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Z-scheme heterostructure-based photocatalysts consist of a reduction photocatalyst and an oxidation photocatalyst, enabling them to possess a high capacity for both reduction and oxidation. However, the coupling reaction between photocatalytic H 2 generation through water reduction and sterilization using Z-scheme systems has been rarely reported. Herein, 1D W 18 O 49 nanowires embedded over 2D g-C 3 N 4 nanosheets are well-constructed as an integrated Z-scheme heterojunction. Experimental results and density functional theory calculations not only demonstrate the achievement of efficient interfacial charge separation and transport, leading to prolonged lifetime of photogenerated charge carriers, but also directly confirm the mechanism of Z-scheme charge transfer. As expected, the optimized W 18 O 49 /g-C 3 N 4 nanostructure exhibits superior photocatalytic sterilization activity against Staphylococcus aureus as well as excellent H 2 generation performance under visible-light irradiation (λ ≥ 420 nm). Due to its nontoxic nature, W 18 O 49 /g-C 3 N 4 holds great potential in eradicating bacterial infections in living organisms.

Mitigation of undesired byproducts from ozonation of dissolved organic matter (DOM) such as aldehydes and ketones is currently hampered by limited knowledge of their precursors and formation pathways. Here, the stable oxygen isotope composition of H 2 O 2 formed simultaneously with these byproducts was studied to determine if it can reveal this missing information. A newly developed procedure, which quantitatively transforms H 2 O 2 to O 2 for subsequent 18 O/ 16 O ratio analysis, was used to determine the δ 18 O of H 2 O 2 generated from ozonated model compounds (olefins and phenol, pH 3-8). A constant enrichment of 18 O in H 2 O 2 with a δ 18 O value of ∼59‰ implies that 16 O- 16 O bonds are cleaved preferentially in the intermediate Criegee ozonide, which is commonly formed from olefins. H 2 O 2 from the ozonation of acrylic acid and phenol at pH 7 resulted in lower 18 O enrichment (δ 18 O = 47-49‰). For acrylic acid, enhancement of one of the two pathways followed by a carbonyl-H 2 O 2 equilibrium was responsible for the smaller δ 18 O of H 2 O 2 . During phenol ozonation at pH 7, various competing reactions leading to H 2 O 2 via an intermediate ozone adduct are hypothesized to cause lower δ 18 O in H 2 O 2 . These insights provide a first step toward supporting pH-dependent H 2 O 2 precursor elucidation in DOM.

A study of different grapevine tissues and organs (root, stem, leaf, fruit) water isotope fractionation models from high-quality wine grapes produced in the Helan Mountains, a key wine-producing area in northwestern China, was undertaken. Results showed that δ 2 H values of local groundwater sources were more negative than rivers and precipitation. Soil water δ 2 H and δ 18 O values were significantly higher than those of other environmental water sources. Water from the soil surface layer (0-30 cm, δ 2 H and δ 18 O values) was more positive than the deeper layer (30-60 cm), indicating that soil water has undergone a positive fractionation effect. δ 2 H and δ 18 O values of tissues and organs from different grape varieties followed a similar pattern but were more negative than the local atmospheric precipitation line (slope between 4.1 to 5.2). The 2 H and 18 O fractionation relationship in grapevine organs was similar, and 18 O has a higher fractionation effect than 2 H. δ 2 H and δ 18 O values showed a strong fractionation effect during the transportation of water to different grape organs (trend of stem > fruit > leaf). This study showed that 18/16 O fractionation in grapes is more likely to occur under drought conditions and provides a theoretical basis to improve traceability accuracy and origin protection of wine production areas.

The oxygen-18 isotopic (18O) composition in CO2 provides an important insight into the variation of rate in isotopic fractionation reaction regulated by carbonic anydrase (CA) metalloenzyme. This work aims to employ an 18O-isotope ratio-based analytical method for quantitative estimation of CA activity in erythrocytes for clinical testing purposes. Here, a new method has been developed that contains the measurements of 18O/16O isotope ratios during oxygen-18 isotopic exchange between 12C16O16O and H218O of an in vitro biochemical reaction controlled by erythrocytes CA and estimation of enzymatic activity of CA from the isotopic composition of CO2. We studied the enrichments of 18O-isotope of CO2 with increments of CA activities during isotopic fractionation reaction. To check the influence of subject-specific body temperature, pH, H218O, and cellular produced CO2 on this reaction, we performed the in vitro experiments in closed containers with variations of those parameters. Finally, we mimicked the exchange reaction at 5% [CO2], 5‰ [H2 18O], pH of 7.4, and temperature of 37 °C to create the physiological environment equivalent to that of the human body and monitored the exchange kinetics with variations of CA activities, and subsequently, we derived the quantitative relation between the 18O-isotope of CO2 and CA activity in erythrocytes. This assay may be applicable for rapid and simple quantification of carbonic anhydrase activity which is very important to prevent the carbonic-anhydrase-associated disorders in human. [ABSTRACT FROM AUTHOR]

The sources and chemistry of gaseous nitrous acid (HONO) in the environment are of great interest. HONO is a major source of atmospheric hydroxyl radical (OH), which impacts air quality and climate. HONO is also a major indoor pollutant that threatens human health. However, the large uncertainty of HONO sources and chemistry hinders an accurate prediction of the OH budget. Isotopic analysis of HONO may provide a tool for tracking the sources and chemistry of HONO. In this study, a modified annular denuder system (ADS) was developed to quantitatively capture HONO for offline nitrogen and oxygen isotopic analysis (δ15N and δ18O) using the denitrifier method. The ADS method was tested using laboratory generated HONO (400 ppbv to 1 ppmv) and validated by parallel HONO collection with a standard, basic impinger (BI) method. The ADS system shows complete capture of HONO without isotopic fractionation. The uncertainty (1σ) based on repeated measurements across the entire analytical procedure is 0.6‰ for δ15N and 0.5‰ for δ18O. The ADS method was also tested in roadside collections of ambient HONO (0.4-1.3 ppbv) for isotopic analysis and was found to be robust for low concentration collections over 3 and 12 h collection times. In order to ensure ability to use this method in the laboratory and in the field, storage conditions for the collected HONO samples were tested and samples can be stored with consistent δ15N and δ18O for 60 days. This method enables future work to utilize the isotopic composition of HONO for studying HONO chemical formation pathways, as well as atmospheric sources and chemistry. [ABSTRACT FROM AUTHOR]

Grasslands throughout the world are responding in diverse ways to changing climate and environmental conditions. In this study we analyze indicators of phosphorus limitation including phosphorus concentrations, phosphorus to nitrogen, and carbon ratios, oxygen isotope ratios of phosphate in vegetation, and phosphatase enzyme activity in soil to shed light on potential effects of climate change on phosphorus availability to grassland vegetation. The study was conducted at the Jasper Ridge Global Change Experiment (JRGCE), California where manipulations mimicking increases in temperature, water, nitrogen and carbon-dioxide have been maintained for over 15 years. We compare our results to an earlier study conducted 3 years after the start of the experiment, in order to assess any change in the response of phosphorus over time. Our results suggest that a decade later the measured indicators show similar or only slightly stronger responses. Specifically, addition of nitrogen, the principle parameter controlling biomass growth, increased phosphorus demand but thresholds that suggest P limitation were not reached. A study documenting changes in net primary productivity (NPP) over time at the JRGCE also could not identify a progressive effect of the manipulations on NPP. Combined these results indicate that the vegetation in these grassland systems is not very sensitive to the range of climate parameters tested. [ABSTRACT FROM AUTHOR]

We present an ultrasensitive absorption spectrometer based on a 30 Hz/s stability, sub-kHz line width laser source coupled to a high-stability cavity-ring-down-spectroscopy setup. It provides direct and precise measurements of the isotopic ratios δ17O and δ18O in CO2. We demonstrate the first optical absorption measurements of 17O anomalies in CO2 with a precision better than 10 ppm, matching the requirements for paleo-environmental applications. This illustrates how optical absorption methods have become a competitive alternative to state-of-the-art isotopic ratio mass spectrometry techniques. [ABSTRACT FROM AUTHOR]

This paper describes calibration strategies in laboratory conditions that can be applied to ensure accurate measurements of the isotopic composition of the CO2 in ultradry air, expressed as d13C and d18O on the VPDB scale, with either FT-IR (in this case a Vertex 70 V (Bruker)) or an isotope ratio infrared spectrometer (IRIS) (in this case a Delta Ray (Thermo Fisher Scientific)). In the case of FT-IR a novel methodology using only two standards of CO2 in air with different mole fractions but identical isotopic composition was demonstrated to be highly accurate for measurements of d13C and d18O with standard uncertainties of 0.09% and 1.03%, respectively, at a nominal CO2 mole fraction of 400 µmol mol-1 in air. In the case of the IRIS system, we demonstrate that the use of two standards of CO2 in air of known but differing d13C and d18O isotopic composition allows standard uncertainties of 0.18% and 0.48% to be achieved for d13C and d18O measurements, respectively. The calibration strategies were validated using a set of five traceable primary reference gas mixtures. These standards, produced with whole air or synthetic air covered the mole fraction range of (378-420) µmol mol-1 and were prepared and/or value assigned either by the National Institute of Standards and Technology (NIST) or the National Physical Laboratory (NPL). The standards were prepared using pure CO2 obtained from different sources, namely, combustion; Northern Continental and Southern Oceanic Air and a gas well source, with d13C values ranging between -35% and -1%. The isotopic composition of all standards was value assigned at the Max Planck Institute for Biogeochemistry Jena (MPI-Jena). [ABSTRACT FROM AUTHOR]

Quantification of contributions from various sources of CO2 is important for understanding the atmospheric CO2 budget. Considering the number and diversity of sources and sinks, the widely used proxies such as concentration and conventional isotopic compositions (δ13C and δ18O) are not always sufficient to fully constrain the CO2 budget. Additional constraints may help in understanding the mechanisms of CO2 production and consumption. The anomaly in triple oxygen isotopes or 17O excess (denoted by Δ17O) and molecules containing two rare isotopes, called clumped isotopes, are two recently developed tracers with potentials to independently constrain some important processes that regulate CO2 in the atmosphere. The clumped isotope for CO2, denoted by Δ47, is the excess of 13C16O18O over a random distribution of isotopes in a CO2 molecule. We measured the concentrations of δ13C, δ18O, Δ17O, and Δ47 in air CO2 samples collected from the Hsuehshan tunnel (length: 12.9 km), and applied linear and polynomial regressions to obtain the fossil fuel end-members for all these isotope proxies. The other end-members, the values of all these proxies for background air CO2, are either assumed or taken as the values obtained over the tunnel and ocean. The fossil fuel (anthropogenic) CO2 end-member values for δ13C, δ18O, Δ17O, and Δ47 are estimated using the two component mixing approach: the derived values are -26.76 ± 0.25‰, 24.57 ± 0.33‰, -0.219 ± 0.021‰, and 0.267 ± 0.036‰, respectively. These four major CO2 isotope tracers along with the concentration were used to estimate the anthropogenic contribution in the atmospheric CO2 in urban and suburban locations. We demonstrate that Δ17O and Δ47 have the potential to independently estimate anthropogenic contribution, and the advantages of these two over the conventional isotope proxies are discussed. [ABSTRACT FROM AUTHOR]

Alpine river and lake systems on the Tibetan Plateau are highly sensitive indicators and amplifiers of global climate change and important components of the carbon cycle. Dissolved organic matter (DOM) encompasses organic carbon in aquatic systems, yet knowledge about DOM variation throughout the river-lake aquatic continuum within alpine regions is limited. We used optical spectroscopy, ultrahigh-resolution mass spectrometry (Fourier transform ion cyclotron resonance mass spectrometry), and stable water isotopic measurements to evaluate linkages between DOM composition and hydrological connection. We investigated glacial influences on DOM composition throughout the watershed of Selin Co, including upstream glacier-fed rivers and downstream-linked lakes. We found that the dissolved organic carbon concentration increased, whereas specific ultraviolet absorbance (SUVA 254 ) decreased along the river-lake continuum. Relative to rivers, the downstream lakes had low relative abundances of polyphenolic and condensed aromatic compounds and humic-like substances but increased relative abundances of aliphatics and protein-like compounds. SUVA 254 decreased while protein-like components increased with enriched stable water isotope δ 2 H-H 2 O, indicating that DOM aromaticity declined while autochthonous production increased along the flow paths. Glacier meltwater contributed to elevated relative abundances of aliphatic and protein-like compounds in headwater streams, while increased relative abundances of aromatics and humic-like DOM were found in glacier-fed lakes than downstream lakes. We conclude that changes in hydrological conditions, including glacier melt driven by a warming climate, will significantly alter DOM composition and potentially their biogeochemical function in surface waters on the Tibetan Plateau.

Understanding the interaction processes between the mainstream and its tributaries and detailing the rates of contribution of water and nutrients from two different waterbodies in the confluence zone are essential for water management in the Three Gorges Reservoir (TGR). The stable isotope ratios of hydrogen (dD) and oxygen (d18O) were applied to explore the interactions between the TGR mainstream and a typical tributary, the Daning River. The results of the model calculations showed that approximately 78.9% of the water and 88% of the nitrate in the confluence zone were from the TGR mainstream. The dynamic vertical distributions of the mixing ratios, major ion contributions, and flow velocities indicated that the water mass from the Yangtze River mainstream flowed backward from the confluence zone up to the tributary along the surface and upper-middle layers, whereas water from the tributary flowed into the mainstream through the lower-middle and bottom layers. This study demonstrates the value of hydrogen and oxygen isotope tracers in accurately describing water mass mixing processes and estimating the rates of contribution of different nutrient sources in the confluence zone, which will provide valuable information for controlling algal blooms in the future. [ABSTRACT FROM AUTHOR]

Manganese is biogeochemically cycled between aqueous Mn(II) and Mn(IV) oxides. Aqueous Mn(II) often coexists with Mn(IV) oxides, and redox reactions between the two (e.g., comproportionation) are well known to result in the formation of Mn(III) minerals. It is unknown, however, whether aqueous Mn(II) exchanges with structural Mn(III) in manganese oxides in the absence of any mineral transformation (similar to what has been reported for aqueous Fe(II) and some Fe(III) minerals). To probe whether atoms exchange between a Mn(III) oxide and water, we use a 17O tracer to measure oxygen isotope exchange between structural oxygen in manganite (γ-MnOOH) and water. In the absence of aqueous Mn(II), about 18% of the oxygen atoms in manganite exchange with the aqueous phase, which is close to the estimated surface oxygen atoms (~11%). In the presence of aqueous Mn(II), an additional 10% (for a total of 28%) of the oxygen atoms exchange with water, suggesting that some of the bulk manganite mineral (i.e., beyond surface) is exchanging with the fluid. Exchange of manganite oxygen with water occurs without any observable change in mineral phase and appears to be independent of the rapid Mn(II) sorption kinetics. These experiments suggest that Mn(II) catalyzes manganese oxide recrystallization and illustrate a new pathway by which these ubiquitous minerals interact with their surrounding fluid. [ABSTRACT FROM AUTHOR]

Accurate 182Hf-182W chronology of early planetary differentiation relies on highly precise and accurate tungsten isotope measurements. WO3- analysis by negative thermal ionization mass spectrometry requires W17O16O2-, W17O216O-, W18O16O2-, W17O3-, W17O18O16O-, and W18O216O isotopologue interference corrections on W16O3- species (Harper et al. Geochim. Cosmochim. Acta 1996, 60, 1131; Quitte et al. Geostandard. Newslett. 2002, 26, 149; Trinquier et al. Anal Chem. 2016, 88, 1542; Touboul et al. Nature 2015, 520, 530; Touboul et al Int J. Mass Spedrom. 2012, 309, 109). In addition, low ion beam intensity counting statistics combined with Faraday cup detection noise limit the precision on the determination of 18O/16O and 17O/16O relative abimdances. Mass dependent variability of 18O/16O over the course of an analysis and between different analyses calls for oxide interference correction on a per integration basis, based on the in-run monitoring of the 18O/16O ratio (Harper et al. Geochim. Cosmochim. Acta 1996, 60, 1131; Quitte et aL Geostandard. Newslett. 2002, 26, 149; Trinquier et al. Anal. Chem. 2016, 88, 1542). Yet, the 17O/16O variation is normally not being monitored and, instead, inferred from the measured I80/160 variation, assuming a δ17O-δ18O Terrestrial Fractionation Line (Trinquier et al. Anal. Chem. 2016, 88, 1542). The purpose of the present study is to verify the validity of this assumption. Using high resistivity amplifiers, UO2 and 238U18O2 ion beams down to 1.6 fA have been monitored simultaneously with 235,238U16O2 species in a uranium certified reference material. This leads to a characterization of O isotope fractionation by thermal ionization mass spectrometry in variable loading and running conditions (additive-to-sample ratio, PO2 pressure, presence of ionized metal and oxide spedes). Proper determination of O isotope composition based on the simultaneous analysis of the 18O/16O and 17O/16O ratios could prevent tens of ppm bias or more on the 182W/184W and l83W/l84W ratios. [ABSTRACT FROM AUTHOR]

Atmospheric dust deposition can be a significant source of phosphorus (P) in some tropical forests, so information on the origins and solubility of atmospheric P is needed to understand and predict patterns of forest productivity under future climate scenarios. We characterized atmospheric dust P across a seasonal cycle in a tropical lowland rain forest on Barro Colorado Nature Monument (BCNM), Republic of Panama. We traced P sources by combining remote sensing imagery with the first measurements of stable oxygen isotopes in soluble inorganic phosphate (δ18OP) in dust. In addition, we measured soluble inorganic and organic P concentrations in fine (<1 μm) and coarse (>1 μm) aerosol fractions and used this data to estimate the contribution of P inputs from dust deposition to the forest P budget. Aerosol dry mass was greater in the dry season (December to April, 5.6-15.7 μg m-3) than the wet season (May to November, 3.1-7.1 μg m-3). In contrast, soluble P concentrations in the aerosols were lower in the dry season (980-1880 μg P g-1) than the wet season (1170-3380 μg P g-1). The δ18OP of dry-season aerosols resembled that of nearby forest soils (~19.5?), suggesting a local origin. In the wet season, when the Trans-Atlantic Saharan dust belt moves north close to Panama, the δ18OP of aerosols was considerably lower (~15.5?), suggesting a significant contribution of long-distance dust P transport. Using satellite retrieved aerosol optical depth (AOD) and the P concentrations in aerosols we sampled in periods when Saharan dust was evident we estimate that the monthly P input from long distance dust transport during the period with highest Saharan dust deposition is 88 ± 31 g P ha-1 month-1, equivalent to between 10 and 29% of the P in monthly litter fall in nearby forests. These findings have important implications for our understanding of modern nutrient budgets and the productivity of tropical forests in the region under future climate scenarios. [ABSTRACT FROM AUTHOR]

We present a novel method for high precision measurement of 186Os/188Os and 187Os/188Os ratios, applying isobaric oxide interference correction based on in-run measurements of oxygen isotopic ratios. For this purpose, we set up a static data collection routine to measure the main Os16O3- ion beams with Faraday cups connected to conventional 1011 amplifiers, and 192Os16O217O- and 192Os16O218O- ion beams with Faraday cups connected to 1012 amplifiers. Because of the limited number of Faraday cups, we did not measure 184Os16O3- and 189Os16O3- simultaneously in-run, but the analytical setup had no significant influence on final 186Os/188Os and 187Os/188Os data. By analyzing UMd, DROsS, an in-house Os solution standard, and several rock reference materials, including WPR-1, WMS-1a, and Gpt-5, the in-run measured oxygen isotopic ratios were proven to present accurate Os isotopic data. However, 186Os/188Os and 187Os/188Os data obtained with in-run O isotopic compositions for the solution standards and rock reference materials show minimal improvement in internal and external precision, compared to the conventional oxygen correction method. We concluded that, the small variations of oxygen isotopes during OsO3- analytical sessions are probably not the main source of error for high precision Os isotopic analysis. Nevertheless, use of run-specific O isotopic compositions is still a better choice for Os isotopic data reduction and eliminates the requirement of extra measurements of the oxygen isotopic ratios. [ABSTRACT FROM AUTHOR]

An experimental study has been carriedout on the reactions of state selected O+(4S, 2D, 2P) ions with methane with the aims of characterizingthe effects of both the parent ion internal energy and collision energyon the reaction dynamics and determining the fate of oxygen speciesin complex media, in particular the Titan ionosphere. Absolute crosssections and product velocity distributions have been determined forthe reactions of 16O+or 18O+ions with CH4or CD4from thermal to5 eV collision energies by using the guided ion beam (GIB) technique.Dissociative photoionization of O2with vacuum ultraviolet(VUV) synchrotron radiation delivered by the DESIRS beamline at theSOLEIL storage ring and the threshold photoion photoelectron coincidence(TPEPICO) technique are used for the preparation of purely state-selectedO+(4S, 2D, 2P) ions. Acomplete inversion of the product branching ratio between CH4+and CH3+ions in favor of thelatter is observed for excitation of O+ions from the 4S ground state to either the 2D or the 2P metastable state. CH4+and CH3+ions, which are by far the major products for the reactionof ground state and excited states, are strongly backward scatteredin the center of mass frame relative to O+parent ions.For the reaction of O+(4S), CH3+production also rises with increasing collision energy butwith much less efficiency than with O+excitation. We foundthat a mechanism of dissociative charge transfer, mediated by an initialcharge transfer step, can account very well for all the observations,indicating that CH3+production is associatedwith the formation of H and O atoms (CH3++H + O) rather than with OH formation by an hydride transfer process(CH3++ OH). Therefore, as the CH4+production by charge transfer is also associated withO atoms, the fate of oxygen species in these reactions is essentiallythe O production, except for the reaction of O+(4S), which also produces appreciable amounts of H2O+ions but only at very low collision energy. The productionof O atoms and the nature of the states in which they are formed arediscussed for the reactions of O+ions with CH4and N2. [ABSTRACT FROM AUTHOR]

Weexplore the deconvolution of water–nitrate correlationsby the first-order difference approach involving neutron diffractionof heavy- and null-aqueous solutions of KNO3under 14N/15N and natON/18ONsubstitutions to achieve a full characterization ofthe first water coordination around the nitrate ion. For that purposewe performed isobaric–isothermal simulations of 3.5 mKNO3aqueous solutions at ambient conditionsto generate the relevant radial distribution functions required inthe analysis (a) to identify the individual partial contributionsto the total neutron-weighted distribution function, (b) to isolateand assess the contribution of NO3–···K+pair formation, (c) to test the accuracy of the neutrondiffraction with isotope substitution based coordination calculationsand X-ray diffraction based assumptions, and (d) to describe the watercoordination around both the nitrogen and oxygen sites of the nitrateion. [ABSTRACT FROM AUTHOR]

The NdO+ technique has been considerably refined in recent years for high-predsion measurement of Nd isotope ratios in low-level samples (1-5 ng Nd). As oxygen isotopic compositions may vary significantly with experimental conditions such as filament material, ionization enhancer and the ambient oxygen in the ion source, great "care" should be taken for using correct oxygen isotopic compositions to do the isobaric oxide corrections for the "conventional" NdO+ method. Our method presented here for NdO+ data reduction and PrO+ interference corrections uses the oxygen isotope composition determined in each cycle of the NdO+ measurements. For that purpose, we measured the small ion signals of 150Nd17O+ and 150Nd 18O+ with amplifiers equipped with 1012 Ω2 feedback resistors, and those of Nd 16O+ ion beams with 1011 Ω amplifiers. Using 1012 Ω amplifiers facilitates a precise measurement of the very small 150Nd 17O+ and 150Nd l8O+ ion signals and calculation of highly accurate and precise 143Nd / 144Nd isotope ratios. The 143Nd /144Nd ratios for JNdi-1 standards and several whole-rock reference materials determined with the method on 4 ng of Nd loads are consistent with previously reported values within analytical error, with internal and external precision (2 RSE and 2 RSD) of better than 20 and 30 ppm, respectively. [ABSTRACT FROM AUTHOR]

The enthalpy of oxygen atom transfer (OAT) to V[(Me3SiNCH2CH2)3N], 1, forming OV[(Me3SiNCH2CH2)3N], 1-O, and the enthalpies of sulfur atom transfer (SAT) to 1 and V(N[t-Bu]Ar)3, 2 (Ar = 3,5-C6H3Me2), forming the corresponding sulfides SV[(Me3SiNCH2CH2)3N], 1-S, and SV(N[t-Bu]Ar)3, 2-S, have been measured by solution calorimetry in toluene solution using dbabhNO (dbabhNO = 7-nitroso-2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) and Ph3SbS as chalcogen atom transfer reagents. The V-O BDE in 1-O is 6.3 ± 3.2 kcal·mol-1 lower than the previously reported value for 2-O and the V-S BDE in 1-S is 3.3 ± 3.1 kcal·mol-1 lower than that in 2-S. These differences are attributed primarily to a weakening of the V-Naxial bond present in complexes of 1 upon oxidation. The rate of reaction of 1 with dbabhNO has been studied by low temperature stopped-flow kinetics. Rate constants for OAT are over 20 times greater than those reported for 2. Adamantyl isonitrile (AdNC) binds rapidly and quantitatively to both 1 and 2 forming high spin adducts of V(III). The enthalpies of ligand addition to 1 and 2 in toluene solution are -19.9 ± 0.6 and -17.1 ± 0.7 kcal·mol-1, respectively. The more exothermic ligand addition to 1 as compared to 2 is opposite to what was observed for OAT and SAT. This is attributed to less weakening of the V-Naxial bond in ligand binding as opposed to chalcogen atom transfer and is in keeping with structural data and computations. The structures of 1, 1-O, 1-S, 1-CNAd, and 2-CNAd have been determined by X-ray crystallography and are reported. [ABSTRACT FROM AUTHOR]

Numerical models have been developed to elucidate air pollution caused by sulfate aerosols (SO 4 2- ). However, typical models generally underestimate SO 4 2- , and oxidation processes have not been validated. This study improves the modeling of SO 4 2- formation processes using the mass-independent oxygen isotopic composition [ 17 O-excess; Δ 17 O(SO 4 2- )], which reflects pathways from sulfur dioxide (SO 2 ) to SO 4 2- , at the background site in Japan throughout 2015. The standard setting in the Community Multiscale Air Quality (CMAQ) model captured SO 4 2- concentration, whereas Δ 17 O(SO 4 2- ) was underestimated, suggesting that oxidation processes were not correctly represented. The dust inline calculation improved Δ 17 O(SO 4 2- ) because dust-derived increases in cloud-water pH promoted acidity-driven SO 4 2- production, but Δ 17 O(SO 4 2- ) was still overestimated during winter as a result. Increasing solubilities of the transition-metal ions, such as iron, which are a highly uncertain modeling parameter, decreased the overestimated Δ 17 O(SO 4 2- ) in winter. Thus, dust and high metal solubility are essential factors for SO 4 2- formation in the region downstream of China. It was estimated that the remaining mismatch of Δ 17 O(SO 4 2- ) between the observation and model can be explained by the proposed SO 4 2- formation mechanisms in Chinese pollution. These accurately modeled SO 4 2- formation mechanisms validated by Δ 17 O(SO 4 2- ) will contribute to emission regulation strategies required for better air quality and precise climate change predictions over East Asia.

The microbial oxidation of sulfide is a key reaction of the microbial sulfur cycle, recycling sulfur in its most reduced valence state back to more oxidized forms usable as electron acceptors. Under anoxic conditions, nitrate is a preferential electron acceptor for this process. Two enzymatic pathways have smv-s-so been proposed for sulfide oxidation under nitrate reducing conditions, the sulfide.-quinone oxidoreductase (SQR) pathway and the Sox (sulfur oxidation) system. In experiments with the n, model strains Thiobacillus denitrificans and Sulfurimonas deni-xps / trificans, both pathways resulted in a similar small sulfur and smy-s-so,' oxygen isotope fractionation of -2.4 to - 3.6%c for 34s and -2.4 to -3.4%c for800 . A similar pattern was detected during the oxidation of sulfide in a column percolated with sulfidic, nitrate amended groundwater. In experiments with 18O-labeled water, a strong oxygen isotope fractionation was observed for T. denitrificans and S. denitrificans, indicating a preferential incorporation of 180-depleted oxygen released as water by nitrate reduction to nitrogen. The study indicates that nitrate-dependent sulfide oxidation might be monitored in the environment by analysis of 180-depleted sulfate. [ABSTRACT FROM AUTHOR]

The analysis of the stable hydrogen and oxygen isotope composition of water using cavity ring-down spectroscopy (CRDS) instruments utilizing infrared absorption spectroscopy have been comprehensively tested. However, potential limitations of infrared spectroscopy for the analysis of highly saline water have not yet been evaluated. In this study, we assessed uncertainty arising from elevated salt concentrations in water analyzed on a CRDS instrument and the necessity of a correction procedure. We prepared various solutions of mixed salts and separate solutions with individual salts (NaCl, KCl, MgCl2, and CaCl2) using deionized water with a known stable isotope composition. Most of the individual salt and salt mixture solutions (some up to 340 g L-1) had d-values within the range usual for CRDS analytical uncertainty (0.1‰ for d 18O and 1.0‰ for δ ²H). Results were not compromised even when the total load of salt in the vaporizer reached ~38.5 mg (equivalent to build up after running ~100 ocean water samples). Therefore, highly saline mixtures can be successfully analyzed using CRDS, except highly concentrated MgCl2 solutions, without the need for an additional correction if the vaporizer is frequently cleaned and MgCl2 concentration in water is relatively low. [ABSTRACT FROM AUTHOR]

The measurement of δ2H and δ18O in water samples by laser absorption spectroscopy (LAS) are adopted increasingly in hydrologic and environmental studies. Although LAS instrumentation is easy to use, its incorporation into laboratory operations is not as easy, owing to extensive offline data manipulation required for outlier detection, derivation and application of algorithms to correct for between-sample memory, correcting for linear and nonlinear instrumental drift, VSMOW-SLAP scale normalization, and in maintaining long-term QA/QC audits. Here we propose a series of standardized water-isotope LAS performance tests and routine sample analysis templates, recommended procedural guidelines, and new data processing software (LIMS for Lasers) that altogether enables new and current LAS users to achieve and sustain long-term δ2H and δ18O accuracy and precision for these important isotopic assays. [ABSTRACT FROM AUTHOR]

Trinitite is the melt glass produced subsequent the first nudear bomb test conducted on July 16, 1945, at White Sands Range (Alamagordo, NM). The geological background of tbe latter consists of arikosic sand that was fiised with radioactive debris and anthropogenic materials at ground zero subsequent detonation of the device. Postdetonation materials from historic nuclear weapon test sites provide ideal samples for development of novel forensic methods for attribution and studying the chemical/isotopic effects of the explosion on the natural geological environment In particular, the latter effects can be evaluated relative to their spatial distribution from ground zero. We report here δ18O(‰) values for nonmelted, precursor minerals phases (quartz, feldspar, caldte), "feldspathic-rich" glass, "average" melt glass, and bulk (natural) urunelted sand from the Trinity site. Prior to oxygen isotope analysis, grains/crystals were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to determine their corresponding major element composition. δ18O values for bulk trinitite samples exhibit a large range (l 1.2--15.5‰) and do not correlate with activity levels for activation product 152Eu; the latter levels are a function of their spatial distribution relative to ground zero. Therefore, the slow neutron flux associated with the nuclear eipiosion did not perturb the 18O/ 16O isotope systematics. The oxygen isotope values do correlate with the abundances of major elements derived from precursor minerals present within the arkosic sand. Hence, the O isotope ratios documented here for trinitite_melt glass can be attributed to a mixture of the respective sipiatures for precursor minerals at the Trinity site prior to the nudear explosion. [ABSTRACT FROM AUTHOR]

The isotopic composition of carbon dioxide provides a powerful tool and has been widely used for constraining the sources and sinks of atmospheric CO2. In this work, we demonstrate a simple, rapid, and clean way for measuring the triple oxygen isotope ratio of carbon dioxide with high precision. The method depends on isotope exchange between O2 and CO2 in the presence of platinum at high temperature and allows rapid measurement of Δ17O of CO2. The method has been established and confirmed through several tests by using artificially made CO2 with known Δ17O values. The analytical precision obtained for determining Δ17O in CO2 is 0.045‰ (1 - σ standard deviation). [ABSTRACT FROM AUTHOR]

Characterization of protein cross-linking, particularly without prior knowledge of the chemical nature and site of cross-linking, poses a significant challenge, because of their intrinsic structural complexity and the lack of a comprehensive analytical approach. Toward this end, we have developed a generally applicable workflow—XChem-Finder—that involves four stages: (1) detection of cross-linked peptides via 18O-labeling at C-termini; (2) determination of the putative partial sequences of each cross-linked peptide pair using a fragment ion mass database search against known protein sequences coupled with a de novo sequence tag search; (3) extension to full sequences based on protease specificity, the unique combination of mass, and other constraints; and (4) deduction of cross-linking chemistry and site. The mass difference between the sum of two putative full-length peptides and the cross-linked peptide provides the formulas (elemental composition analysis) for the functional groups involved in each cross-linking. Combined with sequence restraint from MS/MS data, plausible cross-linking chemistry and site were inferred, and ultimately confirmed, by matching with all data. Applying our approach to a stressed IgG2 antibody, 10 cross-linked peptides were discovered and found to be connected via thioethers originating from disulfides at locations that had not been previously recognized. Furthermore, once the cross-link chemistry was revealed, a targeted cross-link search yielded 4 additional cross-linked peptides that all contain the C-terminus of the light chain. [ABSTRACT FROM AUTHOR]

Selenate (SeO42-) reduction to elemental selenium is an important Se immobilization process in subsurface environments that could be mediated by Fe(II)-rich minerals or selenate-respiring microorganisms. We report the kinetic isotope effects for 18O within selenate during abiotic reactions with iron-bearing hydroxides within laboratory experiments. Selenate was reduced to Se(0) by a green rust (chloride interlayer type) and ferrous hydroxide, the two known environmentally relevant mineral reductants for selenate. Reaction kinetics are described by a rapid, low-fractionating uptake step caused by diffusive exchange between selenate and chloride followed by a slower, high-fractionating reduction step caused by electron transfer from structural Fe(II). The dual-phase kinetics cannot be described with the traditional Rayleigh fractionation model; however, well after the initial uptake step, the extent of selenate reaction is well correlated with δ18O values in accordance with the Rayleigh model. Selenate-18O enrichment (εO) was nearly identical for reaction with chloride green rust (22.7 ± 2.2‰) and ferrous hydroxide (22.1 ± 1.1‰) which suggests a common reduction mechanism by structural Fe(II). The minor enrichment due to anion exchange alone (1.4 ± 0.2‰) was confirmed using iowaite, a nonredox active Mg(II)-Fe(III) layered double hydroxide. Our eO results may contribute to Se isotope forensics to identify selenate reduction within field sites and to possibly distinguish between abiotic and biotic reduction processes. [ABSTRACT FROM AUTHOR]

The input of phosphorus (P) through atmospheric deposition can be a major source of P to fresh water bodies and may strongly affect their biogeochemistry. In Lake Kinneret (LK), northern Israel, dust deposition provides a significant fraction of the bioavailable P input. Here, we demonstrate that the oxygen isotopic composition of resin-extractable inorganic phosphate (δ18OP) in dust particles can be used to identify the phosphate source. Samples of soils with both natural vegetation and agricultural cover were collected upwind of LK and found to have distinct δ18OP value ranges (17.4-18.2‰ and 19.3-22.1‰, respectively). The δ18OP values for dust, collected continuously over LK during June 2011 to March 2012, were in the same range as agricultural soils. The dust concentration in the air decreased from the dry to the wet season and was correlated with a decrease in P concentration in air, yet no correlation was found between these parameters and dust δ18OP. Dust deposited during short-term desert dust events was characterized by a combination of high δ18OP values ranging from 22.2‰ to 22.7‰ and high concentrations of dust in the air. The data we present demonstrates a new application of δ18OP measurements for direct estimation of dust-P sources to lakes, as well as the potential for tracing dust-P on larger scales. [ABSTRACT FROM AUTHOR]

We show that natural abundance oxygen-17 NMR of solids could be obtained in minutes at a moderate magnetic field strength by using dynamic nuclear polarization (DNP). Electron spin polarization could be transferred either directly to 17O spins or indirectly via 1H spins in inorganic oxides and hydroxides using an oxygen-free solution containing a biradical polarization agent (bTbK). The results open up a powerful method for rapidly acquiring high signal-to-noise ratio solid-state NMR spectra of 17O nuclear spins and to probe sites on or near the surface, without the need for isotope labeling. [ABSTRACT FROM AUTHOR]

Mechanisms controlling mineral stabilities in contact with injected supercritical fluids containing water are relatively unknown. In this paper, we discuss carbonation reactions occurring with forsterite (Mg2SiO4) exposed to variably wet supercritical CO2 (scCO2). Transformation reactions were tracked by in situ high-pressure X-ray diffraction in the presence of scCO2 containing dissolved water. Under modest pressures (90 bar) and temperatures (50 °C), scCO2 saturated with water converted >70 wt % forsterite to a hydrated magnesium carbonate, nesquehonite (MgCO33H2O), and magnesite (MgCO3) after 72 h. However, comparable tests with scCO2 at only partial water saturation showed a faster carbonation rate but significantly less nesquehonite formation and no evidence of the anhydrous form (MgCO3). The presence and properties of a thin water film, observed by in situ infrared (IR) spectroscopy and with isotopically labeled oxygen (18O), appears to be critical for this silicate mineral to carbonate in low water environments. The carbonation products formed demonstrated by temperature and water-content dependence highlights the importance of these kinds of studies to enable better predictions of the long-term fate of geologically stored CO2. [ABSTRACT FROM AUTHOR]

The stable isotopes of hydrogen (δ2H) and oxygen (δ18O) in human urine are measured during studies of total energy expenditure by the doubly labeled water method, measurement of total body water, and measurement of insulin resistance by glucose disposal among other applications. An ultrasensitive laser absorption spectrometer based on off-axis integrated cavity output spectroscopy was demonstrated for simple and inexpensive measurement of stable isotopes in natural isotopic abundance and isotopically enriched human urine. Preparation of urine for analysis was simple and rapid (approximately 25 samples per hour), requiring no decolorizing or distillation steps. Analysis schemes were demonstrated to address sample-to-sample memory while still allowing analysis of 45 natural or 30 enriched urine samples per day. The instrument was linear over a wide range of water isotopes (δ2H = −454 to +1702 ‰ and δ18O = −58.3 to +265 ‰). Measurements of human urine were precise to better than 0.65 ‰ 1σ for δ2H and 0.09 ‰ 1σ for δ18O for natural urines, 1.1 ‰ 1σ for δ2H and 0.13 ‰ 1σ for δ18O for low enriched urines, and 1.0 ‰ 1σ for δ2H and 0.08 ‰ 1σ for δ18O for high enriched urines. Furthermore, the accuracy of the isotope measurements of human urines was verified to better than ±0.81 ‰ in δ2H and ±0.13 ‰ in δ18O (average deviation) against three independent isotope-ratio mass spectrometry laboratories. The ability to immediately and inexpensively measure the stable isotopes of water in human urine is expected to increase the number and variety of experiments which can be undertaken. [ABSTRACT FROM AUTHOR]

Analyses of δ18-O of nitrate (NO3-) have been widely used in partitioning NO3- sources. However the δ18-O value of NO3- produced from nitrification (microbial NO3-) is commonly estimated using the δ18-O of environmental water and molecular oxygen in a 2:1 ratio. Here our laboratory incubation of nine temperate forest soils across a 1500 m elevation gradient demonstrates that microbial NO3- has lower δ18-O values than the predicted using the 2:1 ratio (by 5.2-9.5% at low elevation sites), in contrast to previous reports showing higher δ18-O values (up to +15%) than their predicted values. Elevated δ18-O values of microbial NO3- were observed at high elevation sites where soil was more acidic, perhaps due to accelerated O-exchange between nitrite, an intermediate product of nitrification, and water. Lower δ18-O of microbial NO3- than the predicted and from previous observations suggests that the contribution of anthropogenic N inputs, such as fertilizer and atmospheric deposition, to a given ecosystem and the progress of denitrification in nitrogen removal are greater than we know. More than half of the δ18-O of stream NO3- lower than the predicted value along the elevation gradient also indicate the impropriety using the 2:1 ratio for differentiating NO3- sources. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is considered the ultimate limiting nutrient for plants in most natural systems and changes in the distribution of inorganic and organic P forms during soil development have been well documented. In particular, microbial activity has been shown to be an important control on P cycling but its contribution in building up the pool of plant-available P during soil development is still poorly quantified. To determine the importance of different biological processes on P cycling, we analyzed the isotopic composition of oxygen in phosphate (δ18O-Pi) from the parent material, soil microorganisms, the available P pool, and from the vegetation along a 150-year soil chronosequence of a glacier forefield. Our results show that at all sites, δ18O-Pi of microbial Pi is within the range expected for the temperature-dependent equilibrium between phosphate and water. In addition, the isotopic signature of available Pi is close to the signature of microbial Pi, independently of the contribution of parent material Pi, vegetation Pi or Pi released from organic matter mineralization. Thus, we show that phosphate is cycled through soil microorganisms before being released to the available pool. This isotopic approach demonstrates for the first time in the field and over long time scales, and not only through controlled experiments, the role of the microbial activity in cycling of P in soils. [ABSTRACT FROM AUTHOR]

Oxygen isotope fractionation is applied for the first time to probe the catalytic oxidation of water using a widely studied ruthenium complex, [RuII(tpy)(bpy)(H2O)](ClO4)2 (bpy = 2,2'-bipyridine; tpy = 2,2';6",2"-terpyridine). Competitive oxygen-18 kinetic isotope effects (18O KIEs) derived from the ratio of 16,16O2 to 16,18O2 formed from natural-abundance water vary from 1.0132 ± 0.0005 to 1.0312 ± 0.0004. Experiments were conducted with cerium(IV) salts at low pH and a photogenerated ruthenium(III) tris(bipyridine) complex at neutral pH as the oxidants. The results are interpreted within the context of catalytic mechanisms using an adiabatic formalism to ensure the highest barriers for electron-transfer and proton-coupled electron-transfer steps. In view of these contributions, O-O bond formation is predicted to be irreversible and turnover-limiting. The reaction with the largest 18O KIE exhibits the greatest degree of O-O coupling in the transition state. Smaller 18O KIEs are observed due to multiple rate-limiting steps or transition-state structures which do not involve significant O-O motion. These findings provide benchmarks for systematizing mechanisms of O-O bond formation, the critical step in water oxidation by natural and synthetic catalysts. In addition, the measurements introduce a new tool for calibrating computational studies using relevant experimental data. [ABSTRACT FROM AUTHOR]

The rate of oxygen isotope exchange between selenate and water was investigated at conditions of 10 to 80 °C and pH -0.6 to 4.4. Oxygen isotope exchange proceeds as a first-order reaction, and the exchange rate is strongly affected by reaction temperature and pH, with increased rates of isotope exchange at higher temperature and lower pH. Selenate speciation (HSeO4- vs SeO42-) also has a significant effect on the rate of isotope exchange. The half-life for isotope exchange at example natural conditions (25 °C and pH 7) is estimated to be significantly in excess of 106 years. The very slow rate of oxygen isotope exchange between selenate and water under most environmental conditions demonstrates that selenate-δ18O signatures produced by biogeochemical processes will be preserved and hence that it will be possible to use the value of selenate-δ18O to investigate the biogeochemical behavior of selenate, in an analogous fashion to the use of sulfate-δ18O to study the biogeochemical behavior of sulfate. [ABSTRACT FROM AUTHOR]

The stable oxygen isotope compositions of soil phosphate (d18Op) were suggested recently to be a tracer of phosphorus cycling in soils and plants. Here we present a survey of bioavailable (resin-extractable or resin-P) inorganic phosphate d18Op across natural and experimental rainfall gradients, and across soil formed on sedimentary and igneous bedrock. In addition, we analyzed the soil HCl-extractable inorganic d18Op, which mainly represents calcium-bound inorganic phosphate. The resin-P values were in the range 14.5-21.2‰. A similar range, 15.6-21.3‰, was found for the HCl-extractable inorganic d18Op, with the exception of samples from a soil of igneous origin that show lower values, 8.2-10.9‰, which indicate that a large fraction of the inorganic phosphate in this soil is still in the form of a primary mineral. The available-P d18Op values are considerably higher than the values we calculated for extracellular hydrolysis of organic phosphate, based on the known fractionation from lab experiments. However, these values are close to the values expected for enzymatic-mediated phosphate equilibration with soil-water. The possible processes that can explain this observation are (1) extracellular equilibration of the inorganic phosphate in the soil; (2) fractionations in the soil are different than the ones measured at the lab; (3) effect of fractionation during uptake; and (4) a flux of intercellular-equilibrated inorganic phosphate from the soil microbiota, which is considerably larger than the flux of hydrolyzed organic-P. [ABSTRACT FROM AUTHOR]

We apply a triple isotope approach for nitrate that utilizes Δ17O as a conservative tracer, in combination with δ18O and δ15N, to assess source/sink dynamics of groundwater nitrate beneath alluvial washes in a semiarid urban setting. Other studies have used δ18O and δ15N to determine nitrate sources and cycling, but the atmospheric δ18O signature can be overprinted by biogeochemical processes. In this study, δ18O and δ15N values of nitrate were coupled with δ17O values of nitrate to quantify atmospheric nitrate inputs and denitrification amounts. Results show generally low groundwater nitrate concentrations (<0.2 mmol/L) throughout the basin; high nitrate concentrations (up to 1 mmol/L) with evidence for some denitrification were detected in areas where effluent was the predominant source of recharge to groundwater. Furthermore, the denitrification was inferred from elevated δ18O and δ15N values which were reinforced by increases in observed δ17O values. Finally, relatively low, but significant atmospheric nitrate concentrations were measured in groundwater (up to 6% of total nitrate). This study concludes that the triple isotope approach improves determination of the proportion of atmospheric nitrate and the significance of denitrification in natural waters, allowing us to develop a conceptual model of the biogeochemical processes controlling nitrogen in an urban setting. [ABSTRACT FROM AUTHOR]

The dynamics of O(³P) + CO2 collisions at hyperthermal energies were investigated experimentally and theoretically. Crossed-molecular-beams experiments at 〈Ecoll〈 = 98.8 kcal mol-1 were performed with isotopically labeled 12C18O2 to distinguish products of nonreactive scattering from those of reactive scattering. The following product channels were observed: elastic and inelastic scattering (16O(³P) + 12C18O2), isotope exchange (18O + 16O12C18O), and oxygen-atom abstraction (18O16O + 12C18O). Stationary points on the two lowest triplet potential energy surfaces of the O(³P) + CO2 system were characterized at the CCSD(T)/aug-cc-pVTZ level of theory and by means of W4 theory, which represents an approximation to the relativistic basis set limit, full-configuration-interaction (FCI) energy. The calculations predict a planar CO3(C2v, ³A") intermediate that lies 16.3 kcal mol-1 (W4 FCI excluding zero point energy) above reactants and is approached by a C2v transition state with energy 24.08 kcal mol-1. Quasi-classical trajectory (QCT) calculations with collision energies in the range 23-150 kcal mol-1 were performed at the B3LYP/6-311G(d) and BMK/6-311G(d) levels. Both reactive channels observed in the experiment were predicted by these calculations. In the isotope exchange reaction, the experimental center-of-mass (c.m.) angular distribution, T(ϑc.m. ), of the 16O12C18O products peaked along the initial CO2 direction (backward relative to the direction of the reagent O atoms), with a smaller isotropic component. The product translational energy distribution, P(ET), had a relatively low average of 〈ET〈 = 35 kcal mol-1, indicating that the 16O12C18O products were formed with substantial internal energy. The QCT calculations give c.m. P(ET) and T(ϑc.m.) distributions and a relative product yield that agree qualitatively with the experimental results, and the trajectories indicate that exchange occurs through a short-lived CO3* intermediate. A low yield for the abstraction reaction was seen in both the experiment and the theory. Experimentally, a fast and weak 16O18O product signal from an abstraction reaction was observed, which could only be detected in the forward direction. A small number of QCT trajectories leading to abstraction were observed to occur primarily via a transient CO3 intermediate, albeit only at high collision energies (149 kcal mol-1). The oxygen isotope exchange mechanism for CO2 in collisions with ground state O atoms is a newly discovered pathway through which oxygen isotopes may be cycled in the upper atmosphere, where O(³P) atoms with hyperthermal translational energies can be generated by photodissociation of O3 and O2. [ABSTRACT FROM AUTHOR]

We have investigated the structure, adsorption, electronic states, and charge transfer of small water aggregates on the surface of a graphene layer using density functional theory. Our calculations were focused on water adsorbates containing up to five water molecules interacting with one and both sides of a perfect freestanding sheet. Different orientations of the aggregates with respect to the graphene sites were considered. The results show that the adsorption energy of one water molecule is primarily determined by its orientation, although it is also strongly dependent on the implemented functional scheme. Despite its intrinsic difficulties with dispersion interactions, the Perdew and Wang's exchange-correlation functional may be a viable alternative to investigate the adsorption of large molecular aggregates on a graphene surface. Although water physisorption is expected to occur in the regime of droplets, we found no induced impurity states close to the Fermi level of graphene interacting with small water clusters. In order to investigate the donor/acceptor tendency of the water clusters on graphene, we have performed a Bader charge analysis. Considering the charge transfer mechanism, we have noticed that it should preferentially occur from water to graphene only when the oxygen atom is pointing toward the surface. Otherwise, and in the case of larger adsorbed clusters, charge transfers systematically occur from graphene to water. [ABSTRACT FROM AUTHOR]

Photolysis of 17,18O-labeled water in the presence of molecular oxygen is accompanied by transfer of 17O and 18O isotopes from water to oxygen, demonstrating that photoinduced oxidation of water does occur. The reaction exhibits the following isotope effect: oxidation of H217O is faster by 2.6% (in the Earth's magnetic field) and by 6.0% (in the field 0.5 T) than that of H218O. The effect is supposed to arise in the two spin-selective, isotope-sorting reactions￣recombination and disproportionation￣in the pairs of encountering HO2 radicals. The former is spin allowed from the singlet state; the latter occurs only in the triplet one. Nuclear spin sorting produced by these reactions proceeds in opposite directions with the dominating contribution of recombination, which provides observable 17O/18O isotope fractionation in favor of magnetic isotope 17O. Neither isotope exchange nor the reaction itself occurs in the dark. [ABSTRACT FROM AUTHOR]

The stable carbon and oxygen isotope compositions of carbonate minerals are utilized throughout the earth and environmental sciences for various purposes. Here, we demonstrate the first application of a prototype instrument, based on off-axis integrated cavity output laser spectroscopy, to measure the carbon and oxygen isotope composition of CO2 gas evolved from the acidification of carbonate minerals. The carbon and oxygen isotope ratios were recorded from absorption spectra of 12C16O16O, 13C16O16O, and 12C16O18O in the near-infrared wavelength region. The instrument was calibrated using CaCO3 minerals with known δ13CVPDB and δ18OVSMOW values, which had been previously calibrated by isotope ratio mass spectrometry relative to the international isotopic standards NBS 18 and NBS 19. Individual analyses are demonstrated to have internal precision (1 SE) of better than 0.15‰ for δ13C and 0.6‰ for δ18O. Analysis of four carbonate standards of known isotopic composition over 2 months, determined using the original instrumental calibration, indicates that analyses are accurate to better than 0.5‰ for both δ13C and δ18O without application of standard-sample-standard corrections. [ABSTRACT FROM AUTHOR]

Iridium half-sandwich complexes of the types Cp*lr(N-C)X, [Cp*Ir(N-N)X]X, and [Cplr(N-N)X]X are catalyst precursors for the homogeneous oxidation of water to dioxygen. Kinetic studies with cerium(IV) ammonium nitrate as primary oxidant show that oxygen evolution is rapid and continues over many hours. In addition, [Cp*lr(H2O)3]SO4 and [(Cp*lr)2(μ-OH)3]OH can show even higher turnover frequencies (up to 20 min-1 at pH 0.89). Aqueous electrochemical studies on the cationic complexes having chelate ligands show catalytic oxidation at pH > 7; conversely, at low pH, there are no oxidation waves up to 1.5 V vs NHE for the complexes. H218O isotope incorporation studies demonstrate that water is the source of oxygen atoms during cerium(IV)-driven catalysis. DFT calculations and kinetic experiments, including kinetic-isotope- effect studies, suggest a mechanism for homogeneous iridium-catalyzed water oxidation and contribute to the determination of the rate-determining step. The kinetic experiments also help distinguish the active homogeneous catalyst from heterogeneous nanoparticulate iridium dioxide. [ABSTRACT FROM AUTHOR]

The isotopic signature of oxygen in phosphate (δ18Op) of various soil fractions may shed light on P transformations, including phosphorus (P) recycling by soil microorganisms, uptake by plants and P adsorption, precipitation and release by oxides and minerals, thus increasing our understanding on P cycling and lability in soils. We developed and tested a protocol to extract and purify inorganic phosphate (Pi) from different soil fractions distinguished by binding strength and precipitate it as silver phosphate (Ag3PO4) for δ18Op analysis. Soil P is extracted sequentially using water, NaHCO3, NaOH and HCl and Pi in each solution is purified and precipitated as Ag3PO4. The unique characteristics and possible interferences of the soil solution extracts are addressed. Two agricultural soil samples receiving reclaimed wastewater or fresh water were analyzed, and results indicate that all soil fractions analyzed have been impacted to some degree by biologically enzyme mediated cycling of P in the soil. [ABSTRACT FROM AUTHOR]

A series of cyclic nucleotide analogues to HepDirect prodrugs were prepared by a three-component reaction of protected thymine, phosphoryl chloride, and 5-aryl-α-D-xylofuranoses derivatives. One of the cyclic nucteotides showed NMR data that suggest a predominant twisted conformation; however, in spite of having an aryl group at the C4 position within the crystal lattice, the cyclic nucleotide had a chair conformation with the aryl group axially oriented. By analyzing the unprecedented X-ray structure, it was observed that the oxygen atom from the phoshoryl group (P=O) is found in close proximity to the o-hydrogen atom of the aryl group (2.51 Å), suggesting thus an attractive nonbonding electrostatic interaction, which might be the driving force that overcomes the steric diaxial interactions imposed by the aryl group. Theoretical studies (NBO) for two model compounds showed that there are indeed interactions between tilled (donor) Lewis-type NBOs and empty (acceptor) non-Lewis NBOs corresponding to the nO→σ*C-H interaction. Additionally, conversion of a diastereomeric mixture of cyclic nucleotides into the more stable diastereomeric cyclic nucleotide was observed and explained by spontaneous isomerization in the phosphorinane ring. This finding supports the recently established hypothesis for the mode of action of prodrug cleavage, for which the anomeric effect plays an important role. [ABSTRACT FROM AUTHOR]

In many nitrate (NO3-) source partitioning studies, the δ18O value for NO3- produced from nitrification is often assumed to reflect the isotopic compositions of environmental water (H2O) and molecular oxygen (O2) in a 2:1 ratio. Most studies that have measured or observed this microbial endmember have found that the δ18O-NO3- was more positive (up to +15‰ higher) than the assumed value. Current understanding of the mechanism(s) responsible for this discrepancy is limited. Incubations of one temperate forest soil (organic) and two agricultural soils (mineral) were conducted with 18O-labeled H2O to apportion the sources of oxygen in NO3- generated from nitrification. The NO3- produced in all soils had δ18O values that could not be explained by a simple endmember mixing ratio of 2:1. A more comprehensive model describing the formation of microbial NO3- was developed, which accounts for oxygen exchange between H2O and NO2-, and includes terms for kinetic and equilibrium isotope effects. Oxygen isotope exchange (i.e., the fraction of NO3-oxygen that originates from the abiotic exchange of H2O and NO2- varied widely between the temperate forest soil (0.37) and the two agricultural soils (0.52 and 0.88). At present, the microbial endmember for nitrification cannot be successfully predicted. [ABSTRACT FROM AUTHOR]

Since the discovery of mass-independent isotope effects in stratospheric and tropospheric gases, the analysis of triple oxygen isotope abundance in carbon dioxide gained in importance. However, precise triple oxygen isotope detennination in carbon dioxide is a challenging task due to mass-interference of 17O and 13C variations. Here, we present a novel analytical technique that allows us to determine slight deviations of CO2 from the terrestrial fractionation line rITE]. Our approach is based on isotopic equilibration between CO2 gas and CeO2 powder at 685 °C and subsequent mass spectrometric analysis of ceria powder by infrared-laser fluorination. We found that βCO2-CeO2, the exponent in the relation α17/16 = (α18/16)β, amounts to 0.5240 ± 0.0011 at 685 °C. The oxygen isotope anomaly of CO2 (Δ17O) can be determined for a single analysis of CeO2 with a precision of ±0.05 ‰ (1σ). Our CO2-CeO2 equilibration procedure is performed with an excess of CO2 so that one analysis of Δ17O on CO2 requires at least 3.5 mmol of CO2 gas. Our new technique allows accurate and precise determination of in CO2 and opens up a new field for investigating triple oxygen isotope abundance in various types of natural CO2. [ABSTRACT FROM AUTHOR]

Oxidative transformations using molecular oxygen are widespread in nature but remain a major challenge in chemical synthesis. Limited mechanistic understanding presents the main obstacle to exploiting O2 in "bioinspired" industrial processes, Isotopic methods are presently being applied to characterize reactions of natural abundance O2 including its coordination to reduced transition metals and cleavage of the O—O bond. This review describes the application of competitive oxygen-18 isotope effects, together with Density Functional Theory, to examine O2 reductive activation under catalytically relevant conditions. The approach should be generally useful for probing small-molecule activation by transition-metal complexes. [ABSTRACT FROM AUTHOR]

The Changjiang River is the largest freshwater river in China. Here, the sources and variability in nitrate of the Changjiang River are assessed for the first time using dual isotopic approach. Water samples were collected once in August 2006 from the main channel of the Changjiang and its major tributaries. The concentrations and isotopic composition of nitrate were then analyzed for the waters in the Changjiang River. The δ15N and δ18O of NO3- ranges from 7.3‰ to 12.9‰ and 2.4‰ to 11.2‰ in the Changjiang River waters, respectively. The ranges of isotopic compositions of nitrate suggested that nitrification (including "modified fertilizer") and urban sewage effluent are the major sources of nitrate in the Changjiang River. The high δ18O-NO3- values were observed in the water of the upper reaches, indicated that the current drought might be one important reason for shifting of isotopes in the special sampling period. In addition, there was a strong positive relationship between δ15N-NO3- and δ18O-NO3-, which indicated that denitrification added to the enrichment of heavy isotopes of nitrate. [ABSTRACT FROM AUTHOR]

Solid octaaqua(κ2-perchIorato)thonum(lV) perchlorate hydrate, [Th(H20)8(ClO4)](ClO4)3-H2O, 1, and aquaoxonium hexaaqualris(κO-trifluoromethanesultonato)thorium(lV) trisaquahexakis(κO'trifluoromethanesultonato)thorinate(IV), H5O2[Th(H2O)6(OSO2CF3)3][Th(H2O)3(OSO2CF3)6], 2, were crystallized from concentrated perchloric and tritluoromethanesulfonic acid solutions, respectively. 1 adopts a severely distorted tricapped trigonal prismatic configuration with an additional oxygen from the perchlorate ion at a longer distance. 2 consists of individual hexaaquatris(κO-tritluoromethanesulfonato)thorium(IV) and trisaquahexakis(κO-trifluoromethanesulfonato)thorinate(lV) ions and an aquaoxonium ion bridging these two ions through hydrogen bonding. The hydrated thorium(IV) ion is nine-coordinated in aqueous solution as determined by extended X-ray absorption tine structure (EXAFS) and large angle X-ray scattering (LAXS). The LAXS studies also showed a second hydration sphere of about 18 water molecules, and traces of a 3rd hydration sphere. Structural studies in aqueous solution of the hydrolysis products of thorium(lV) have identified three different types of hydrolysis species: aμ2O-hydroxo dimer, [Th2(OH)2(H2O)12]6+, a μ2O-hydroxo tetramer, [Th4(OH)8(H2O)16]8+ , and a μ3 O-oxo hexamer, [Th6O8(H2O)n]8+'. Detailed structures of these three hydrolysis species are given. A compilation of reported solid state structures of actinoid(lV) compounds with oxygen donor ligands show a strong correlation between the An-C bond distance and the coordination number. The earlier reported U–C bond distance in the hydrated uranium(lV) ion in aqueous soistion, confirmed in this study, is related to nine-coordination. The hydrated tn- and tetravalent actinoid ions in aqueous solution all seem to be nine- coordinated. The trivalent ions show a significant difference in bond distance to prismatic and capping water molecules in assumed tricapped trigonal prismatic configuration, while the tetravalent ions seem to form more regular structures, probably because of higher polarization. [ABSTRACT FROM AUTHOR]