The nature of molecular oxygen binding and activation in Mn-O2 complex

Non-empirical calculations of CASSCF energies, electric dipole moments, Einstein coefficients, matrix elements of the operator of spin-orbital interaction between states of different multiplicity in a model complex ^6,8[Mn-O₂] of C _2v symmetry have been made in 3-21G, 6-31G, 6-31G** basis sets. The crosssections of the potential energy surface (PES) of the ground and excited states were built. It is found that oxygen bonding to manganese is possible when excited atoms of manganese collide with molecular oxygen, singlet oxygen with Mn[⁶ S _5/2] atoms, or in a close contact O₂[X³Σ _g ^? ] + Mn[⁶ S _5/2] and is determined by charge transfer states ^6,8CTS(Mn⁺O ₂ ^? ). Mechanisms of singlet oxygen activation/deactivation are determined by a considerably increased probability of electric dipole transitions b ¹Σ _g ⁺ ?a ¹Δ_g, a ¹Δ_g?X³Σ _g ^? , b ¹Σ _g ⁺ ?X³Σ _g ^? induced in oxygen in the collision process.     

Singlet Sigma: The "Other" Singlet Oxygen in Solution

The lowest excited electronic state of molecular oxygen, O₂(a^1-DL_g), is often called simply singlet oxygen. This singlet delta state is an acknowledged and well-studied intermediate in many solution-phase photosystems. However, the second excited electronic state of oxygen, O₂(b¹δ_g+), is also a singlet. It has recently become possible to monitor this singlet sigma state in solution, which, in combination with studies of the singlet delta state, contributes to a better understanding of a variety of general problems in chemistry.     

Solute re-encounter probabilities from dissociative oxciplex relaxation

Measurements of the quantum yield of self-sensitized 1,3-diphenylisobenzofuran peroxidation as a function of dissolved oxygen of added azulene concentrations indicate that oxygen quenching of the sensitizer singlet state produces both triplet and ground states of the sensitizer in addition to O₂(¹Δ_g) and O₂(³Σ^?_g). This partitioning of quenching products may be due to the competitive relaxation of the initially formed complex (oxciplex), or to sequential relaxation of different oxciplex states in which symmetry and spin barriers are negotiated by complex dissociation and re-encounter of the solute pair in the required configuration. The latter interpretation provides re-encounter probabilities for the processes M(T₁) + O₂(¹Δ_g) → M(T₁) + O₂(³Σ^?_g) and M(T₁) + O₂(³Σ^?_g) → M(S_o) + O₂(¹Δ_g) from which estimated rate constants are compatible with theoretical expectation.     

Excited state formation in the H + O2 system

This paper describes emission in the near-infrared, produced in a mixture containing atomic hydrogen (or deuterium) and molecular oxygen. The bands observed are ascribed to the formation of O₂(¹Σ_g⁺), of electronically and vibrationally excited HO₂, and perhaps O₂(¹Δ_g), in the reaction system. Possible excitation mechanisms are presented.     

Anisotropy parameters for molecules excited by electron impact

Rotationally resolved excitation of the a¹Δ_g and b¹∑ _g ⁺ states of O₂ is considered. Results for the relevant state multipoles are given. The anisotropy produced by electron impact is visualised by figures of the angular distribution of the molecular axes in the excited states.     

Relaxation mechanism of the 1Δg state of liquid O2

Collisional deactivation of the first excited electronic ¹Δ_g(υ = 0) state of O₂ involves intersystem crossing to higher vibrational levels (υ < 5) of the electronic ground state ³Σ^?_g. It is followed by rapid vibrational-vibrational energy exchange which populates the first excited ³Σ^?_g(υ = 1) vibrational level. The suggested relaxation mechanism is supported by experimental results on the time dependence of the populations of the ¹Δ_g(υ = 0) and ³Σ^?_g(υ = 1) states in liquid natural O₂ and ¹⁸O₂.     

Generation of high O2 (a1Δg) concentrations in O2/H2 mixtures

The effect of hydrogen on the concentration of singlet oxygen in the a¹Δ_g and b¹Σ states, generated from a microwave discharge in O₂ and in an O₂/Ar mixture, was studied in flow reactors. The addition of hydrogen, in a range of 0.01–1 of concentration of the O₂, increased the yields of singlet oxygen by factor of 5–20. In addition to the higher O₂ (a and b) concentrations, the addition of hydrogen removed the usual NO₂ fluorescence, making observation of the O₂(b → X) transition at 762 nm much easier in the flow reactor. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 12–17, 2006     

Oxygen Quenching of nπ* Triplet Phenyl Ketones: Local Excitation and Local Deactivation

We have studied the charge‐transfer‐induced deactivation of nπ* excited triplet states of benzophenone derivatives by O₂(³Σ), and the charge‐transfer‐induced deactivation of O₂(¹Δ_g) by ground‐state benzophenone derivatives in CH₂Cl₂ and CCl₄. The rate constants for both processes are described by Marcus electron‐transfer theory, and are compared with the respective data for a series of biphenyl and naphthalene derivatives, the triplet states of which have ππ* configuration. The results demonstrate that deactivation of the locally excited nπ* triplets occurs by local charge‐transfer and non‐charge‐transfer interactions of the oxygen molecule with the ketone carbonyl group. Relatively large intramolecular reorganization energies show that this quenching process involves large geometry changes in the benzophenone molecule, which are related to favorable Franck‐Condon factors for the deactivation of ketone‐oxygen complexes to the ground‐state molecules. This leads to large rate constants in the triplet channel, which are responsible for the low efficiencies of O₂(¹Δ_g) formation observed with nπ* excited ketones. Compared with the deactivation of ππ* triplets, the non‐charge‐transfer process is largely enhanced, and charge‐transfer interactions are less important. The deactivation of singlet oxygen by ground‐state benzophenone derivatives proceeds via interactions of O₂(¹Δ_g) with the Ph rings.     

Production of electronically excited NF by the reaction of fluorine atoms with HN3

Electronically excited NF in both the a¹Δ and b ¹Σ⁺ states hasbeen observed from the reaction of fluorine atoms with HN₃. The results suggest that fluorine atoms first abstract the hydrogen atom from HN₃, then react with the remaining N₃ to form NF^≠(a¹Δ). NF^*(b¹Σ⁺) is produced by a subsequent energy pooling reaction between NF^≠(a¹Δ) and vibrationally excited HF. The rate of the F + N₃ reaction is estimated to be ≈ 10¹² and ³ mole^?1 s^?1.     

Kinetics of Active Oxygen Species with Implications for Atmospheric Ozone Chemistry

Photochemical processes involving singlet oxygen (O₂(a¹Δ)), oxygen atoms_, and ozone are critical in determining atmospheric ozone concentrations. Here we report on kinetic measurements and modeling that examine the importance of the reactions of vibrationally excited ozone. Oxygen atoms and O₂(a¹Δ) were produced by UV laser photolysis of ozone. Time‐resolved absorption spectroscopy was used for O₃ concentration measurements. It was found that vibrationally excited ozone formed by O + O₂ + M → O₃(ν) + M recombination reacts effectively with O₂(a¹Δ) and O atoms. The reaction O₃(υ) + O₂(a¹Δ) → O + 2O₂ results in a reduction of the ozone recovery rate due to O atom regeneration, whereas the reaction O₃(υ) + O → 2O₂ removes two odd oxygen species, resulting in incomplete ozone recovery. The possible impact of these reactions on the atmospheric O₂(a¹Δ) and O₃ budgets at altitudes in the range of 80–100 km is considered.     

Radiative Transitions of Singlet Oxygen: New Tools, New Techniques and New Interpretations

The near-IR phosphorescence of singlet delta oxygen, O₂(a¹Δg), has provided a wealth of information since it was first observed in solution-phase systems. The techniques employed and the quality of the data obtained have unproved significantly over the years that, in turn, presently makes it possible to address a wide variety of problems using both steady-state and time-resolved measurements. The development of spectroscopic methods to monitor other transitions in oxygen, specifically those that involve the singlet sigma state, O₂(b¹σ_g⁺), and the incorporation of high-level computational methods provides access to an even broader range of fundamental issues. The expertise presently available to monitor radiative transitions in oxygen, coupled with the current understanding of the effect of solvent on these transitions as achieved through state-of-the-art theoretical modeling makes it possible to consider the next step forward: the incorporation of spatial resolution and the construction of the singlet oxygen microscope.     

Intracellular Modulation of Excited‐State Dynamics in a Chromophore Dyad: Differential Enhancement of Photocytotoxicity Targeting Cancer Cells

The photosensitized generation of reactive oxygen species, and particularly of singlet oxygen [O₂(a¹Δ_g)], is the essence of photodynamic action exploited in photodynamic therapy. The ability to switch singlet oxygen generation on/off would be highly valuable, especially when it is linked to a cancer‐related cellular parameter. Building on recent findings related to intersystem crossing efficiency, we designed a dimeric BODIPY dye with reduced symmetry, which is ineffective as a photosensitizer unless it is activated by a reaction with intracellular glutathione (GSH). The reaction alters the properties of both the ground and excited states, consequently enabling the efficient generation of singlet oxygen. Remarkably, the designed photosensitizer can discriminate between different concentrations of GSH in normal and cancer cells and thus remains inefficient as a photosensitizer inside a normal cell while being transformed into a lethal singlet oxygen source in cancer cells. This is the first demonstration of such a difference in the intracellular activity of a photosensitizer.     

Uric Acid: A Less‐than‐Perfect Probe for Singlet Oxygen

Uric acid and/or its monoanion has long been used as chemical‐trapping agents to demonstrate the presence of singlet oxygen, O₂(a¹Δ_g), in aqueous systems. “Oxidative bleaching” of uric acid, generally monitored through changes in the uric acid absorption spectrum, is often used in support of claims for the intermediacy of O₂(a¹Δ_g). The bleaching of uric acid has also been used to quantify photosensitized O₂(a¹Δ_g) yields in selected systems. Unfortunately, experiments performed to these ends often neglect processes and phenomena that can influence the results obtained. For the present study, we experimentally examined the behavior of uric acid under a variety of conditions relevant to the photoinitiated creation and subsequent removal of O₂(a¹Δ_g). Although the oxidative destruction of uric acid can indeed be a useful tool in some cases, we conclude that caution must be exercised such as not to incorrectly interpret the data obtained.     

Non-exponential decay of the fluorescence from the 1Δg state in liquid oxygen at high excitation intensities

The ¹Δ_g state of liquid oxygen (natural O₂ and ¹⁸O₂) was excited by intense Nd: YAG laser pulses. The observed fluorescence decay is non-exponential and depends on pump laser intensity. Various decay channels of the ¹Δ_g state are discussed. The energy pooling reaction ¹Δ_g + ¹Δ_g → ¹Σ⁺_g + ³Σ^?_g is verified experimentally. Values f constant of liquid natural O₂ and ¹⁸O₂ are given.     

Collisional deactivation of O2(1Σg+)

Relaxation rates for O₂(¹Σ_g⁺) by nonradiative pathways have been determined using the fast-flow technique. O₂(¹Σ_g⁺) is formed from O₂(¹Δ_g) by an energy pooling process. O₂(¹Δ_g) is generated by passing purified oxygen through a microwave discharge. Oxygen atoms are removed by distilling mercury vapor through the discharge zone. It has been observed that the wall loss rate for O₂(¹Σ_g⁺) decreases with increasing pressure of oxygen and thus appears to be diffusion controlled. Quenching rate constants for O₂, N₂, and He have been determined and found to be (1.5 ± 0.1) × 10⁴, (1.0 ± 0.05) × 10⁶ and (1.2 ± 0.1) × 10⁵ l./mol·sec, respectively.     

Oxidation of 7-ethyl-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone (echinochrome A) by atmospheric oxygen 1. Structure of dehydroechinochrome

The nondestructive oxidation of 7-ethyl-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone by atmospheric oxygen in the ground (triplet ³Σ_g^–) and excited (singlet ¹Δ_g) states in different solvents (acetone, dioxane, ethanol, aqueous ethanol, water) at room temperature involves the initial formation of 7-ethyl-5,6,8-trihydroxy-2,3-dioxo-2,3-dihydro-1,4-naphthoquinone (dehydroechinochrome) accompanied by the release of an H₂O₂ molecule into the reaction medium. Dehydroechinochrome, being highly susceptible to hydration, successively reacts with two H₂O molecules to form 7-ethyl-2,2,3,3,5,6,8-heptahydroxy-2,3-dihydro- 1,4-naphthoquinone as the relatively stable final product. In this form, it can be isolated from the mixture of reaction products.     

Electronically excited oxygen states in exoemission and heterogeneous catalytic oxidation reactions

The role played by electronically excited oxygen in exoemission and heterogeneous catalysis is considered. The results obtained in a study of thermally stimulated exoemission from the surface of Al₂O₃ and SiO₂ are compared with data of temperature-programmed desorption of singlet oxygen O₂(¹Δ_g) from the surface of Al₂O₃ and HZSM-5 zeolites with different SiO₂/Al₂O₃ ratios. The role played by electronically excited oxygen states in heterogeneous catalysis is discussed on the basis of our own and literature data. Thermally stimulated exoemission after the action of an electron flow is considered taking into account electron-stimulated desorption and the available data on electron bombardment in catalysis.     

Yields of singlet nitrenes from halogen atom—azide molecule reactions

Chemiluminescence from the a¹Δ and b¹Σ⁺ excited electronic states of nitrogen halide diatomics is observed when HN₃ is allowed to react with mixtures of halogen atoms in a discharge-flow apparatus. Excited NF (a¹Δ) is produced by the F + HN₃ reaction, and NCl (a¹Δ, b¹Σ⁺) and NBr (a¹Δ, b¹Σ⁺) are produced by the F, Cl, + HN₃ and F, Br + HN₃ reactions, respectively. In the low-density limit, the yield of NF(a¹Δ) was found to be near unity. The yields of the b¹Σ⁺ states of NCl and NBr were determined to have a lower limit of ca. 10%. A number of results from these experiments, including direct observation of N₃ radicals in the flow, support a hypothetical mechanism in which N₃ acts as an intermediate. A second possible mechanism proceeding via an HNF intermediate cannot be ruled out.     

The photodissociation of vibrationally excited ozone in the upper atmosphere

The photodissociation of vibrationally excited O₃ in the sunlit mesosphere is investigated. Dissociation rate coefficients for specific vibrationally excited states of O₃ are calculated for the conditions of an overhead sun. Possible vibrational enhancements of the O(¹D) and O₂(a¹Δ_g) production rates are assessed. It is shown that such enhancements should make only minor contributions to the daytime production of these species in the mesosphere and lower thermosphere.     

Singlet oxygen luminescence as a probe of the stilbene triplet surface

The quenching of stilbene triplet by oxygen in benzene produces singlet oxygen, O₂(¹Δ_g), with an efficiency of 18±5%. This value, together with the corresponding rate constant, shows that the O₂(¹Δ_g) produced cannot be the consequence of exclusive oxygen quenching of a low concentration of planar trans-triplet in rapid equilibrium with the perpendicular form.