Review of Chemical Reactivity of Singlet Oxygen with Organic Fuels and Contaminants.

Singlet oxygen represents a form of reactive oxygen species (ROS), produced by electronic excitation of molecular triplet oxygen. In general, highly reactive oxygen-bearing molecules remain the backbone of diverse ground-breaking technologies, driving the waves of scientific development in environmental, biotechnology, materials, medical and defence sciences. Singlet oxygen has a relatively high energy of about 94 kJ/mol compared to the ground state molecular O2 and therefore initiates low-temperature oxidation of electron-rich hydrocarbons. Such reactivity of singlet oxygen has inspired a wide array of emerging applications in chemical, biochemical and combustion phenomena. This paper reviews the intrinsic properties of singlet oxygen, emphasising the physical aspects of its natural occurrences, production techniques, as well as chemical reactivity with organic fuels and contaminants. The review assembles critical scientific studies on the implications of singlet oxygen in initiating chemical reactions, identifying, and quantitating the consequential effects on combustion, fire safety, as well as on the low-temperature treatment of organic wastes and contaminants. Moreover, the content of this review appraises computational efforts, such as DFT quantum mechanical modelling, in developing mechanistic (i. e., both thermodynamic and kinetic) insights into the reaction of singlet oxygen with hydrocarbons.

Kinetics of Photo-Oxidation of Oxazole and its Substituents by Singlet Oxygen.

Oxazole has critical roles not only in heterocycle (bio)chemistry research, but also as the backbone of many active natural and medicinal species. These diverse and specialised functions can be attributed to the unique physicochemical properties of oxazole. This contribution investigates the reaction of oxazole and its derivatives with singlet oxygen, employing density functional theory DFT-B3LYP calculations. The absence of allylic hydrogen in oxazole eliminates the ene-mode addition of singlet oxygen to the aromatic ring. Therefore, the primary reaction pathway constitutes the [4 + 2]-cycloaddition of singlet oxygen to oxazole ring, favouring an energetically accessible corridor of 57 kJ/mol to produce imino-anhydride which is postulated to convert to triamide end-product in subsequent steps. The pseudo-first-order reaction rate for substituted oxazole (e.g., 4-methyl-2,5-diphenyloxazole, 1.14 × 106 M-1 s-1) appears slightly higher than that of unsubstituted oxazole (0.94 × 106 M-1 s-1) considering the same initial concentration of the species at 300 K, due to the electronic effect of the functional groups. The global reactivity descriptors have justified the relative influence of the functional groups along with their respective physiochemical properties.

Probing the Reactivity of Singlet Oxygen with Cyclic Monoterpenes.

Monoterpenes represent a class of hydrocarbons consisting of two isoprene units. Like many other terpenes, monoterpenes emerge mainly from vegetation, indicating their significance in both atmospheric chemistry and pharmaceutical and food industries. The atmospheric recycling of monoterpenes constitutes a major source of secondary organic aerosols. Therefore, this contribution focuses on the mechanism and kinetics of atmospheric oxidation of five dominant monoterpenes (i.e., limonene, α-pinene, ß-pinene, sabinene, and camphene) by singlet oxygen. The reactions are initiated via the ene-type addition of singlet oxygen (O2 1Δg) to the electron-rich double bond, progressing favorably through the concerted reaction mechanisms. The physical analyses of the frontier molecular orbitals agree well with the thermodynamic properties of the selected reagents, and the computed reaction rate parameters. The reactivity of monoterpenes with O2 1Δg follows the order of α-pinene > sabinene > limonene > ß-pinene > camphene, i.e., α-pinene and camphene retain the highest and lowest reactivity toward singlet oxygen, with rate expressions of k(T) (M-1 s-1) = 1.13 × 108 exp(-48(kJ)/RT(K)) and 6.93 × 108 exp(-139(kJ)/RT(K)), respectively. The effect of solvent on the primary reaction pathways triggers a slight reduction in energy, ranging between 12 and 34 kJ/mol.

Destruction of dioxin and furan pollutants via electrophilic attack of singlet oxygen.

Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) remain of particular concern owing to their extensive toxicity towards health and accumulation in the environment. Atmospheric oxidation (by ambient oxygen molecules) of this class of persistent environmental pollutants has little to no kinetic feasibility due to very sizable activation energies in the entrance channel. The current control measures involve energy-intensive source incineration of contaminated materials at high temperatures as high as 850 °C. This study finds an alternative low-energy approach of destroying dioxin-like compounds, proposing that advanced oxidation by highly reactive singlet oxygen (O21Δg, originated from chemical, surface-mediated and photochemical processes) can initiate low-temperature remediation of these pollutants. This contribution completes the first milestone in mapping out the mechanisms of the electrophilic addition of singlet oxygen to unsubstituted and chlorinated dibenzo-p-dioxin (DBD) and dibenzofuran (DBF) structures, according to density functional theory DFT-B3LYP method in conjunction with the 6-311+g(d,p) basis set, as well as energy refinements based on the approximate spin-projection scheme. The [2+2]-cycloaddition mechanism appears dominant for singlet oxidation of dibenzo-p-dioxin with a fitted rate constant of k(T) = 5.01 × 10-14 exp(-98000/RT). On the other hand, the addition of singlet oxygen to the aromatic ring of dibenzofuran primarily transpires via [4+2]-cycloaddition channel with a fitted rate constant of k(T) = 2.16 × 10-13 exp(-119000/RT). The results suggest that application of singlet oxygen can reduce the energy cost of recycling halogenated and flame retarded materials.

Reaction of phenol with singlet oxygen.

Photo-degradation of organic pollutants plays an important role in their removal from the environment. This study provides an experimental and theoretical account of the reaction of singlet oxygen O2(1Δg) with the biodegradable-resistant species of phenol in an aqueous medium. The experiments combine customised LED-photoreactors, high-performance liquid chromatography (HPLC), and electron paramagnetic resonance (EPR) imaging, employing rose bengal as a sensitiser. Guided by density functional theory (DFT) calculations at the M062X level, we report the mechanism of the reaction and its kinetic model. Addition of O2(1Δg) to the phenol molecule branches into two competitive 1,4-cycloaddition and ortho ene-type routes, yielding 2,3-dioxabicyclo[2.2.2]octa-5,7-dien-1-ol (i.e., 1,4-endoperoxide 1-hydroxy-2,5-cyclohexadiene) and 2-hydroperoxycyclohexa-3,5-dien-1-one, respectively. Unimolecular rearrangements of the 1,4-endoperoxide proceed in a facile exothermic reaction to form the only experimentally detected product, para-benzoquinone. EPR revealed the nature of the oxidation intermediates and corroborated the appearance of O2(1Δg) as the only active radical participating in the photosensitised reaction. Additional experiments excluded the formation of hydroxyl (HO˙), hydroperoxyl (HO2˙), and phenoxy intermediates. We detected for the first time the para-semibenzoquinone anion (PSBQ), supporting the reaction pathway leading to the formation of para-benzoquinone. Our experiments and the water-solvation model result in the overall reaction rates of kr-solvation = 1.21 × 104 M-1 s-1 and kr = 1.14 × 104 M-1 s-1, respectively. These results have practical application to quantify the degradation of phenol in wastewater treatment.

New Mechanistic Insights: Why Do Plants Produce Isoprene?

In this article, we argue that the primary role of isoprene is to remove the singlet delta oxygen (O2 1Δg) that forms inside plants by ultraviolet excitation rather than to provide heat protection or scavenge ozone, OH, or other reactive oxygen species (ROS) in the gas phase. By deploying a quantum chemical framework, we address for the first time the exact mode of isoprene reactions with O2 1Δg, the most prominent ROS that causes damage to leaves. Initial reactions of isoprene with O2 1Δg comprise its addition at the two terminal carbon atoms. The two primary open-shell adducts that appear in these reactions undergo 1,2-cycloaddition to generate methyl vinyl ketone and methacrolein, the sole products detected from in-house (i.e., inside of plants) oxidation of isoprene. Formation of other products, comprising the peroxy O-O bonds, is kinetically insignificant. Furthermore, these adducts are thermodynamically too unstable to diffuse outside of plants. Oxidation of isoprene with O2 1Δg does not produce new ROS (such as OH or HO2), supporting the well-documented role of isoprene as an effective ROS scavenger. Deploying a solvation model reduces the energy requirements for the primary pathways in the range of 10-56 kJ/mol. The present results indicate that plants attach significant value to the in-home protection against O2 1Δg by investing carbon and energy into the formation of isoprene, in spite of the appearance of the cytotoxic methyl vinyl ketone as one of the reaction products. (The same chemical species also form in unrelated gas-phase reactions involving isoprene and other ROS.) This finding explains the primary reason for the appearance of the dynamic biosphere-atmosphere exchange of methyl vinyl ketone.