and 1 O 2 (many are determined in organic solvents), ,, and a few thousand rate constants with HO • available for various target compounds, ,,, whereas fewer number of rate constants concerning the oxidation of compounds by MnO 4 – , FeO 4 2– , , ClO 2 , and HOCl are documented. HO • is generally accepted to be nonselective, with the rate constants ranging from 10 8 –10 10 M –1 s –1 , while SO 4 •– shows a broad rate range from 10 5 –10 10 M –1 s –1 for different compounds .…”

“…In comparison, CO 3 •– , Cl 2 •– , O 3 , 1 O 2 , MnO 4 – , FeO 4 2– , ClO 2 , and HOCl are sensitive to electron-rich C, N, and S sites, and are thought to be more selective with varied rate constants from ∼0 to 10 9 M –1 s –1 . ,,,,,,, It is important to address that the target compound normally bears more than one functional group, in which the oxidation might occur with different rate constants and even via different mechanisms. QSAR has been developed to resolve the complex mechanisms and predict the bimolecular rate constants by establishing the suitable descriptors of structures, such as redox potential, Hammett constants, or energy of the highest occupied molecular orbitals ( E HOMO ), and the lowest unoccupied molecular orbitals ( E LUMO ) with the reactivity. ,,,, …”