Metal cluster topology. 1. Osmium carbonyl clusters

Important theoretical approaches to metal cluster bonding including the Wade-Mingos skeletal electron pair method, the Teo topological electron count, the King-Rouvray graph theory derived method, and Lauher's extended Hückel calculations are shown to agree in their apparent skeletal electron counts for the most prevalent metal cluster polyhedra including the tetrahedron, the trigonal bipyramid (both ordinary and elongated), square pyramid, octahedron, bicapped tetrahedron, pentagonal bipyramid, and capped octahedron. The graph theory derived method is used to treat osmium carbonyl clusters containing from five to eleven osmium atoms. In this connection most osmium carbonyl clusters can be classified into the following types: (1) Clusters exhibiting edge- localized bonding containing multiple tetrahedral chambers (e.g., Os₅(CO)₁₆, Os₆(CO)₁₈, H₂Os₇(CO)₂₀ and HOs₈(CO)₂₂⁻); (2) Capped octahedral clusters derived from osmium carbonyl fragments of the type Os_6+p(CO)_19+2p (p = 0, 1, 2, and 4) (e.g., Os₆- (CO)₁₈²⁻, Os₇(CO)₂₁, Os₈(CO)₂₂²⁻, and H₄Os₁₀- (CO)₂₄²⁻). Other more unusual osmium carbonyl clusters such as the planar Os₆(CO)₁₇ [P(OCH₃)₃]₄, the Os₉ cluster [Os₉(CO)₂₁C₃H₂R]⁻, and the Os₁₁ cluster Os₁₁C(CO)₂₇²⁻ can also be treated satisfactorily by these methods. The importance of the number of ligands around isoelectronic Os_n systems in determining the cluster polyhedron is illustrated by the different cluster polyhedra found for each member of the following isoelectronic pairs: HOs₆- (CO)₁₈⁻/H₂Os₆(CO)₁₈. Os₇(CO)₂₁/H₂Os₇(CO)₂₀, Os₈(CO)₂₂²⁻/HOs₈(CO)₂₂⁻. The tendency for osmium carbonyl clusters frequently to form polyhedra exhibiting edge-localized rather than globally delocalized bonding relates to the facility for osmium carbonyl vertices to contribute more than three internal orbitals to the cluster bonding. In this way Wade's well-known analogy between boron hydride clusters and metal clusters, which assumes exactly three internal orbitals for each vertex atom, is frequently no longer followed in the case of osmium carbonyl clusters.     

Modifying La0.6Sr0.4MnO3 Perovskites with Cr Incorporation for Fast Isothermal CO2‐Splitting Kinetics in Solar‐Driven Thermochemical Cycles

Perovskites are promising oxygen carriers for solar‐driven thermochemical fuel production due to higher oxygen exchange capacity. Despite their higher fuel yield capacity, La_0.6Sr_0.4MnO₃ perovskite materials present slow CO₂‐splitting kinetics compared with state‐of‐the‐art CeO₂. In order to improve the CO production rates, the incorporation of Cr in La_0.6Sr_0.4MnO₃ is explored based on thermodynamic calculations that suggest an enhanced driving force toward CO₂ splitting at high temperatures for La_0.6Sr_0.4Cr_xMn_1?xO₃ perovskites. Here, reported is a threefold faster CO fuel production for La_0.6Sr_0.4Cr_0.85Mn_0.15O₃ compared to conventional La_0.6Sr_0.4MnO₃, and twofold faster than CeO₂ under isothermal redox cycling at 1400 °C, and high stability upon long‐term cycling without any evidence of microstructural degradation. The findings suggest that with the proper design in terms of transition metal ion doping, it is possible to adjust perovskite compositions and reactor conditions for improved solar‐to‐fuel thermochemical production under nonconventional solar‐driven thermochemical cycling schemes such as the here presented near isothermal operation.     

Anodization of aluminum and silicon in plasma of a non-self-sustained glow discharge

The results of anodization of aluminum and silicon in an oxygen plasma are presented. The plasma was generated by a non-self-sustained glow discharge with a hollow cathode excited by an electron beam at the oxygen pressure of 20 Pa. The density of the current flowing through the anodized specimen did not exceed 1.5 mA/cm², and its temperature was 200–250°C. Continuous Al₂O₃ and SiO₂ films were formed on the aluminum and silicon surfaces. The growth rate of the oxide layers was 150–200 nm/h for Al₂O₃ and 400–800 nm/h for SiO₂.     

Preparation and characterization of amino and carboxyl functionalized core-shell Fe3O4/SiO2 for L-asparaginase immobilization: A comparison study

Abstract

Magnetic nanoparticles are well known as facile and effective support for enzyme immobilization since they have a high surface area, large surface-to-volume ratio, easy separation, a fast and high enzyme loading. This study aims to provide insights on whether acidic or basic modified particles are more effective for L-asparaginase (ASNase) immobilization. Therefore, amino (Fe₃O₄/SiO₂/NH₂) and carboxyl-functionalized (Fe₃O₄/SiO₂/COOH) particles were prepared. The functional groups, crystalline structure, magnetic properties, morphology, chemical composition and thermal behaviour of the prepared modified nanoparticles were examined via Fourier-transform infra-red spectroscopy (FTIR), X-ray diffraction (XRD), vibrating-sample magnetometer (VSM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDAX). Under the optimum conditions, the immobilized enzymes were more stable within a certain range of temperatures and pH values in comparison to free enzyme. On the other hand, the immobilized enzymes showed greater stability after incubation for 3?h at 50?°C. The free enzyme maintained only 30% of its initial activity for 4?weeks at 4?°C, while Fe₃O₄/SiO₂/NH₂/ASNase and Fe₃O₄/SiO₂/COOH/ASNase retained more than 78.9% and 56.5% of initial activities under the same conditions, respectively. Moreover, Fe₃O₄/SiO₂/NH₂/ASNase (77.2%) and Fe₃O₄/SiO₂/COOH/ASNase (57.4%) displayed excellent operational stability after 17 repeated cycles. These findings suggested that the Fe₃O₄/SiO₂/NH₂ and Fe₃O₄/SiO₂/COOH may be utilized as efficient and sustainable supports to developed immobilized ASNase in several biotechnological applications.     

Synthesis and characterisation of the carboxylate linked osmium clusters [{Os3H(CO)10}2(CO2CH2CO2)], [{Os3H(CO)10}2(CO2C2H4CO2)], [{Os3H(CO)10}2(C4O4)] and [{Os3H(CO)10}2(C4O4){Co2(CO)6}]

Reactions between the activated cluster [Os₃(CO)₁₀(NCMe)₂] and malonic acid, succinic acid and dicarboxylic acetylene, respectively, lead to the formation of the linked cluster complexes [{Os₃H(CO)₁₀}₂(CO₂CH₂CO₂)] (1), [{Os₃H(CO)₁₀}₂(CO₂C₂H₄CO₂)] (2), and [{Os₃H(CO)₁₀}₂(C₄O₄)] (3) in good yield. Cluster 3 was subsequently treated with [Co₂(CO)₈] and this results in the addition of a “Co₂(CO)₆” group giving [{Os₃H(CO)₁₀}₂(C₂O₄){Co₂(CO)₆}] (4). The X-ray crystal structures are reported for 2–4. In each structure the two triangular triosmium units are linked by the carboxylate groups and within each complex the carboxylate groups are chelating and bridge two osmium atoms.     

FT-IR study of bridging carbonyl structure of H2FeRu3(CO)13 trapped in noble gas matrices and absorbed on oxide surfaces

Infrared spectra ranging from 4000 to 180 cm⁻¹ of H₂FeRu₃(CO)₁₃ isolated in argon and nitrogen matrices were recorded. Minimum two conformations of H₂FeRu₃(CO)₁₃ and two fragments (Ru₃(CO)₁₂ and Fe(CO)₅) have been individuated. By quantitative treatment of the spectral range of bridging carbonyl stretching modes performing on spectra of H₂FeRu₃(CO)₁₃ matrix-isolated and adsorbed on SiO₂ and Al₂O₃ supports, two main double bridging structures of different geometries have been identified and angles between bridging carbonyls have been estimated. Additional double and single bridging carbonyls, characteristic of the individual supported cluster systems, have also been detected.     

Transesterification of sunflower oil to biodiesel on ZrO2 supported La2O3 catalyst

ZrO₂ supported La₂O₃ catalyst prepared by impregnation method was examined in the transesterification reaction of sunflower oil with methanol to produce biodiesel. It was found that the catalyst with 21 wt% loaded La₂O₃ and calcined at 600 °C showed the optimum activity. The basic property of the catalyst was studied by CO₂-TPD, and the results showed that the fatty acid methyl ester (FAME) yield was related to their basicity. The catalyst was also characterized by TG–DTA, XRD, FTIR, SEM and TEM, and the mechanism for the formation of basic sites was discussed. It was also found that the crystallite size of support ZrO₂ decreased by loading of La₂O₃, and the model of the solid-state reaction on the surface of La₂O₃/ZrO₂ catalyst was proposed. Besides, the influence of various reaction variables on the conversion was investigated.     

Reactivity of ruthenium carbonyls on metal oxide surfaces: effects of the surface acid-base chemistry

The reactivity of tetraruthenium carbonyl clusters supported on metal oxides (SiO₂, TiO₂, γ-Al₂O₃, and MgO) under various atmospheres has been investigated by infrared and ultraviolet-visible spectroscopies and electron microscopy. [H₄Ru₄(CO)₁₂] physisorbed on SiO₂ easily decomposes and aggregates to form metal particles. The tetraruthenium clusters supported on TiO₂ are oxidized by surface hydroxyl groups, giving mononuclear complexes; virtually the same chemistry occurs, but less readily, on γ-Al₂O₃. The tetramthenium clusters supported on MgO, in contrast, are highly resistant to oxidation. The strong basicity of this support leads to an increased stability of small polynuclear complexes, hindering the formation of oxidized complexes and aggregated metallic structures. The unique character of the basic support appears to be related to the tendency of the support to form and stabilize cluster anions.     

Growth of the metal framework in linear ruthenium and osmium carbonyls

Formation of the linear chain ruthenium and osmium carbonyls by successive linkage of mononuclear [M(CO)₄Cl₂] units and by opening trinuclear clusters [M₃(CO)₁₂] and [FeM₂(CO)₁₂] (M = Ru, Os) with chlorine gas have been studied by computational DFT methods. Energetically the formation of dinuclear [M₂(CO)₈Cl₂] from [M(CO)₄Cl₂] units is the most demanding step. The following chain growth by adding new mononuclear units proceeds more easily with nearly constant energy per step. Cluster opening by chlorine gas to obtain trinuclear [M₃(CO)₁₂Cl₂] is a facile reaction for both ruthenium and osmium clusters as well as for mixed metal clusters. Mixed metal clusters [FeOs₂(CO)₁₂] and [FeRu₂(CO)₁₂] open preferably between iron-osmium or iron-ruthenium bonds producing linear trinuclear Fe-M-M-type of compound. In the case of mixed metal Os-Ru clusters, the cleavage of Os-Ru bond is not clearly preferred. Fragmentation of the cluster to shorter units cis(Cl)-[M(CO)₄Cl₂] or [M₂(CO)₈Cl₂] with equatorial chlorides is highly favorable and competes with the cluster opening. No preferences on the bond type (Os-Ru, Os-Os, or Ru-Ru) that are broken can be found in the case of mixed metal Os-Ru clusters.     

Syntheses and structures of the heterometallic clusters [(Ph3PAu)3Re(CO)4], [(Ph3PAu)4Re(CO)4], [(Ph3PAu)6AuRe2(CO)8], and [(Ph3PAu)6Re(CO)3]

The reaction of [HRe₃(CO)₁₂]²⁻ with an excess of Ph₃PAuCl in CH₂Cl₂ yields [(Ph₃PAu)₄Re(CO)₄]⁺ as the main product, which crystallizes as [(Ph₃PAu)₄Re(CO)₄]PF₆ · CH₂Cl₂ (1 · CH₂Cl₂) after the addition of KPF₆.The crystal structure determination reveals a trigonal bipyramidal Au₄Re cluster with the Re atom in equatorial position.If [(Ph₃PAu)₄Re(CO)₄]⁺ is reacted with PPh₄Cl, a cation [Ph₃PAu]⁺ is eliminated as Ph₃PAuCl, and the neutral cluster [(Ph₃PAu)₃Re(CO)₄] (2) is formed.It combines with excess [(Ph₃PAu)₄Re(CO)₄]⁺ to afford the cluster cation, [(Ph₃PAu)₆AuRe₂(CO)₈]⁺. It crystallizes from CH₂Cl₂ as[(Ph₃PAu)₆AuRe₂(CO)₈]PF₆ · 4CH₂Cl₂ (3 · 4CH₂Cl₂). In [(Ph₃PAu)₃Re(CO)₄] the metal atoms are arranged in form of a lozenge while in [(Ph₃PAu)₆AuRe₂(CO)₈]⁺ two Au₄Re trigonal bipyramids are connected by a common axial Au atom.The treatment of [(Ph₃PAu)₄Re(CO)₄]⁺ with KOH and Ph₃PAuCl in methanol yields the cluster cation [(Ph₃PAu)₆Re(CO)₃]⁺, which crystallizes with from CH₂Cl₂ as [(Ph₃PAu)₆Re(CO)₃]PF₆ · CH₂Cl₂ (4 · CH₂Cl₂). The metal atoms in this cluster form a pentagonal bipyramid with the Re atom in the axial position.     

Planar tetrameric metal cluster formation at room temperature induced by π?π interactions: 3D supramolecular host containing 1D channel filled with solvent methanol

A novel tetrameric metal cluster, [La₂(phen)₃(2,3-pdc)(NO₃)₄(H₂O)]₂·(CH₃OH)₂ (2,3-pdcH₂ = pyridine-2,3-dicarboxylic acid, phen = 1,10-phenanthroline), have been synthesized at room temperature from water-methanol mixture by mixing the reactants in stoichiometric ratio. 2,3-Pdc and their π?π interactions played a vital role on the construction of the core. The auxiliary ligand, phen, which blocked the outermost periphery of the molecule and their hydrophobic π?π interactions facilitate the formation of the tetrameric metal clusters. The tetrameric metal clusters are connected by supramolecular interactions to form 3D supramolecular metal organic host (MOSH) producing supramolecular channels along a-axis. These supramolecular channels are filled up by solvent methanol molecules. The luminescent investigations reveal that cluster complex exhibits strong blue emission.     

Differential carbon monoxide sensitivity of cytochrome C oxidase in the leaves of c(3) and c(4) plants

CO sensitivity of cytochrome a₃ in the leaves of a number of C₃ and C₄ plants was monitored by the nitrate reductase assay under differing CO to O₂ ratios. All the C₃ plants were relatively insensitive to CO and required a high CO to O₂ ratio of 40 to promote significant nitrate reductase activity. However, when treated with 2 millimolar 2,4-dinitrophenol, these leaves readily responded to CO even at low CO to O₂ ratios of 10 or less. On the other hand, the leaves of all C₄ plants tested, belonging to the three subgroups, were highly sensitive to CO, even at CO to O₂ ratios of 5 or less. In these leaves, the uncoupler was without any effect, probably because the mitochondria, either from mesophyll or bundle sheath cells or both, lacked tight respiratory control.     

Flavin-mediated reduction of ferric leghemoglobin from soybean nodules

The reduction of ferric leghemoglobin (Lb³⁺) from soybean (Glycine max (L.) Merr.) nodules by riboflavin, FMN and FAD in the presence of NAD(P)H was studied in vitro. The system NAD(P)H + flavin reduced Lb³⁺ to oxyferrous (Lb²⁺ · O₂) or deoxyferrous (Lb²⁺) leghemoglobin in aerobic or anaerobic conditions, respectively. In the absence of O₂ the reaction was faster and more effective (i.e. less NAD(P)H oxidized per mole Lb³⁺ reduced) than in the presence of O₂; this phenomenon was probably because O₂ competes with Lb³⁺ for reductant, thus generating activated O₂ species. The flavin-mediated reduction of Lb³⁺ did not entail production of superoxide or peroxide, indicating that NAD(P)H-reduced flavins were able to reduce Lb³⁺ directly. The NAD(P)H + flavin system also reduced the complexes Lb³⁺ · nicotinate and Lb³⁺ · acetate to Lb²⁺ · O₂, Lb²⁺ or Lb²⁺ · nicotinate, depending on the concentrations of ligands and of O₂. In the presence of 200 M nitrite most Lb remained as Lb³⁺ in aerobic conditions but the nitrosyl complex (Lb²⁺ · NO) was generated in anaerobic conditions. The above-mentioned characteristics of the NAD(P)H + flavin system, coupled with its effectiveness in reducing Lb³⁺ at physiological levels of NAD(P)H and flavins in soybean nodules, indicate that this mechanism may be especially important for reducing Lb³⁺ in vivo.Abbreviations and Terminology FLbR ferric leghemoglobin reductase - Hb²⁺ /Hb³⁺ hemoglobin containing Fe²⁺ /Fe²⁺ - Lb²⁺ /Lb³⁺ leghemoglobin containing Fe²⁺ /Fe³⁺ - Lb³⁺ · nicotinate/acetate Lb in which nicotinate or acetate are complexed to Lb³⁺ - Lb²⁺ · O₂/CO/NO/nicotinate Lb in which O₂, CO, NO or nicotinate are complexed to Lb²⁺ - Rfl riboflavin - SOD superoxide dismutase (EC 1.15.1.1) Published as Paper No. 9237, Journal Series, Nebraska Agricultural Research DivisionWe thank M.B. Crusellas for his skillful drawings. M. Becana thanks the Spanish Ministry of Education and Science/Fulbright Commission for financial support.     

Stimulation of photosynthesis by 2% oxygen at low temperatures is restored by phosphate

The effect of phosphate feeding on the influence of low (2%) oxygen on photosynthetic carbon assimilation has been investigated in leaf discs of spinach (Spinacia oleracea L.) at 12°C. The following observations were made. First, after the transition from 20% O₂ to 2% O₂, the rate of CO₂ uptake was inhibited at CO₂ concentrations between about 250 and about 800 l CO₂·l^-1. Second, phosphate feeding stimulated the rate of CO₂ uptake in 20% O₂ at higher concentrations of CO₂ (500–900 l·l^-1). Third, phosphate feeding stimulated the rate of CO₂ uptake in 2% O₂ at all but the highest (900 l·l^-1) and lowest 74 (l·l^-1) concentrations of CO₂ employed. Phosphate thereby restored the stimulation of photosynthesis by 2% O₂ and it did so over a wide range of lower temperatures. Fourth, oscillatory behaviour, however generated, was dampened by phosphate feeding, even at very low concentrations of CO₂. Contents of leaf metabolites were measured during the transition to 2% O₂ in control and phosphate-fed leaf discs. During this period the ratio glycerate-3-phosphate/triose phosphate rose steeply, but fell again only in the phosphate-treated leaf discs. These data, taken together with measured ATP/ADP ratios, showed that assimilatory power, the ratio [ATP]·[NAD(P)H]/[ADP]·[Pi]·[NAD(P)], decreased when leaves were exposed to 2% O₂, but that this decrease was minimised by previous feeding of phosphate. The mechanism of phosphate limitation is discussed in the light of the results.Abbreviations C_i intercellular concentration of CO₂ - RuBP ribulose-1,5-bisphosphate     

Comparative study of three magnetic nano-particles (FeSO4, FeSO4/SiO2, FeSO4/SiO2/TiO2) in plasmid DNA extraction

Recent updates on Magnetic Nano-Particles (MNPs) based separation of nucleic acids have received more attention due to their easy manipulation, simplicity, ease of automation and cost-effectiveness. It has been indicated that DNA molecules absorb on solid surfaces via hydrogen-bonding, and hydrophobic and electrostatic interactions. These properties highly depend on the surface condition of the solid support. Therefore, surface modification of MNPs may enhance their functionality and specification. In the present study, we functionalized Fe₃O₄ nano-particle surface utilizing SiO₂ and TiO₂ layer as Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/TiO₂ and then compare their functionality in the adsorption of plasmid DNA molecules with the naked Fe₃O₄ nano-particles. The result obtained showed that the purity and amount of DNA extracted by Fe₃O₄ coated by SiO₂ or SiO₂/TiO₂ were higher than the naked Fe₃O₄ nano-particles. Furthermore, we obtained pH 8 and 1.5 M NaCl as an optimal condition for desorption of DNA from MNPs. The result further showed that, 0.2 mg nano-particle and 10 min at 55 °C are the optimal conditions for DNA desorption from nano-particles. In conclusion, we recommended Fe₃O₄/SiO₂/TiO₂ as a new MNP for separation of DNA molecules from biological sources.     

The oxygen uptake-power regression in cyclists and untrained men: implications for the accumulated oxygen deficit

The regression of oxygen uptake (O₂) on power output and the O₂ demand predicted for suprapeak oxygen uptake (O_2peak) exercise (power output = 432 W) were compared in ten male cyclists [C, mean O_2peak = 67.9 (SD 4.2) ml · kg^–1 · min^–1] and nine active, yet untrained men [UT, mean O_2peak = 54.1 (SD 6.5) ml · kg^–1 · min^–1]. The O₂-power regression was determined using a continuous incremental cycle test (CON4), performed twice, which comprised several 4-min exercise periods progressing in intensity from approximately 40%–85% O_2peak. Minute ventilation (_E), heart rate (HR), respiratory exchange ratio (R), blood lactate concentration ([1a^–]_b) and rectal temperature (T _re) were measured at rest and during CON4. The slope of the O₂-power regression was greater (P 0.05) in C [12.4 (SD 0.7) ml · min^–1. W^–1] compared to UT [11.7 (SD 0.4) ml · min^–1 W^–1]; as a result, the O₂ demand (at 432 W) was also higher (P 0.05) in C [5.97 (SD 0.23) l · min^–1] than UT [5.70 (SD 0.15) 1 · min^–1]. ExerciseR and [la^–]_b were lower (P 0.05) in C .in comparison to UT at all power outputs, whereas _E and HR were relatively lower (P 0.05) in C at power outputs approximating 180 W, 220 W and 270 W. Differences in fat metabolism estimated over the first three power outputs accounted for approximately 19% of the difference in O₂-power slopes between the groups and up to 46% of the difference in O₂ at a given intensity. Although the O₂-power regressions were linear for C [r = 0.997 (SD 0.001)] and UT [r = 0.997 (SD 0.001)], the O₂-power slope was higher at power outputs at or above the lactate threshold (13.2 ml · min^–1 · W^–1 than at lower intensities (11.6 ml · min^–1 · W^–1) in C, an effect which was less profound in UT. As a result, the exclusion of O₂ at the highest power outputs completely abolished the difference in O₂-power slopes between C and UT. Thus, the relatively higher O₂ during incremental exercise in C can be almost entirely attributed to the higher O₂ cost of cycling at higher power outputs. In addition, the presence of non-linear responses in O₂ at higher intensities also confirms the invalidity of describing the O₂ response across a wide range of power outputs using a linear function, and challenges the validity of predicting the O₂ demand of more intense exercise by a linear extrapolation of this same function.     

From U(IV) double-decker phthalocyanine with stacked UPc2 sandwiches to unstacked ones

The uranium diphthalocyanine complex with the composition of [UPc₂]·2DBU·(CH₃)₂CO has been obtained by recrystallisation of [UPc₂](I₃)_2/3 in DBU (1,8-diazabicyclo[4.5.0]undec-7-ene) during slow dilution with acetone. The [UPc₂]·2DBU·(CH₃)₂CO complex crystallises in the Pna2₁ space group of the orthorhombic system with four molecules in the unit cell. In contrast to the stacked structure of [UPc₂](I₃)_2/3 the structure of [UPc₂]·2DBU·(CH₃)₂CO contains separated and unstacked UPc₂ sandwiches. The [UPc₂]·2DBU·(CH₃)₂CO complex dissolved in benzene, in 1-choronaphthalene and in DBU has been characterised by the UV-Vis spectroscopy. Some remarks on the stability of the complex in these solutions as well as its transformation to uranium monophthalocyanine have been made.     

Catalytic mechanisms of Au<Subscript>11</Subscript> and Au<Subscript>11-n</Subscript>Pt<Subscript>n</Subscript> (<Emphasis Type="Italic">n</Emphasis>=1–2) clusters: a DFT investigation on the oxidation of CO by O<Subscript>2</Subscript>

The oxidation of CO catalyzed by clusters of Au₁₁, Au₁₀Pt and Au₉Pt₂ was investigated using the M06 functional suite of the density functional theory. Au and Pt atoms were described with the double-ζ valence basis set Los Alamos National Laboratory 2-double-z (LanL2DZ), whereas the standard 6-311++G(d,p) basis set was employed for the C and O atoms. Our theoretical model showed that (1) after coordination to Au and Au-Pt cluster, O₂ and CO are apparently activated, and Mulliken charges show that the gold atoms in the active sites of Au11 are negatively charged; (2) Au-Pt clusters with 11 atoms can effectively catalyze the oxidation of CO by O₂; (3) Au₁₁ exhibits good catalytic performance for the oxidation of CO; (4) oxidation of CO occurs preferably on the Au–Pt active sites in Pt-doped clusters, and the single-center mechanisms are more favorable energetically than the two-center mechanisms; (5) after adsorption, an O₂ molecule oxidates two CO molecules via stepwise mechanisms; and (6) the catalytic processes are highly exothermic.     

Hydrothermal synthesis and structure of [Ni(tpyrpyz)2]2[Mo4O12F2][Mo6O19] · 2H2O, a material exhibiting an unusual tetranuclear polyoxofluoromolybdate cluster [Mo4O12F2]

The hydrothermal reaction of a solution of Ni(CH₃CO₂)₂ · 4H₂O, MoO₃, tetra-4-pyridylpyrazine, H₂O₃PCH₃, and HF at 200 °C for 96 h yields orange crystals of [Ni(tpyrpyz)₂]₂[Mo₄O₁₂F₂][Mo₆O₁₇] · 2H₂O (1 · 2H₂O). The structure consists of discrete {Ni(tpyrpyz)₂}²⁺ cations and {Mo₆O₁₉}²⁻ and {Mo₄O₁₂F₂}²⁻ anionic clusters. The hexamolybdate is the well-documented octahedron of octahedra, that is, six {MoO₆} octahedra in a compact edge-sharing arrangement. The novel oxyfluoride cluster {Mo₄O₁₂F₂}²⁻ features two {MoO₄F₂} octahedra, sharing the edge defined by the fluoride ligands; the octahedral Mo sites corner-share to two {MoO₄} tetrahedra in the μ₂-O, O^′ bridging mode.     

Temperature insensitive O2 in blood of the tree frogChiromantis petersi

Summary Respiratory gas exchange and blood respiratory properties have been studied in the East-African tree frogChiromantis petersi. This frog is unusually xerophilous, occupies dry habitats and prefers body temperatures near 40°C and direct solar exposure. Total O₂ uptake was low at 81 l O₂·g^–1·h^–1±19.0 (SD) at 25°C increasing to 253.5 l O₂·g^–1·h^–1±94.8 (SD) at 40°C giving aQ ₁₀ value of 2.1. Skin O₂ uptake at 25°C was 38.5% of total. The gas exchange ratio was 0.71 for whole body gas exchange, 0.61 for the lungs and 1.02 for the skin at 25°C.Blood O₂ affinity was low with aP ₅₀ of 47.5 mmHg at 25°C and pH 7.65. Then _H-value at 25°C increased from 2.7 aroundP ₅₀ to 5.0 at O₂ saturations exceeding 70–80%. Surprisingly, blood O₂ affinity was nearly insensitive to temperature expressed by a H value of ±1.0 kcal·mole between 25 and 40°C.The adaptive significance of the low O₂ affinity, the increase ofn _H with O₂ saturation and the temperature insensitive O₂-Hb binding is discussed in relation to the high and fluctuating body temperatures ofChiromantis.