Laser Flash Photolysis of 1,2-Diketopyracene and a Theoretical Study of the Phenolic Hydrogen Abstraction by the Triplet State of Cyclic α-Diketones

A photochemical and theoretical study of the triplet reactivity of furano- and pyrano-1,4-naphthoquionones towards tyrosine and tryptophan derivatives

The photochemical reactivity of the triplet state of pyrano- and furano-1,4-naphthoquinone derivatives (1 and 2) has been examined employing nanosecond laser flash photolysis. The quinone triplets were efficiently quenched by l-tryptophan methyl ester hydrochloride, l-tyrosine methyl ester hydrochloride, N-acetyl-l-tryptophan methyl ester and N-acetyl-l-tyrosine methyl ester, substituted phenols and indole (k q ∼109 L mol-1 s-1). For all these quenchers new transients were formed in the quenching process. These were assigned to the corresponding radical pairs that resulted from a coupled electron/proton transfer from the phenols, indole, amino acids, or their esters, to the excited state of the quinone. The proton coupled electron transfer (PCET) mechanism is supported by experimental rate constants, isotopic effects and theoretical calculations. The calculations revealed differences between the hydrogen abstraction reactions of phenol and indole substrates. For the latter, the calculations indicate that electron transfer and proton transfer occur as discrete steps.

Mechanism of Photoinduced Triplet Intermolecular Hydrogen Transfer between Cycloxydim and Chlorothalonil

The possible reaction mechanisms for the experimentally observed hydrogen transfer between the herbicide cycloxydim (CD) and the triplet fungicide chlorothalonil (CT) were identified with density functional theory (DFT) and time-dependent density function theory (TDDFT) computations. Excited energy transfer (EET) calculations indicate that reactants for intermolecular hydrogen transfer were formed via energy transfer from triplet CT to ground state CD. Three possible reaction pathways after EET were identified, and hydrogen transfer from the hydroxyl group on the cyclohexane ring of CD to CT exhibited the lowest energy barrier. Natural population analysis (NPA) along the reaction pathways has confirmed that the pathways involved either electron transfer induced proton transfer or coupled electron-proton transfer, leading to different potential energy profiles. Electrostatic potential (ESP) study substantiated the reaction mechanisms in different pathways. This study suggests an explanation for the accelerated photodegradation of CD by CT and provides a pipeline for future studies of photoinduced intermolecular hydrogen transfer.

Photodegradation mechanism of sulfonamides with excited triplet state dissolved organic matter: a case of sulfadiazine with 4-carboxybenzophenone as a proxy

Excited triplet states of dissolved organic matter ((3)DOM*) are important players for photodegradation sulfonamide antibiotics (SAs) in sunlit natural waters. However, the triplet-mediated reaction mechanism was poorly understood. In this study, we investigated the reaction adopting sulfadiazine as a representative SA and 4-carboxybenzophenone (CBBP)as a proxy of DOM. Results showed that the excited triplet state of CBBP ((3)CBBP*) is responsible for the photodegradation of sulfadiazine. The reaction of (3)CBBP* with substructure model compounds verified there are two reaction sites (amino-or sulfonyl-N atoms) of sulfadiazine. Density functional theory calculations were performed, which unveiled that electrons transfer from the N reaction sites to the carbonyl oxygen atom of (3)CBBP* moiety, followed by proton transfers, leading to the formation of sulfadiazine radicals. Laser flash photolysis experiments were performed to confirm the mechanism. Thus, this study identified that the photodegradation mechanism of SAs initiated by (3)DOM*, which is important for understanding the photochemical fate, predicting the photoproducts, and assessing the ecological risks of SAs in the aquatic environment.

Non-hydroxyl radical mediated photochemical processes for dye degradation

The UV/diketone processes can degrade dyes faster and more selectively than the UV/H₂O₂ process, due to their non-hydroxyl radical nature.

Assessment of theoretical methods for the study of hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation (IUPAC Technical Report)

Global climate change is a major concern as it leads to an increase in the average temperature of the earth’s atmosphere. The existence and persistence of some gaseous species in the atmosphere contribute to global warming. Experimental techniques are used to study the kinetics and degradation of global warming gases. However, quantum mechanical methods are also useful for the kinetic and radiative forcing study of global warming species and can precede experimental investigations. Research has also been targeted to develop more adapted procedures using ab initio and density functional theory (DFT) methods. This report provides a global perspective, in simplified manner, of the theoretical studies of the degradation of gas species in the atmosphere with an emphasis on the hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation. En route, the results obtained from these studies are analysed and compared with experimental data where available. Our analyses indicate that the theoretical predictions are in agreement with experimental findings but the predicted parameters are dependent on the method being used. Theoretical methods are used to predict the thermodynamic parameters of reactions, and, with relevance to this report, the global warming potential (GWP) index can also be calculated. This report can be useful for future investigations involving global warming gaseous species while providing suggestions on how computations can fill in data gaps when experimental data are unavailable.