Production of singlet oxygen atoms by photodissociation of oxywater

Unraveling the interaction between singlet state atomic oxygen O(1D) and water: toward the formation of oxywater and hydrogen peroxide

We performed high-level quantum mechanical calculations to explore the interaction of atomic oxygen in the ground triplet state, O(3P), and the excited singlet state, O(1D), with water. We reported the potential energy curves for a few lowest electronic states when an atomic oxygen approaches the oxygen of a water molecule. Our results predict the formation of a singlet oxywater species as the product of O(1D) and H2O which lies about 149.33 kJ mol-1 below the total energy of a singlet oxygen atom and a water molecule. Our calculations predict that an O(3P) atom interacting with a water molecule forms a triplet oxywater complex with a shallow minimum on the triplet potential energy surfaces. We examined the transition of the singlet state oxywater species to hydrogen peroxide through the unimolecular reaction pathway, a (1,2)-hydrogen shift. We reported the structural properties, vibrational frequencies, and dipole moments of oxywater species, the transition state, and hydrogen peroxide. We also reported the energy barrier for the transition, and we provided an estimate for the respective reaction rate constant. In addition, we investigated the impact of solvents on the reaction pathway using an implicit solvation model of water. We predict that a singlet state oxywater species has a longer lifetime in a water environment than in the gas phase.

The Boundary between Two Modes of Gas Evolution: Oscillatory (H2 and O2) and Conventional Redox (O2 Only), in the Hydrocarbon/H2O2/Cu(II)/CH3CN System

During the oxidation of hydrocarbons using hydrogen peroxide solutions, the evolution of gaseous oxygen is a side and undesirable process, in which the consumption of the oxidizer is not associated with the formation of target products. Therefore, no attention is paid to the systematic study of the chemical composition of the gas and the mechanisms of its formation. Filling this gap, the authors discovered a number of new, previously unidentified, interesting facts concerning both gas evolution and the oxidation of hydrocarbons. In a 33% H2O2/Cu2Cl4·2DMG/CH3CN system, where DMG is dimethylglyoxime (Butane-2,3-dione dioxime), and is at 50 °C, evidence of significant evolution of gaseous hydrogen, along with the evolution of gaseous oxygen was found. In the authors’ opinion, which requires additional verification, the ratio of gaseous hydrogen and oxygen in the discussed catalytic system can reach up to 1:1. The conditions in which only gaseous oxygen is formed are selected. Using a number of oxidizable hydrocarbons with the first adiabatic ionization potentials (AIPs) of a wide range of values, it was found that the first stage of such a process of evolving only gaseous oxygen was the single electron transfer from hydrogen peroxide molecules to trinuclear copper clusters with the formation, respectively, of hydrogen peroxide radical cations H2O2•+ and radical anions Cu3Cl5•− (AIP = 5 eV). When the conditions for the implementation of such a single electron transfer mechanism are exhausted, the channel of decomposition of hydrogen peroxide molecules into gaseous hydrogen and oxygen is switched on, which is accompanied by the transition of the system to an oscillatory mode of gas evolution. In some cases, the formation of additional amounts of gaseous products is provided by the catalytically activated decomposition of water molecules into hydrogen and oxygen after the complete consumption of hydrogen peroxide molecules in the reaction of gaseous oxygen evolution. The adiabatic electron affinity of various forms of copper molecules involved in chemical processes is calculated by the density functional theory method.

Molecular oxygen generation from the reaction of water cations with oxygen atoms

The oxywater cation (H₂OO⁺), previously shown to form barrierlessly in the gas phase from water cations and atomic oxygen, is proposed here potentially to possess a ²A″ ←⁴A″ excitation leading to the H₂⋯O₂ ⁺ complex. This complex could then easily decompose into molecular hydrogen and the molecular oxygen cation. The present quantum chemical study shows that the necessary electronic transition takes place in the range of 1.92 eV (645 nm), in the orange-red range of the visible and solar spectrum, and dissociation of the complex only requires 5.8 kcal/mol (0.25 eV). Such a process for the abiotic, gas phase formation of O₂ would only need to be photocatalyzed by visible wavelength photons. Hence, such a process could produce O₂ at the mesosphere/stratosphere boundary as climate change is driving more water into the upper atmosphere, in the comet 67P/Churyumov-Gerasimenko where surprisingly high levels of O₂ have been observed, or at gas-surface (ice) interfaces.