Oxygen Dianions Research Articles

Dynamic d-symmetry Bose condensate of a planar-large-bipolaron liquid in cuprate superconductors

Planar-large-bipolarons can form if the ratio of the surrounding mediums’ static to high-frequency dielectric constants is especially large, ε0/ε∞ >> 2. A large-bipolaron in p-doped La2CuO4 is modelled as two electrons being removed from the out-of-plane orbitals of four oxygen ions circumscribed by four copper ions of a CuO2 layer. These oxygen dianions relax inwardly as they donate electrons to the surrounding outwardly relaxing copper cations. This charge transfer generates the strong in-plane electron–lattice interaction needed to stabilise a large-bipolaron with respect to decomposing into polarons. The lowest-energy radial in-plane optic vibration of a large-bipolaron’s four core oxygen ions with their associated electronic charges has d-symmetry. Electronic relaxation in response to multiple large-bipolarons’ atomic vibrations lowers their frequencies to generate a phonon-mediated attraction among them which fosters their condensation into a liquid. This liquid features distinctive transport and optical properties. A large-bipolaron liquid’s superconductivity can result when it undergoes a Bose condensation yielding macroscopic occupation of its ground state. The synchronised vibrations of large-bipolarons’ core-oxygen ions with their electronic charges generate this Bose condensate’s dynamic global d-symmetry.

Native structure of photosystem II at 1.95Å resolution viewed by femtosecond X-ray pulses.

Photosynthesis converts light energy into biologically useful chemical energy vital to life on Earth. The initial reaction of photosynthesis takes place in photosystem II (PSII), a 700-kilodalton homodimeric membrane protein complex that catalyses photo-oxidation of water into dioxygen through an S-state cycle of the oxygen evolving complex (OEC). The structure of PSII has been solved by X-ray diffraction (XRD) at 1.9 ångström resolution, which revealed that the OEC is a Mn4CaO5-cluster coordinated by a well defined protein environment. However, extended X-ray absorption fine structure (EXAFS) studies showed that the manganese cations in the OEC are easily reduced by X-ray irradiation, and slight differences were found in the Mn-Mn distances determined by XRD, EXAFS and theoretical studies. Here we report a 'radiation-damage-free' structure of PSII from Thermosynechococcus vulcanus in the S1 state at a resolution of 1.95 ångströms using femtosecond X-ray pulses of the SPring-8 ångström compact free-electron laser (SACLA) and hundreds of large, highly isomorphous PSII crystals. Compared with the structure from XRD, the OEC in the X-ray free electron laser structure has Mn-Mn distances that are shorter by 0.1-0.2 ångströms. The valences of each manganese atom were tentatively assigned as Mn1D(III), Mn2C(IV), Mn3B(IV) and Mn4A(III), based on the average Mn-ligand distances and analysis of the Jahn-Teller axis on Mn(III). One of the oxo-bridged oxygens, O5, has significantly longer distances to Mn than do the other oxo-oxygen atoms, suggesting that O5 is a hydroxide ion instead of a normal oxygen dianion and therefore may serve as one of the substrate oxygen atoms. These findings provide a structural basis for the mechanism of oxygen evolution, and we expect that this structure will provide a blueprint for the design of artificial catalysts for water oxidation.

Theory of chemical bonds in metalloenzymes XIX: labile manganese oxygen bonds of the CaMn4O5 cluster in oxygen evolving complex of photosystem II

Spin polarisation effects of labile manganese–oxygen bonds in the X-ray diffraction structure of the oxygen-evolving complex (OEC) of photosystem II (PSII) at 1.9 Å resolution have been investigated by the UB3LYP computations on the basis of three different theoretical models with and without hydrogen bonds: quantum-mechanical (QM) Model I, QM(Model II)/MM and QM Model III. The spin densities on the manganese and oxygen atoms of the CaMn4O5 cluster revealed by these computations have elucidated internal, semi-internal and external reductions of high-valent manganese ions in the CaMn4O5 cluster in OEC of PSII. The internal reduction of Mn(IV) ions by the back charge transfer from oxygen dianions is remarkable in the small QM Model I, whereas it is significantly reduced in the case of more realistic QM Model III including hydrogen bonding stabilisations of oxygen dianions. However, semi-internal reduction of the CaMn4O5 cluster with remote amino acid residues such as Asp61 anion occurs even in QM Model III, indicating the necessity of large QM parts for redox-active systems such as OEC of PSII. The computational results have clearly demonstrated important roles of confinement effects of the CaMn4O5 cluster with labile Mn–O bonds with protein. These computational results have been applied to molecular design of artificial robust catalysts for water oxidation by use of sunlight.

Theory of chemical bonds in metalloenzymes XVI. Oxygen activation by high-valent transition metal ions in native and artificial systems

Activation mechanisms of oxygen dianion and hydroxide by high-valent transition metal ions for the oxygen–oxygen (O–O) bond formation of water splitting reaction have been investigated on the theoretical grounds, together with experimental results. First of all, broken-symmetry MO formulations are revisited to elucidate the instability of the dπ–pπ bond in high-valent metal–oxo M(X)O (M=Mn, Fe, Ru, etc.; X=IV, V) systems that react with hydroxide anion (or radical) or water to afford hydroperoxide anion (or peroxide). The triplet instability of these bonds entails strong or intermediate diradical characters: •M(X−1)O• and ••M(X−2)O••; the broken-symmetry (BS) molecular orbitals (MO) resulted from strong electron correlation, leading to the concept of electron localizations and local spins. As a continuation of these theoretical results, the BS MO interaction diagrams, namely orbital and spin correlation diagrams, for one-electron and electron-pair transfer mechanisms for the O–O bond formation have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of mechanisms of oxygenation reactions by p450, methane mono- oxygenase (MMO) and homolytic radical coupling mechanisms by oxygen evolving complex (OEC) of PSII. The spin alignments in high-valent M(X)O systems are found directly corresponding to possible mechanisms of the O–O bond formation between Mn(X)O and hydroxide (OH) anion via one-electron, electron-pair transfer and their superposed (chameleon) processes. The broken-symmetry (BS) UB3LYP calculations of the model systems have been performed to confirm these mechanisms for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms and orbital and spin correlation diagrams. Implications of the theoretical results are discussed in relation to three scenarios of the O–O bond formation for water oxidation: (a) HO–OH, (b) OOH and (c) O–O, and water oxidation of OEC of PSII.

Theoretical insight in to hydrogen-bonding networks and proton wire for the CaMn4O5 cluster of photosystem II. Elongation of Mn–Mn distances with hydrogen bonds

Quantum mechanical (QM) and QM/molecular mechanics (MM) calculations of three different cluster models have been performed to shed light on hydrogen bonding networks and proton wires for proton release pathways (PRP) of water oxidation reactions in the oxygen evolving complex (OEC) of photosystem II (PSII). Positions of all the hydrogen atoms in an extended QM Model III including the second coordination sphere for the active-site CaMn4O5 complex of OEC of PSII have been optimized assuming the geometry of heavy atoms determined by the recent high-resolution X-ray diffraction (XRD) experiment of PSII refined to 1.9 A resolution. Full geometry optimizations of the first coordination sphere model (QM Model I) embedded in the Model III and QM (QM Model I plus seven water molecules, namely QM Model II)/MM models, together with full QM Model III, have also been conducted to elucidate confinement effects for geometrical parameters of the CaMn4O5 cluster by proteins. Computational results by these methods have elucidated the O⋯O(N), O⋯H distances and O(N)–H⋯O angles for hydrogen bonds in proton release paths (PRP) I and II that construct a proton wire from Asp61 toward His190. The hydrogen-bonding structures revealed have also been examined in relation to the possibilities of protonation of bridge oxygen dianions within the CaMn4O5 cluster. The optimized inter-atomic distances by QM Models I and III, together with QM(Model II)/MM, have elucidated the elongation of the Mn–Mn distances with hydrogen bonds and variations of the Mnd–O(5) length with confinement effects by protein. Implications of the computational results are discussed in relation to the available EXAFS experiments, and internal, semi-internal and external reductions of Mn ions and long Mn–Mn distances of the high-resolution (SP8) XRD, and rational design of artificial catalysts for water oxidation that are current topics in the field of OEC of PSII.

Synthesis, Structure and Raman Data of the New Binuclear Zirconium Complex [Ph4P]2[(ZrCl4Py)2O], with a Zr‐O‐Zr Bridge

AbstractThe new Zirconium(IV) coordination compound [Ph4P]2[(ZrCl4Py)2O] (Ph = phenyl, Py = pyridine) was synthesized by dissolving ZrCl4, [Ph4P]Cl and a stoichiometric amount of NaOH/Na mixture in pyridine or pyridine/organic solvent mixtures. The title phase was obtained as colourless crystals. The crystal structure of [Ph4P]2[(ZrCl4Py)2O] was determined. It crystallizes monoclinic, P21/c, Z = 4, a = 13.412(2), b = 13.461(2), c = 16.442(3) Å, β = 102.72(1)°. The structure consists of isolated tetraphenylphosphonium cations and [(ZrCl4Py)2O]2− complex anions. The centrosymmetric complex anion contains a linear Zr–O–Zr bridge. Each Zr atom is coordinated by one oxygen dianion, the N atom of one pyridine ring and four chloro ligands in a distorted octahedral geometry. The Raman spectrum of [Ph4P]2[(ZrCl4Py)2O] is also reported. Most of the observed frequencies can be assigned to vibrations of the tetraphenylphosphonium cations and the pyridine rings.

Small silicon–oxygen dianions in the gas phase

Cs + sputtering of a silicon target with simultaneous exposure of the Si surface to O 2 gas has produced small silicon–oxygen dianions Si n O 2 n + 1 2 - (2 ⩽ n ⩽ 8) as well as Si 5 O 10 2 - . These dianions were observed by mass spectrometry for ion flight times of a few 10 −5 s. The two smallest species, Si 2 O 5 2 - and Si 3 O 7 2 - , have been identified unambiguously by their isotopic abundances. Ab initio electronic structure calculations are used to study the geometrical structure of the three most abundant species Si n O 2 n + 1 2 - ( n = 3, 4, 5), and for all three dianions several stable isomers are found. The structural motifs of the low-energy isomers are analyzed, and include in particular structures with trivalent oxygen atoms akin to impurities in solid silicon.

Sputtered gas-phase dianions detected by high-sensitivity mass spectrometry

The detection of small doubly-charged molecular anions by means of highly sensitive mass spectrometry is discussed. The production of these gas-phase dianions is accomplished by sputtering the specimen with Cs + ions with an energy of a few keV. It is demonstrated that dianions can be detected most easily when the molecular ion has an odd total mass; then, the dianions will show up at half-integral mass numbers in the mass spectrum. In addition, the agreement of the relative abundances of several isotopomers of a dianion with the nominal isotopic pattern corroborates the identification of a dianionic species in the mass spectrum. These features are exemplified by monitoring mixed silicon–oxygen dianions of the general form Si n O 2 n + 1 2 - (with 2 ⩽ n ⩽ 8) in a low-energy mass spectrometer. They were formed by sputtering a silicon wafer at an elevated oxygen partial pressure in the vicinity of the sample’s surface. The flight time through the mass spectrometer of ∼15–30 μs establishes a lower limit with respect to the intrinsic lifetimes of these dianions. Emission energy spectra of various singly- and doubly-charged ions illustrate the occurrence of fragmentation processes. The yields of the doubly- and singly-charged mixed silicon–oxygen anions increase with the ratio of the O 2 arrival rate to the Cs + flux density, but tend to saturate when this ratio approaches unity. The benefits of high-energy accelerator mass spectrometry in dianion detection are illustrated for LiF 3 2 - and CaF 4 2 - .

Observation of small gas-phase metal–oxygen dianions

Novel small metal–oxygen dianions have been produced in the gas phase by sputtering of metallic targets with simultaneous oxygen flooding. These exotic species were observed in a mass spectrometer for ion flight times of >10 μs. Both Cs + bombardment of an elemental Be metal surface and of a copper-based Cu(Be) alloy foil, with exposure of these sputtered surfaces to O 2 gas, produced the novel dianions of Be n - 1 O n 2 - with n = 4, 6. Be 3 O 4 2 - at m/ z 45.5 and 46.5 was unambiguously identified by its isotopic abundance. In addition, CuBe 2 O 4 2 - at m/ z 72.5 and 73.5 was detected for O 2 flooding of Cs + sputtered Cu(Be).

Synthesis of Strontium and Barium Bis{tris[(trimethylsilyl)methyl]zincates} via the Transmetalation of Bis[(trimethylsilyl)methyl]zinc

The transmetalation of bis[(trimethylsilyl)methyl]zinc in THF with distilled strontium and barium yielded bis(tetrahydrofuran)strontium and tetrakis(tetrahydrofuran)barium bis{tris[(trimethylsilyl)methyl]zincates} (1 and 2, respectively). The reaction of barium with Zn(CH2SiMe3)2 in toluene gave bis(η6-toluene)barium bis{tris[(trimethylsilyl)methyl]zincate} (3) with rather short Ba−C bond lengths of 3.028(5) and 3.055(5) Å. All of these trialkylzincate anions coordinate as bidentate ligands with M−C−Zn three-center two-electron bonds. When carried out in the nondonor solvent heptane, the transmetalation reaction yielded barium bis{tris[(trimethylsilyl)methyl]zincate} (4), which reacts with THF to give the complex 2 and a small amount of [(THF)3Ba]2(μ4-O)2[(Me3SiCH2ZnCH2)2SiMe2]2 (5). The central moiety is a centrosymmetric Ba2O2 cycle with Ba−O distances of 2.514(2) and 2.570(3) Å. The oxygen dianion is surrounded, distorted tetrahedrally by two barium and two zinc atoms.

Syntheses and crystal structures of three indium pyrazolyl complexes two of which contain novel bridging features

The complexes [(HB(Pz*)3)In(µ-Pz*)2(µ-OH)In((Pz*)3BH)]I·2MeCN 1, [In(HB(Pz*)3)I2(HPz*)]·0.5C6H12 2, and [In2(Pz**)4I2ONa2(THF)2] 3, where HPz* is 3,5-dimethylpyrazole and HPz** is 3,5-di-tert-butylpyrazole, have been prepared and their crystal structures determined. The presence in 1 of bridging Pz* and in 2 of monodentate HPz* given that this was not a starting material is attributed to the moisture sensitivity of tris(pyrazolyl)borates. The indium ions in 1 are bridged by two Pz* groups and an hydroxide ion, whereas in 3 the only bridge directly between the indium ions is an oxygen dianion. The four-co-ordinate donor set of the indium ion in 3 is formed by the bridging oxygen dianion, an iodide ion and the nitrogens of two Pz** groups. The remaining nitrogens of the Pz** co-ordinate to a sodium ion, which itself effectively bridges Pz** groups on adjacent indium ions.

Primary cathodic steps of electrogenerated chemiluminescence of lanthanide(III) chelates at oxide-covered aluminum electrodes in aqueous solution

The primary processes occurring at cathodically polarized oxide-covered aluminum electrode are discussed in detail. It is pointed out that more energetic cathodic processes can be induced in aqueous media at thin insulating film-coated electrodes than at any semiconductor or active metal electrode. It is proposed that tunnel emission of hot electrons with energies well above the level of the conduction band edge of water occur, and the thermalization and solvation of the emitted electrons can result in generation of hydrated electrons. The cathodically pulse-polarized oxide-covered aluminum also generates a strong oxidant (or oxidants) at the oxide/electrolyte interface, and it is proposed that this species is the hydroxyl radical which is generated either by cathodic high field-induced ejection of self-trapped holes as oxygen dianions (i.e. oxide radical ions) into the electrolyte solution, or by the action of anion vacancies and/or F +-centers as the primary oxidants capable of oxidizing hydroxide ions or the hydroxyl groups of the hydroxylated surface on the oxide film. These radicals, hydrated electrons/hydroxyl radicals, can act as mediating reductants/oxidants in reduction/oxidation of solutes. The formation of the primary species is monitored by electrochemiluminophores which cannot be cathodically excited at active metal electrodes in fully aqueous solutions, but which can be chemically excited in aqueous media in the simultaneous presence of highly reducing and highly oxidizing radicals.

Doubly-charged positive ions following loss of oxygen dianion under conditions of field desorption mass spectrometry

A number of cyclic aromatic anhydrides have been found to exhibit doubly-charged positive ions in their field desorption mass spectrum which correspond to the formal loss of oxygen dianion followed by clustering with a neutral molecule. Admixtures of two anhydrides, which both exhibit this phenomenon, produce heterogeneous doubly-charged cluster ions. Admixture with anhydrides which do not show the effect, produces only homogeneous clusters at reduced intensity.

Multiple Charge‐Transfer Reactions in Zirconia Electrolytic Cells: NO x Reduction on Platinum

This paper investigates the electrochemical activity of nitrogen, , and on platinum paste electrodes in zirconia oxygen sensors. Open‐circuit voltages and steady‐state current‐voltage characteristics were measured at 600°–800°C as a function of the partial pressures of oxygen, nitrogen, and nitrogen oxides. The open‐circuit voltage and the current density both increased monotonically with the and pressures in the sensor feed stream; both were most sensitive to the concentration. These data are well described by a reaction network in which two parallel charge‐transfer reactions occur on platinum: oxygen reduction to oxygen dianions; and reduction to and oxygen dianions. The electrochemical behavior is not consistent with a reaction network in which and are reduced catalytically, nor is it consistent with networks in which the oxides are completely reduced to nitrogen and oxygen dianions. These results indicate that and are electroactive on platinum. They also demonstrate that oxygen exchange cannot always be assumed to be the dominant charge‐transfer reaction in zirconia electrolytic cells.