Origin of the oxygen in the oxidation of triphenylphosphine by potassium persulfate—application of the 18O isotope effect in 31P NMR

Potassium persulfate oxidizes triphenylphosphine to triphenylphosphine oxide in 60% aqueous acetonitrile. It has been suggested that the oxygen of the product, triphenylphosphine oxide, might originate from solvent water, following nucleophilic attack on an intermediate phosphonium ion. We have investigated the origin of the oxygen in the oxidation of triphenylphosphine by potassium persulfate in 60% aqueous acetonitrile containing 20% [¹⁸O]water. The product was analyzed by using the ¹⁸O isotope effect in ³¹P NMR spectroscopy. The magnitude of the ¹⁸O isotope-induced shift was determined by synthesizing triphenylphosphine [¹⁸O]oxide and was found to be 0.038 ppm upfield. The product of the oxidation reaction in 20% [¹⁸O]water displayed no ¹⁸O isotope effect. The origin of the oxygen in the oxidation reaction is the persulfate ion, consistent with an alternative mechanism involving nucleophilic attack by water at the sulfur atom of a phosphonium peroxysulfate intermediate.     

SIMS investigations on the growth mechanisms of protective chromia and alumina surface scales

The oxidation behaviour of the oxide-dispersion strengthened (ODS) high-temperature alloys MA 956 (an aluminium oxide former) and MA 754 (a chromium oxide former) has been compared with that of two model alloys, Fe-20Cr-5Al and Ni-25Cr. The morphology and composition of the oxide scales were investigated by metallography, X-ray diffraction analysis and scanning electron microscopy. For analysis of the oxide layer growth mechanisms, twostage oxidation experiments with¹⁸O as tracer were used, the distribution of the oxygen isotopes in the oxide scale being determined by SIMS. The ODS alloys show a more selective oxidation than the two model alloys; moreover, the protective oxides on the ODS alloys have a lower growth rate and better adhesion than those on the two model alloys. From the SIMS investigations it can be deduced that the improved properties of the layers on the ODS alloys result from a change in the transport processes in the protective layer; whereas the aluminium and chromium oxide films on the conventional alloys grow by cation and anion transport, the scales on the ODS alloys grow almost exclusively by anion transport. It is shown that the observed properties of the oxide scales on the ODS alloys can be explained by this change in transport mechanism.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Identification of corona discharge-induced SF6 oxidation mechanisms using SF6/18O2/H2 16O and SF6/16O2/H2 18O gas mixtures

The absolute yields of gaseous oxyfluorides SOF₂, SO₂F₂, and SOF₄ from negative, point-plane corona discharges in pressurized gas mixtures of SF₆ with O₂ and H₂O enriched with¹⁸O₂ and H₂ ¹⁸O have been measured using a gas chromatograph-mass spectrometer. The predominant SF₆ oxidation mechanisms have been revealed from a determination of the relative¹⁸O and¹⁶O isotope content of the observed oxyfluoride by-product. The results are consistent with previously proposed production mechanisms and indicate that SOF₂ and SO₂F₂ derive oxygen predominantly from H₂O and O₂, respectively, in slow, gas-phase reactions involving SF₄, SF₃, and SF₂ that occur outside of the discharge region. The species SOF₄ derives oxygen from both H₂O and O₂ through fast reactions in the active discharge region involving free radicals or ions such as OH and O, with SF₅ and SF₄.     

Use of Isotope Effects To Understand the Present and Past of the Atmosphere and Climate and Track the Origin of Life

Stable isotope ratio measurements have been used as a measure of a wide variety of processes, including solar system evolution, geological formational temperatures, tracking of atmospheric gas and aerosol chemical transformation, and is the only means by which past global temperatures may be determined over long time scales. Conventionally, isotope effects derive from differences of isotopically substituted molecules in isotope vibrational energy, bond strength, velocity, gravity, and evaporation/condensation. The variations in isotope ratio, such as ¹⁸O/¹⁶O (δ¹⁸O) and ¹⁷O/¹⁶O (δ¹⁷O) are dependent upon mass differences with δ¹⁷O/δ¹⁸O=0.5, due to the relative mass differences (1 amu vs. 2 amu). Relations that do not follow this are termed mass independent and are the focus of this Minireview. In chemical reactions such as ozone formation, a δ¹⁷O/δ¹⁸O=1 is observed. Physical chemical models capture most parameters but differ in basic approach and are reviewed. The mass independent effect is observed in atmospheric species and used to track their chemistry at the modern and ancient Earth, Mars, and the early solar system (meteorites).     

Effects of catalytic oxidation on the electrochemical performance of common natural graphite as an anode material for lithium ion batteries

We demonstrate for the first time that the reversible capacity of common natural graphite modified by catalytic oxidation can serve as an anode material for lithium ion batteries with above-theoretical capacity of graphite. The enhancement of reversible lithium capacity from 251 to >372 mAh g⁻¹ results from an increase in the number of micropores and nanometer channels, which are formed by both chemical and catalytic oxidation. Lithium can also form alloys with metals used as oxidation catalysts, and these alloys may also contribute to the enhancement of reversible lithium capacity.     

TG Measurements of the Oxidation Kinetics of Fe-Cr Alloy with Regard to its Application as a Separator in SOFC

The high-temperature oxidation behavior of a ferritic alloy (SUS 430) in a SOFC environment, corresponding to the anode (H₂/H₂O gas mixture) and cathode (air) operating conditions, was determined with regard to application of the alloy as a metallic separator material in SOFC. The oxidation kinetics of Fe-16Cr alloy (SUS 430), was studied by thermogravimetry in H₂/H₂O gas mixtures with p_H/p_HO=94/6 and 97/3 and in air, in the temperature range 1023-1223 K, for 3.6 up to 1080 ks. It was found that the protective oxide scale, composed mainly of Cr₂O₃ with uniform thickness and excellent adhesion to the metal substrate, grows in accordance with the parabolic rate law. The dependence of the parabolic rate constant, k_p, of the scale on temperature obeys the Arrhenius equation: k_p=6.8×10^-4 exp (-202.3 kJ mol^-1R^-1T^-1) for H₂/H₂O gas mixtures with p_H/p_HO=94/6. The determined k_p was independent of the oxygen partial pressure in the range from 5.2×10^-22 to 0.21 atm at 1073 K, which means that the rates of growth of the scale on Fe-16Cr alloy in the above-mentioned atmospheres are comparable. The oxidation test results on Fe-16Cr alloy in H₂/H₂O gas mixtures and air demonstrate the applicability of SUS 430 alloys as a separator for SOFC. This revised version was published online in July 2006 with corrections to the Cover Date.     

Trace determination of oxygen in gold-copper alloys and in high purity gold using3He and4He activation analysis

Trace amounts of oxygen in gold-copper alloys and high purity gold have been determined by³He and⁴He activation analysis. Interferences from the matrix have been identified and the factors which can have an effect upon the precision and the accuracy of the method have been investigated. Thus a destructive technique has been choosen. The¹⁸F produced was chemically separated by steam distillation and precipitated as PbClF. The values of the oxygen concentration were found to range between 0.2 and 2 μg·g⁻¹ depending on the copper concentration and the thermal treatment applied to the samples. A possible mechanism of internal oxidation is presented.     

The Positional δ(18O) Values of Extracted and Synthetic Vanillin

The positional δ(¹⁸O) values of vanillin ( 1 ) of different origins have been determined from the global values of 2‐methoxy‐4‐methylphenol ( 4 ), obtained from 1 upon Clemensen reduction, and of 3‐methylanisole ( 5 ), obtained from 4 by removal of the phenolic O‐atom. By these means, it is possible to differentiate samples of 1 of synthetic origin from those extracted from Vanilla plants or produced from lignin by chemical oxidation. The main difference between the samples derived from guaiacol and those possessing the aromatic moiety of natural origin is in the enrichment values of the O‐atoms at C(3) and C(4), while the extractive materials from the pods are distinguished from the product from lignin on the basis of the carbonyl oxygen δ(¹⁸O) values, ranging from +25.5 and +26.2 in the natural material to +19.7‰ in the lignan‐based sample. The values for the phenolic O‐atom vary from +8.9 and +12‰ of the synthetic materials to +6.5, +5.3, and +6.3‰, respectively, of the sample from lignin and the two samples from Vanilla pods,whereas the MeO O‐atoms show the following values for the same compounds: −2.9, −3.2, +3.5, +3.1, and +2.3‰, respectively. This study indicates the significance of the positional δ(¹⁸O) values of polyoxygenated compounds for the definition of their origin.     

EQCM studies on Pd–Ni alloy oxidation in basic solution

Processes of electrochemical oxidation of Pd-rich Pd–Ni alloys in basic solutions were studied with the aim of electrochemical quartz crystal microbalance. Potentials of current peaks of Ni(II)/Ni(III) redox couple are independent of alloy composition. On the other hand, Ni(II)/Ni(III) redox couples formed on Pd–Ni alloys and Ni differ in respect to the structure of involved compounds and the processes of transport of the species accompanying oxidation/reduction reaction. The process of oxidation of Pd exhibits some differences between pure Pd and Pd–Ni alloys. This concerns mainly on participation of adsorbed water/OH⁻ in Pd oxidation process. In the initial stages of Pd oxidation, the source of oxygen is water/OH⁻ from the bulk of the solution. At this stage of the process, the product of Pd oxidation could be described as Pd(OH)₂ or PdOH₂O. With further progress in oxidation process, adsorbed species, water/OH⁻, start to play a decisive role. Hydrous species, i.e. Pd(OH)₂ or PdOH₂O, are also reduced in the final stages of Pd(II) reduction process. This study is dedicated to the 70th birthday of Professor Oleg Petrii.     

A novel isotope analysis of oxygen in uranium oxides: comparison of secondary ion mass spectrometry,glow discharge mass spectrometry and thermal ionization mass spectrometry

The natural variation of the oxygen isotopic composition is used among geologists to determine paleotemperatures and the origin of minerals. In recent studies, oxygen isotopic composition has been recognized as a possible tool for identification of the origin of seized uranium oxides in nuclear forensic science. In the last 10 years, great effort has been made to develop new direct and accurate n(¹⁸O)/n(¹⁶O) measurements methods. Traditionally, n(¹⁸O)/n(¹⁶O) analyses are performed by gas mass spectrometry. In this work, a novel oxygen isotope analysis by thermal ionization mass spectrometry (TIMS), using metal oxide ion species (UO⁺), is compared to the direct methods: glow discharge mass spectrometry (GDMS) and secondary ion mass spectrometry (SIMS). Because of the possible application of the n(¹⁸O)/n(¹⁶O) ratio in nuclear forensics science, the samples were solid, pure UO₂ or U₃O₈ particles. The precision achieved using TIMS analysis was 0.04%, which is similar or even better than the one obtained using the SIMS technique (0.05%), and clearly better if compared to that of GDMS (0.5%). The samples used by TIMS are micrograms in size. The suitability of TIMS as a n(¹⁸O)/n(¹⁶O) measurement method is verified by SIMS measurements. In addition, TIMS results have been confirmed by characterizing the n(¹⁸O)/n(¹⁶O) ratio of UO₂ sample also by the traditional method of static vacuum mass spectrometry at the University of Chicago.     

Synthesis and Catalytic Water Oxidation Activities of Ruthenium Complexes Containing Neutral Ligands

Two dinuclear and one mononuclear ruthenium complexes containing neutral polypyridyl ligands have been synthesised as pre‐water oxidation catalysts and characterised by ¹H and ¹³C NMR spectroscopy and ESI‐MS. Their catalytic water oxidation properties in the presence of [Ce(NH₄)₂(NO₃)₆] (Ce^IV) as oxidant at pH 1.0 have been investigated. At low concentrations of Ce^IV (5 mM ), high turnover numbers of up to 4500 have been achieved. An ¹⁸O‐labelling experiment established that both O atoms in the evolved O₂ originate from water. Combined electrochemical study and electrospray ionisation mass spectrometric analysis suggest that ligand exchange between coordinated 4‐picoline and free water produces Ru aquo species as the real water oxidation catalysts.     

Exploring the Reactivity of Multicomponent Photocatalysts: Insight into the Complex Valence Band of BiOBr

The band structure of multicomponent semiconductor photocatalysts, as well as their reactivity distinction under different wavelengths of light, is still unclear. BiOBr, which is a typical multicomponent semiconductor, may have two possible valence‐band structures, that is, two discrete valence bands constructed respectively from O 2p and Br 4p orbitals, or one valence band derived from the hybridization of these orbitals. In this work, aqueous photocatalytic hydroxylation is applied as the probe reaction to investigate the nature and reactions of photogenerated holes in BiOBr. Three organic compounds (microcystin‐LR, aniline, and benzoic acid) with different oxidation potentials were selected as substrates. Isotope labeling (H₂¹⁸O as the solvent) was used to determine the source of the O atom in the hydroxyl group of the products, which distinguishes the contribution of different hydroxylation pathways. Furthermore, a spin‐trapping ESR method was used to quantify the reactive oxygen species (^.OH and ^.OOH) formed in the reaction system. The different isotope abundances of the hydroxyl O atom of the products formed, as well as the reverse trend of the ^.OH/^.OOH ratio with the oxidative resistance of the substrate under UV and visible irradiation, reveal that BiOBr has two separate valence bands, which have different oxidation ability and respond to UV and visible light, respectively. This study shows that the band structure of semiconductor photocatalysts can be reliably analyzed with an isotope labeling method.     

Inactivation of [Fe?Fe]‐hydrogenase by O2. Thermodynamics and frontier molecular orbitals analyses

The oxidation of H‐cluster in gas phase, and in aqueous enzyme phase, has been investigated by means of quantum mechanics (QM) and combined quantum mechanics–molecular mechanics (QM/MM). Several potential reaction pathways (in the above‐mentioned chemical environments) have been studied, wherein only the aqueous enzyme phase has been found to lead to an inhibited hydroxylated cluster. Specifically, the inhibitory process occurs at the distal iron (Fe_d) of the catalytic H‐cluster (which isalso the atom involved in H₂ synthesis). The processes involved in the H‐cluster oxidative pathways are O₂ binding, e^? transfer, protonation, and H₂O removal. We found that oxygen binding is nonspontaneous in gas phase, and spontaneous for aqueous enzyme phase where both Fe atoms have oxidation state II; however, it is spontaneous for the partially oxidized and reduced clusters in both phases. Hence, in the protein environment the hydroxylated H‐cluster is obtained by means of completely exergonic reaction pathway starting with proton transfer. A unifying endeavor has been carried out for the purpose of understanding the thermodynamic results vis‐à‐vis several other performed electronic structural methods, such as frontier molecular orbitals (FMO), natural bond orbital partial charges (NBO), and H‐cluster geometrical analysis. An interesting result of the FMO examination (for gas phase) is that an e^? is transferred to LUMO_α rather than to SOMO_β, which is unexpected because SOMO_β usually resides in a lower energy rather than LUMO_α for open‐shell clusters. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Surface analysis of a glass leached with stable isotopes

The distribution of oxygen and other elements within the leached layer of a soda-lime glass after a short-time treatment (15–60 min) in pure H₂ ¹⁸O and D₂ ¹⁸O was investigated by means of neutral primary beam secondary ion mass spectrometry (NPB-SIMS). The¹⁸O^– profiles resulting from the penetration of an oxygen containing species are not disturbed by interference of the species H₂ ¹⁶O^–, originating from the residual gas in the vacuum chamber. In agreement with previous results, no isotope effect could be detected. Hydration of the glass leads to a pronounced matrix effect which impedes oxygen analysis in the negative secondary ion mode. The ratio of the maximum¹⁸O^– intensity in the leached layer to the¹⁶O^– intensity in the bulk was recorded as a function of time for one series of experiments. The result is in accordance with the time-dependence of the Na₂O concentration in the leached layer.     

SNMS and GDOES studies of the oxidation behavior of TiAl modified by niobium addition

TiAl-based intermetallic alloys are promising candidates as structural materials for high temperature applications. However, industrial application is hindered by insufficient oxidation resistance at temperatures above 700?°C in air. The oxidation resistance can be improved by the addition of ternary and quaternary alloying elements, such as niobium. In several studies it has been demonstrated that this element can reduce the oxidation rate dramatically although the underlying mechanism is not yet fully understood. In the present study the influence of niobium on the high temperature oxidation behavior at 800?°C of Ti-48Al-2Cr was investigated. Niobium was added by alloying as well as by ion implantation. Some specimens were pre-oxidized prior to ion implantation. Thus, it could be demonstrated that niobium is not only active when present in the bulk alloy, but also when located in the initially formed corrosion scale. Moreover, the implantation experiments revealed that the often suggested “doping mechanism” of the titania lattice by Nb⁵⁺ ions cannot play an important role explaining the beneficial effect of Nb. The morphology and composition of the scales formed during oxidation were studied by glow discharge optical emission spectroscopy (GDOES) and secondary neutral mass spectrometry (SNMS). The latter technique was also used in combination with two-stage oxidation experiments using the isotope tracers ¹⁸O and ¹⁵N.     

Host–guest complexation of antioxidative caffeic and ferulic acid amides with a functionalized cyclophane

Host?guest complexation has been studied by ¹H NMR on the benzyl and phenethyl amides of ferulic and caffeic acids as the guests in chloroform and acetonitrile; the counter host is a cyclophane which integrates four phenylene rings, amino and amide groups in the macrocyclic framework and bears four pendant methyl acetate ester arms. CAPE, one of the best known natural antioxidants, also has been studied for comparison. Among the guests studied, ferulic acid benzyl amide shows NMR shifts due to the formation of a host?guest complex in chloroform. The complexation occurs in two steps with the formation constants K ₁?=?[HG]/[H][G]?=?6?M^?1 and β ₂?=?[HG₂]/[H][G]²?=?87?M^?2. Two guest molecules are bound on the surface of the macrocyclic framework of a host molecule by two hydrogen bonds, NH(host amide)···O=C(guest amide) and C=O(host ester)···HO(guest phenol). The latter hydrogen bond may protect the bioactive site, i.e., phenol OH, of guest molecules captured in the complex against undesirable oxidation. This feature is observed only for ferulic acid benzyl amide in chloroform; the cyclophane ester interacts with this amide, distinctively from the other hydroxycinnamic acid derivatives.     

On the calibration of continuous,high‐precision δ18O and δ2H measurements using an off‐axis integrated cavity output spectrometer

The ¹⁸O and ²H of water vapor serve as powerful tracers of hydrological processes. The typical method for determining water vapor δ¹⁸O and δ²H involves cryogenic trapping and isotope ratio mass spectrometry. Even with recent technical advances, these methods cannot resolve vapor composition at high temporal resolutions. In recent years, a few groups have developed continuous laser absorption spectroscopy (LAS) approaches for measuring δ¹⁸O and δ²H which achieve accuracy levels similar to those of lab‐based mass spectrometry methods. Unfortunately, most LAS systems need cryogenic cooling and constant calibration to a reference gas, and have substantial power requirements, making them unsuitable for long‐term field deployment at remote field sites. A new method called Off‐Axis Integrated Cavity Output Spectroscopy (OA‐ICOS) has been developed which requires extremely low‐energy consumption and neither reference gas nor cryogenic cooling. In this report, we develop a relatively simple pumping system coupled to a dew point generator to calibrate an ICOS‐based instrument (Los Gatos Research Water Vapor Isotope Analyzer (WVIA) DLT‐100) under various pressures using liquid water with known isotopic signatures. Results show that the WVIA can be successfully calibrated using this customized system for different pressure settings, which ensure that this instrument can be combined with other gas‐sampling systems. The precisions of this instrument and the associated calibration method can reach ～0.08‰ for δ¹⁸O and ～0.4‰ for δ²H. Compared with conventional mass spectrometry and other LAS‐based methods, the OA‐ICOS technique provides a promising alternative tool for continuous water vapor isotopic measurements in field deployments. Copyright © 2009 John Wiley & Sons, Ltd.     

Phosphoramidites of Chiral (Rp)-and (Sp)-Configurated d(T[P-18O]-A): Synthesis,Configurational Assignment,and Use as Dimer Blocks in Oligonucleotide Synthesis

The N,N-diisopropylphosphoramidites 10a and 10b of appropriately protected chiral diastereoisomers of d(T[P-¹⁸O]-A) ( 8a and 8b , resp.), chiral by virtue of the isotope ¹⁸O at the P-atom, have been synthesized. The ¹⁸O-isotope was incorporated by oxidation of the phosphite triester 3 with H₂[¹⁸O]/I₂. Separation of the diastereoisomers was accomplished by flash chromatography of the O-3′-deprotected phosphate triesters 5a/b . The absolute configuration at the chiral P-atom was deduced from the methylation products of the fully deprotected diastereoisomers 8a and 8b . Phosphinylation of 5a and 5b yielded the configurationally pure phosphoramidites 10a and 10b , respectively, which were then employed in solid-phase synthesis to yield the self-complementary oligomers d(G-A-G-T-(R_p)-[P-¹⁸O]-A-C-T-C) ( 13 ) and d(G-A-G-T-(S_P)-[P-¹⁸O]-A-C-T-C) ( 14 ), respectively.     

Mapping Aldehyde Dehydrogenase 1A1 Activity using an [18F]Substrate-Based Approach

Aldehyde dehydrogenases (ALDHs) catalyze the oxidation of aldehydes to carboxylic acids. Elevated ALDH expression in human cancers is linked to metastases and poor overall survival. Despite ALDH being a poor prognostic factor, the non-invasive assessment of ALDH activity in vivo has not been possible due to a lack of sensitive and translational imaging agents. Presented in this report are the synthesis and biological evaluation of ALDH1A1-selective chemical probes composed of an aromatic aldehyde derived from N,N-diethylamino benzaldehyde (DEAB) linked to a fluorinated pyridine ring either via an amide or amine linkage. Of the focused library of compounds evaluated, N-ethyl-6-(fluoro)-N-(4-formylbenzyl)nicotinamide 4 b was found to have excellent affinity and isozyme selectivity for ALDH1A1 in vitro. Following ¹⁸F-fluorination, [¹⁸F] 4 b was taken up by colorectal tumor cells and trapped through the conversion to its ¹⁸F-labeled carboxylate product under the action of ALDH. In vivo positron emission tomography revealed high uptake of [¹⁸F] 4 b in the lungs and liver, with radioactivity cleared through the urinary tract. Oxidation of [¹⁸F] 4 b , however, was observed in vivo, which may limit the tissue penetration of this first-in-class radiotracer.