Stable Molybdenum Research Articles

A bifurcated pathway of oxygen atom transfer reactions from a monooxo molybdenum(VI) complex under electrospray ionisation mass spectrometric conditions.

A stable molybdenum(V) complex, LMoOCl2(where L is hydrotris(3,5-dimethylpyrazolyl)borate), has been oxidized under mass spectrometric conditions. The oxidized species reacts with tertiary phosphines and the products have been detected by mass spectrometry. The product distribution has been followed by isotope labeling experiments, and energy dependent electrospray mass spectrometry. These experiments reveal not only oxygen atom transfer but also loss of a chlorine atom from the resulting species.

Efficient mixing of the solar nebula from uniform Mo isotopic composition of meteorites.

The abundances of elements and their isotopes in our Galaxy show wide variations, reflecting different nucleosynthetic processes in stars and the effects of Galactic evolution. These variations contrast with the uniformity of stable isotope abundances for many elements in the Solar System, which implies that processes efficiently homogenized dust and gas from different stellar sources within the young solar nebula. However, isotopic heterogeneity has been recognized on the subcentimetre scale in primitive meteorites, indicating that these preserve a compositional memory of their stellar sources. Small differences in the abundance of stable molybdenum isotopes in bulk rocks of some primitive and differentiated meteorites, relative to terrestrial Mo, suggest large-scale Mo isotopic heterogeneity between some inner Solar System bodies, which implies physical conditions that did not permit efficient mixing of gas and dust. Here we report Mo isotopic data for bulk samples of primitive and differentiated meteorites that show no resolvable deviations from terrestrial Mo. This suggests efficient mixing of gas and dust in the solar nebula at least to 3 au from the Sun, possibly induced by magnetohydrodynamic instabilities. These mixing processes must have occurred before isotopic fractionation of gas-phase elements and volatility-controlled chemical fractionations were established.

On the oxidation behaviour of MoSi 2

Abstract The oxidation behaviour of MoSi 2 has been analyzed on the basis of thermodynamics and kinetics. Thermodynamically speaking silica is in equilibrium with any of the stable molybdenum silicides. At low temperatures, the limited bulk diffusion in MoSi 2 makes it impossible to sustain a sufficient supply of silicon for the formation of a continuous silica layer. This leads to the simultaneous-oxidation of silicon and molybdenum, forming a scale of mixed oxides. The formation of molybdenum oxides leads to ruptures owing to the significant volume change and accelerates the oxidation, leading to the pest phenomenon.

Synthesis and optical properties of highly fluorescent meta– para oligo-phenylenevinylenes

Abstract The synthesis of unsubstituted meta – para oligo-phenylenevinylene ( mp -OPV) was carried out via metathesis polycondensation (acyclic diene metathesis polymerisation ADMET) of commercially available divinylbenzene (55% in ethylvinylbenzene). A stable molybdenum carbene complex of the Schrock type served as initiator. The obtained mp -OPV, which is an intensively yellow powder, with a degree of polymerisation of 2–6 is soluble in CHCl 3 . This oligomer shows a photoluminescence quantum efficiency >50% and λ em lies in the blue-green range.

Catálise eletroquímica da redução do bromato em superfícies modificadas por filmes de molibdênio

In this paper some studies concerning the electroreduction of Mo(VI) in sulphuric acid solutions are described. We have shown that at suitable experimental conditions very stable molybdenum oxide films can be electrochemically deposited at glassy carbon electrodes, the reduction of bromate occurring at less negative potentials on the modified surface. Coulometric experiments have shown that bromide is not the only product of the catalytic bromate reduction by the molybdenum film and species like BrO2 may have part in this process. Based on chronoamperometric curves recorded at -0.60 V, analytical curves have been obtained for the reduction of bromate in the 0.1 - 0.8 mM range, a limit of detection of 20 µM for bromate being determined.

Kinetics of Formation of Molybdenum Selenides from Modulated Reactants and Structure of the New Compound Mo(3)Se.

The kinetics of compound formation in the molybdenum-selenium system has been investigated using elementally modulated reactants to control overall composition and diffusion length. We observed the facile formation of MoSe(2) at low temperatures when the composition was above 50 atom % selenium. No evidence was found for the low-temperature formation of the other known stable molybdenum selenide, the cluster compound Mo(6)Se(8). When the composition of the initially modulated reactant was close to 25% selenium, a previously unreported compound was observed to form. This new compound, Mo(3)Se, has the A-15 crystal structure. The superconducting transition temperature appears to be very sensitive to composition, with a sharp resistive transition at 7 K in one sample and a sharp diamagnetic transition observed in a second sample at 2.2 K. The kinetics of phase formation in this system is analyzed in terms of nucleation kinetics.

Synthesis of thermotropic liquid crystalline all‐Trans‐p‐Phenylenevinylene oligomers via two‐step metathesis

AbstractLiquid crystalline products of oligomeric p‐phenylenevinylene derivates were prepared metathetically using a stable molybdenum carbene complex (Mo(NArMe2)(CHCMe2Ph)[OCMe(CF3)2]2). The synthesis was conducted via two steps: 1. Metathesis polycondensation of 2, 5‐dialkyl‐1, 4‐divinylbenzenes to oligo‐(2, 5‐dialkyl‐1, 4‐phenylenevinylene)s, 2. Endcapping of the vinyl groups via cross‐metathesis using 3‐hexene. The products were characterized by1H‐NMR and UV/Vis spectroscopy, as well as by DSC.

(p,γ)rates of92Mo,94Mo,95Mo,98Mo: Towards an experimentally founded database forp-process studies

The proton capture cross sections of the stable molybdenum isotopes 92, 94, 95, and 98 have been measured by means of the activation method in the proton energy range between 1.5 and 3 MeV. Thin layers of natural molybdenum were irradiated at the Karlsruhe 3.75 MV Van de Graaff accelerator with proton beams of 20{endash}55{mu}A. The activity induced by (p,{gamma}) reactions was measured with a calibrated HPGe detector. In this way, six cross sections for populating ground states and isomers in four different Tc isotopes could be determined simultaneously. The systematic uncertainties of this method are typically 4{percent}. The fact that the resulting cross sections are 2{endash}4 times larger than recent statistical model predictions illustrates the need of experimental data for the yet unexplored field of (p,{gamma}) reactions in the mass region beyond A=90. The present results are important for a quantitative discussion of the production of the abundant p nuclei {sup 92}Mo, {sup 94}Mo, {sup 96}Ru, and {sup 98}Ru during SNIa explosions. They represent also a first step in establishing parameter systematics that allow for more reliable model extrapolations into the region of unstable nuclei of relevance for the p process in SNII. {copyright} {ital 1997} {ital The Americanmore » Physical Society}« less

Effect of microstructural and interface modifications on electrical properties of molybdenum silicide thin films formed by rapid thermal annealing

The microstructural and interface modifications associated with the formation of the most stable molybdenum silicide phase from as-deposited thin films of composition , , and and their effects on electrical properties were studied. The interface modifications formed upon silicidation between silicide and silicon substrate were observed to depend on the silicon content in the starting composition. Large interface depths of the order of 150 - 600 Å were observed in the silicide - silicon thin films. The changes in the interface and microstructure were observed to be due to the out-diffusion of the excess silicon available after the formation of the most stable silicide phase towards the interface at higher annealing temperatures and its distribution within the matrix of the silicide thin film at lower annealing temperatures. The effects of these microstructural modifications were clearly reflected in the electrical properties of the silicide thin films such as sudden drop in resistivity, large ideality factor of Schottky diodes and significant change in Schottky barrier heights.

Investigations of stable molybdenum carbene complexes for the metathesis synthesis of dialkylsubstituted poly( p-phenylenevinylene)s (PPVs)

Abstract Different Schrock-type molybdenum carbene complexes with hexafluoroalkoxy-ligands were examined for their suitability for the acyclic diene metathesis (ADMET) polycondensation of 2,5-dialkyl-1,4-divinylbenzenes (DRDVB). Kinetic studies on cross-metathesis showed that catalysts with (2,6-dimethylphenyl)imido ligands are the most active. 1 H-NMR investigations of the reaction of 2,5-diheptyl-1,4-divinylbenzene using Mo(NAr Me 2 )(CHC(Me) 2 Ph)(OC(Me)(CF 3 ) 2 ) 2 as catalyst show the in situ formation of a new carbene species as well as molybdenacyclobutane intermediate.

Metal-catalyzed gasification of carbon

The catalytic hydrogenolysis(gasification) of carbon by dihydrogen in the presence of molybdenum carbide and platinum has been studied around 900 K. As indicated by X-ray absorption, the catalytic phases during the reaction were stable molybdenum carbide or slightly electron-deficient platinum. The rates of hydrogenolysis with molybdenum carbide were only slightly less than those with platinum, one of the most active metals for the reaction. However, these two types of catalyst showed different kinetic parameters and different behavior of catalyst particles during the reaction.

Determination of stable molybdenum isotopes by thermionic mass spectrometry for tracer studies of molybdenum plant metabolism

A tracer technique based on multiple stable Mo isotopes and thermionic quadrupole mass spectrometry for isotopic analysis of plant tissue was developed and has been applied in the long-term study of foliar absorption of Mo by potato plants. As several tracers have been used the multivariate linear regression methods has been applied to calculate the portions of tracer Mo present in the potato samples from the isotopic ratios measured and to estimate their reproducibilities.

Response to a single oral test of molybdenum stable isotopes for absorption studies in humans

Two volunteer subjects were given orally enriched solutions of Mo-95 and Mo-96 respectively. Blood samples were drawn at various times following the tracer administration.The Mo-95 and Mo-96 content in plasma samples was determined by proton nuclear activation and the response to the single oral test of enriched stable molybdenum isotopes was determined. Assuming a simple two-open-compartment model where the first compartment is the gastrointestinal tract and the other is the plasma, an indicative value of the fractional intestinal absorption for the two subjects is given.The feasibility of direct quantitative measurements of Mo intestinal absorption by the double-tracer technique, using stable tracers, is evidenced.

Transition metal complexes with sulfur ligands LXXXVII. Coordination of CO to high valent molybdenum sulfur centers: syntheses, properties and structure of stable molybdenum(IV) thiolato carbonyl complexes

Thermal thioether- S -desalkylation of the molybdenum( II ) carbonyl complexes [Mo(CO) 2 (PR 3 )(‘ bu S 4 ’)] (R=Ph, Me; ‘ bu S 4 ’ 2− = 1,2-bis(2-mercapto-3,5-di-t-butylphenylthio)ethane(2−)) yields the molybdenum(IV) carbonyl complexes [Mo(CO)(PR 3 )(‘ bu S 2 ’) 2 ] (‘ bu S 2 ’ 2− =3,5-di-t-butyl-1,2-benzenedithiolate(2−)). X-ray structure analysis of [Mo(CO)(PPh 3 )(‘ bu S 2 ’) 2 ] ( 1 ) shows a pseudo trigonal prismatic coordination of the molybdenum center by a S 4 CP donor set. NMR spectra indicate that 1 and [Mo(CO)(PMe 3 )(‘ bu S 2 ’) 2 ] ( 2 ) possess trans octahedral structures in solution. Thermal desalkylation of [Mo(CO) 3 (‘ bu S 4 ’)] at room temperature in high vacuo yields [Mo(CO) 2 (‘ bu S 2 ’) 2 ] ( 4 ), the first molybdenum(IV) carbonyl complex displaying only biologically relevant thiolate donors in its coordination sphere and being stable at room temperature.

Selectivity in the propargylation of polyfunctional amines by (propargylium)-Co 2(CO) 6+ and -(C 5H 5) 2Mo 2(CO) 4+

Abstract [(HCCCH 2 )Co 2 (CO) 6 ]BF 4 ( 1 ) [(HCCCH 2 )Mo 2 (C 5 H 5 ) 2 (CO) 4 ]BF 4 ( 2 ) react with primary and secondary amines in CH 2 Cl 2 solution. In the case of primary amines, the products of mono and dialkylation are obtained. The more stable (less reactive) molybdenum complexes can be employed to alkylate amines in a protic, two-phase medium. The results are interpreted mechanistically and shown to dependent on the thermodynamic stability of the metal-stabilized carbocation.

The application of strongly reducing agents in flow injection analysis Part 6. Molybdenum(III)

The application of molybdenum(III) as reducing agent in flow injection analysis is described. Molybdenum(III), which is unstable to oxidation by air, is generated in-line from the stable molybdenum(VI) by means of a Jones reductor column. With spectrophotometric detection, iodate, uranium(VI), and vanadium(V) and nitrite can be determined in the concentration range 5 x 10−5 x 10−3 M at an injection rate of 3 min−1. Amperometric detection of nitrite is also described.

Organophosphorus acid interferences in flame atomic absorption spectrometry

The interference of the organophosphorus acids, 1-hydroxyethane-1,1-bisphosphonic acid, aminotris(methylenephosphonic acid), ethylenediaminetetrakis(methylenephosphonic acid) and hexamethylenediaminetetrakis(methylenephosphonic acid) on the determination of eighteen metal ions by flame atomic absorption spectrometry is reported. Comparisons with the effect of orthophosphoric acid reveal similarities and distinct differences in their interfering effects. In the air/acetylen flame, depressive interferences are attributed to the formation of phosphates, M 3 (PO 4 ) 2 , or hydroxynpatite-like compounds, M 5 (OH)(PO 4 ) 3 , in the flame aerosol particles for Mg, Ca, Sr, Ba, Mn, Co and Ni. Iron(III) and chromium(III) appear to form stable oxide phosphates, M 2 O 3 •MPO 4 or M 3 O 4 •MPO 4 . Evidence for the formation of stable molybdenum carbides, MoC and MoC 2 , is also presented. In the nitrous oxide/acetylene flame, serious interferences perssisted only for molybdenum but were eliminated by the addition of sodium sulphate.

Determination of the specific activity of fission-based 99Mo using differential pulse polarography

The specific activity of fission-based 99Mo is significantly lower than the theoretical “carrier-free” value because of the contribution from a number of stable molybdenum isotopes which are formed concurrently during irradiation of the uranium target. A rapid analytical technique employing differential pulse polarography was devised to measure the concentrations of molybdenum, and hence the specific activity of 99Mo, in small volumes of radioactive sample. Submicrogram quantities of the analyte can be measured by using the catalytic reduction wave of molybdenum in a nitric acid-ammonium nitrate based supporting electrolyte. Measured specific activities range from 3.1 × 104 to 5.4 × 104 Cig−1; these values were in close agreement with the theoretically computed values.

Accurate spectrophotometric method for the determination of silica in rocks, minerals and related materials

Two main spectrophotometric methods have been used to determine silica: those using the yellow molybdosilicic acids and those using the molybdenum blue complex. Conditions for the production of a stable molybdenum blue complex have been re-investigated. Using suitable conditions, and a modern spectrophotometer with high reproducibility, a rapid and accurate method for the determination of silica has been developed. An important feature of the procedure is the inclusion of an “activation” stage for the silicic acid before complex formation.The method has been applied to a variety of international standards and gives accurate results with a coefficient of variation of 0.5% at the 50% silica level.

Systematics of nuclear charge radii of the stable molybdenum isotopes from muonic atoms

The results of precise measurements of the energies of the 2p32−1s12and 2p12−1s12 muonic X-ray transitions of 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo and 100Mo are reported. The data were analyzed in terms of the Barrett moments 〈rke−αr〉 of the nuclear charge distributions from which the equivalent nuclear radii Rk and the differences ΔRk between neighboring isotopes were computed. Systematic shell effects have been observed at the neutron numbers N = 50 and N = 56.