Complexation of Pu(IV) with the natural siderophore desferrioxamine B and the redox properties of Pu(IV)(siderophore) complexes

The bioavailability and mobility of Pu species can be profoundly affected by siderophores and other oxygen-rich organic ligands. Pu(IV)(siderophore) complexes are generally soluble and may constitute with other soluble organo-Pu(IV) complexes the main fraction of soluble Pu(IV) in the environment. In order to understand the impact of siderophores on the behavior of Pu species, it is important to characterize the formation and redox behavior of Pu(siderophore) complexes. In this work, desferrioxamine B (DFO-B) was investigated for its capacity to bind Pu(IV) as a model siderophore and the properties of the complexes formed were characterized by optical spectroscopy measurements. In a 1:1 Pu(IV)/DFO-B ratio, the complexes Pu(IV)(H2DFO-B)4+, Pu(IV)(H1DFO-B)3+, Pu(IV)(DFO-B)2+, and Pu(IV)(DFO-B)(OH)+ form with corresponding thermodynamic stability constants log beta1,1,2 = 35.48, log beta1,1,1 = 34.87, log beta1,1,0 = 33.98, and log beta1,1,-1 = 27.33, respectively. In the presence of excess DFO-B, the complex Pu(IV)H2(DFO-B)22+ forms with the formation constant log beta2,1,2 = 62.30. The redox potential of the complex Pu(IV)H2(DFO-B)22+ was determined by cyclic voltammetry to be E1/2 = -0.509 V, and the redox potential of the complex Pu(IV)(DFO-B)2+ was estimated to be E1/2 = -0.269 V. The redox properties of Pu(IV)(DFO-B)2+ complexes indicate that Pu(III)(siderophore) complexes are more than 20 orders of magnitude less stable than their Pu(IV) analogues. This indicates that under reducing conditions, stable Pu(siderophore) complexes are unlikely to persist.     

The electrooxidation behavior of bilirubin in DMF and DMF + H_2O mixed solvent

The electrooxidation of Bilirubin (BR) in N,N'-dimethylformamide (DMF) and DMF + H_2O mixed solvent is investigated by voltammetry techniques, in situ rapid scan thin layer spectroelectrochemistry and in situ ESR spectroscopy. The data reveal that the oxidation process of BR undergoes many stages, which are all sped up by the introduction of water. The species of bilirubin taking part in the reaction is found to be changed from BR in DMF into BR' in the mixed solvents and the anodic peak potential is shifted correspondingly from +0.58 V to +0.026-+0.35 V (vs. Ag/AgCl, 1.0 M KCl). Free radicals and the dimerization of them are observed during the oxidation.     

EDTA and mixed-ligand complexes of tetravalent and trivalent plutonium

EDTA forms stable complexes with plutonium that are integral to nuclear material processing, radionuclide decontamination, and the potentially enhanced transport of environmental contamination. To characterize the aqueous Pu(4+/3+)EDTA species formed under the wide range of conditions of these processes, potentiometry, spectrophotometry, and cyclic voltammetry were used to measure solution equilibria. The results reveal new EDTA and mixed-ligand complexes and provide more accurate stability constants for previously identified species. In acidic solution (pH < 4) and at 1:1 ligand to metal ratio, PuY (where Y4- is the tetra-anion of EDTA) is the predominant species, with an overall formation constant of log beta110 = 26.44. At higher pH, the hydrolysis species, PuY(OH)- and PuY(OH)(2)2-, form with the corresponding overall stability constants log beta(11 - 1) = 21.95 and log beta(11 - 2) = 15.29. The redox potential of the complex PuY at pH = 2.3 was determined to be E(1/2) = 342 mV. The correlation between redox potential, pH, and the protonation state of PuY- was derived to estimate the redox potential of the Pu(4+/3+)Y complex as a function of pH. Under conditions of neutral pH and excess EDTA relative to Pu4+, PuY(2)4- forms with an overall formation constant of log beta120 = 35.39. In the presence of ancillary ligands, mixed-ligand complexes form, as exemplified by the citrate and carbonate complexes PuY(citrate)3- (log beta1101 = 33.45) and PuY(carbonate)2- (log beta1101 = 35.51). Cyclic voltammetry shows irreversible electrochemical behavior for these coordinatively saturated Pu4+ complexes: The reduction wave is shifted approximately -400 mV from the reduction wave of the complex PuY, while the oxidation wave is invariant.     

The role of monomers and dimers in the reduction of ruthenium(II) complexes of redox-active tetraazatetrapyridopentacene ligand

A combination of electrochemistry, spectroelectrochemistry, and 1H NMR has been used to study the reduction and solution speciation in acetonitrile of two mononuclear Ru complexes containing the redox-active 9,11,20,22-tetraazatetrapyrido [3,2-a:2',3'-c:3' ',2' '-l:2' ',3' '-n]pentacene (tatpp) ligand. These complexes, [(bpy)2Ru(tatpp)][PF6]2 (1[PF6]2), and [(phen)2Ru(tatpp)][PF6]2 (2[PF6]2) (where bpy is 2,2'-bipyridine and phen is 1,10-phenanthroline), form pi-pi stacked dimers (e.g., pi-{1}24+ and pi-{2}24+) in solution as determined by 1H NMR studies in an extended concentration range (90 - 5000 microM) as well as via simulation of the electrochemical data. The dimerization constant for 12+ in acetonitrile is 2 x 10(4) M(-1) as determined from the NMR data. Slightly higher dimerization constants (8 x 10(4) M(-1)) were obtained via simulation of the electrochemical data and are attributed to the presence of the supporting eletrolyte. Electrochemical and spectroelectrochemical data show that the pi-pi stacked dimers are electroreduced in two consecutive steps at -0.31 and -0.47 V vs Ag/AgCl, which is assigned to the uptake of one electron by each tatpp ligand in pi-{1}24+ to give first pi-{1}23+and then pi-{1}22+. At potentials negative of -0.6 V, the electrochemical data reveal two different reaction pathways depending on the complex concentration in solution. At low concentrations (< or =20 microM), the next electroreduction occurs on a monomeric species (e.g., [(bpy)2Ru(tatpp)]+/0) showing that the doubly reduced pi-pi dimer (pi-{1}22+ and pi-{2}22+) dissociates into monomers. At high concentrations (> or =100 microM), reduction of pi-{1}22+ or pi-{2}22+ induces another dimerization reaction, which we attribute to the formation of a sigma-bond between the radical tatpp ligands and is accompanied by the appearance of a new peak in the absorption spectrum at 535 nm. This new sigma-dimer can undergo one additional tatpp based reduction to form sigma-{1}20 or sigma-{2}20, in which the tatpp-bridged assembly is the site of all four reductions. Finally, potentials negative of -1.2 V result in the electroreduction of the bpy or phen ligands for complexes 12+ or 22+, respectively. For the latter complex 22+, this process is accompanied by the formation of an electrode adsorbed species.     

Preparation, stability, and structural characterization of plutonium(VII) in alkaline aqueous solution

A freshly prepared solution of Pu(VI) in 2 M NaOH was oxidized to Pu(VII), via ozonolysis, while simultaneously collecting X-ray absorption spectra. Analyses of the XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) data, acquired throughout the in situ experiments, show a dioxo coordination environment for Pu(VI), PuO(2)(2+), typical for it and the hexavalent actinyl species of U and Np, and its evolution into a tetraoxo-coordination environment for Pu(VII), PuO(4)(-), like that known for Np(VII). The EXAFS data provide average Pu-O distances of 1.79(1) and 1.88(1) ?, respectively. The second coordination shells, also fit as O atoms, provide Pu-O distances of 2.29-2.32 ? that are independent of the Pu oxidation state. The coordination numbers for the distant O atoms in sums with those for the nearest O atoms are consistent with 6-O environments for both Pu(VI) and Pu(VII) ions in accordance with their previously proposed speciation as [Pu(VI)O(2)(OH)(4)](2-) and [Pu(VII)O(4)(OH)(2)](3-), respectively. This solution speciation accounts precisely for the Pu(VI) and Pu(VII) coordination environments reported in various solid state structures. The Pu(VII) tetraoxo-dihydroxo anion was found to have a half-life of 3.7 h. Its instability is attributed to spontaneous reduction to Pu(VI) and not to a measurable extent of disproportionation. We found no direct evidence for Pu(VIII) in the X-ray data and, furthermore, the stoichiometry of the oxidation of Cr(III) by Pu is consistent with that expected for a valence-pure Pu(VII) preparation by ozonation and, in turn, stoichiometrically equivalent to the established Np(VII)/Cr(III) redox reaction.     

Biphasic kinetic investigations on the evaluation of non-salt forming reductants for Pu(IV) stripping from tributyl phosphate and N,N-dihexyl octanamide solutions in n-dodecane

The relative stability of different oxidation states of actinide elements is influenced by the nature of complexes formed and redox equilibria in aqueous/non-aqueous solutions. The reduction/stripping studies on Pu(IV) ions from loaded organic phases of 1.1 M tributyl phosphate and of 1.1 M N,N-dihexyl octanamide in n-dodecane were studied using organic soluble tert-butyl hydroquinone (TBH) and aqueous soluble reductants like acetaldoxime (AX) and hydroxyurea (HU). These studies were carried out as a function of reductant and nitric acid concentration (0.5–4 M HNO₃) and of time. The changes in Pu oxidation states were followed by spectrophotometry for TBH and by distribution ratio values for AX and HU as reductants. Spectrophotometric investigations using TBH as reductant showed that it was desirable to strip Pu(III) formed after reduction of Pu(IV) in the organic phase, which may otherwise be reconverted to extractable Pu(IV) by in situ generated HNO₂ from oxidative degradation of TBH to tert-butyl quinone. Similarly, the biphasic reduction/stripping of Pu(IV) using AX and HU as reductant rate was affected adversely with increased aqueous phase acidity. This data will help in the accurate simulation of Pu separation processes using these reductants in mixer-settlers/pulsed columns or centrifugal contactors.     

Stabilization of plutonium(III) in the Preyssler polyoxometalate

The Na(+) ion encapsulated within the Preyssler heteropolyoxoanion, [NaP5W30O110](14-), was exchanged with Pu(III) under hydrothermal conditions to obtain [Pu(III)P5W30O110](12-) (abbreviated [PuPA](12-)) with hybrid electrochemical properties resulting from the combination of the key redox behaviors of the Pu cation and the P-W-O anion. The electroanalytical chemistry of this two-center, multielectron redox system in a 1 M HCl electrolyte shows that Pu(III) is oxidized to Pu(IV) at the half-wave potential, E(1/2), of +0.960 V versus Ag/AgCl, which is 0.197 V more positive than the corresponding electrode potential for the Pu(III) aqua ion also in 1 M HCl, indicating the stabilization of the trivalent Pu cation by its encapsulation in the Preyssler polyoxometalate (POM). This effect is uncommon in actinide-POM chemistry, wherein electrode potential shifts of the opposite nature (to more negative values), leading to the stabilization of the tetravalent ions by complexation, are renowned. Moreover, in cyclic voltammetry measurements of the Pu(III) aqua ion and [PuPA](12-), the peak currents, i(p), for the one-electron Pu(III)/Pu(IV) processes show different dependencies with the scan rate, nu. The former shows proportionality with nu(1/2), indicating freely diffusing species, whereas the latter shows proportionality with nu, indicating a surface-confined one. The first of the five successive two-electron, W-centered reduction processes in [PuPA](12-) occurs at E(1/2) = -0.117 V versus Ag/AgCl, which is 1.077 V less than the E(1/2) for the Pu(III)/Pu(IV) oxidation, thereby providing an experimental, electrochemical measure of the highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap, which compares well with values previously obtained by density-functional theory, complete active space-self consistent field, and post-Hartree-Fock calculations for a series of M(n+)-exchanged systems, [MPA](n-15) for 1 < or = n < or = 4 (Fernandez, J. A.; Lopez, X.; Bo, C.; de Graff, C.; Baerends, E. J.; Poblet, J. M. J. Am Chem. Soc. 2007, 129, 12244-12253). It was not possible to prepare the Np-exchanged Preyssler anion in the manner of [PuPA](12-), because of the instability of tri- and tetravalent Np to oxidation and the formation of the neptunyl(V) ion, which also could not be exchanged for Na(+).     

Probing the influence of X-rays on aqueous copper solutions using time-resolved in situ combined video/X-ray absorption near-edge/ultraviolet-visible spectroscopy

Time-resolved in situ video monitoring and ultraviolet-visible spectroscopy in combination with X-ray absorption near-edge spectroscopy (XANES) have been used for the first time in a combined manner to study the effect of synchrotron radiation on a series of homogeneous aqueous copper solutions in a microreactor. This series included both non biologically relevant (pyridine, bipyridine, neocuproine, terpyridine, dimethylpyridine, ammonia, ethylenediamine, and 1,10-phenanthroline) and biologically relevant (histidine, glycine, and imidazole) ligands. It was found that when water is present as solvent, gas bubbles are formed under the influence of the X-ray beam. At the liquid-gas interface of these bubbles, in particular cases colloidal copper nanoparticles are formed. This reduction process was found to be influenced by the type of copper precursor salt (SO(4)(2-), NO(3)(-), and Cl(-)), the ligands surrounding the copper cation, and the redox potential of the copper complexes (ranging between +594 and -360 mV). In other words, in some cases, no reduction was encountered (e.g., ammonia in the presence of SO(4)(2-) and NO(3)(-)), whereas in other cases reduction to either Cu(+) (neocuproine with SO(4)(2-)) or Cu(0) (e.g., histidine and imidazole both with SO(4)(2-), NO(3)(-), and Cl(-)) was observed. These results illustrate the added value of video spectroscopy for the interpretation of in situ XANES studies. Not only do the results give an illustration of the parameters that are important in the redox processes that occur in biological systems, they also show the potential problems associated with studying catalytic processes in aqueous solutions by XANES spectroscopy.     

Determining the distribution of Pu, Np, and U oxidation states in dilute NaCl and synthetic brine solutions

The effect of iron powder (Fe⁰) on the reduction of Pu(VI),Np(V), and U(VI) was investigated in dilute NaCl and synthetic brines. Thetotal concentrations and oxidation states of the actinides in these solutionswere monitored as functions of pC H +, Eh, and time using techniques includingVis/Near IR absorption spectrophotometry, solvent extraction, activity counting,and inductively coupled plasma spectroscopy-mass spectrometry (ICP-MS). Whenconcentrations were too low and the oxidation states could not be directlydetermined by spectrophotometry or solvent extraction, comparing the measuredconcentrations with the solubility of reference systems helped to define thefinal oxidation states. In general, the reduction was more rapid, and couldproceed further, in the dilute NaCl solution than in the brine solutions.The experimental observations can be summarized as follows: (1) in the diluteNaCl solutions (pC H + 7 to 12), all three actinides, Pu(VI), Np(V) and U(VI),were reduced to lower oxidation states (most likely the tetravalent state)within a few days to a few months in the presence of Fe⁰; (2) insynthetic brines containing Fe⁰ (pC H + 8 to 13), the reductionof Pu(VI) was much slower than in the dilute NaCl solution. The dominant oxidationstate of Pu in the brine solution was Pu(V), the concentration of which wascontrolled by the electrochemical potential and could probably be representedby a heterogeneous redox reaction PuO₂ . xH₂O(s) PuO₂ + +e ^—; (3) in synthetic brines containing Fe⁰ (pC H + 8 to 13), Np(V) was probably reduced to Np(IV) and precipitatedfrom the solution; (4) in synthetic brines containing Fe⁰ (pC H+ 8 to 13), no significant reduction of U(VI) was observed within 55 days.     

Versatile diastereoselectivity in formal [3,3]-sigmatropic shifts of substituted 1-alkenyl-3-alkylidenecyclobutanols and their silyl ethers

A new method for the preparation of highly substituted cyclohexenones is reported. [2 + 2] Cycloaddition of 2-silyloxydienes with allenecarboxylate affords the 1-alkenyl-3-alkylidenecyclobutanol silyl ethers. Thermolysis of these compounds affords the methylene cyclohexenyl silyl ethers with excellent exo selectivity (>95:5) when monosubstituted alkenyl groups are used, while the use of disubstituted alkenyl groups gives generally low selectivity ( approximately 2:1). However, rearrangement of the anion of the cyclobutanol (prepared by acidic hydrolysis of the TMS silyl ether) at low temperature gives the endo product with good to excellent diastereoselectivity (5-23:1). Two different mechanistic rationales are given for the two different processes: the first via a diradical and the second via a cleavage intramolecular Michael addition. Thus, the same starting material (e.g., 20) can be converted into either the exo or endo product, 22x or 22n, with good diastereocontrol by just changing the rearrangement conditions.     

Uranium and Plutonium Extraction by N,N-dialkylamides Using Multistage Mixer-settler Extractors

N,N-Dialkylamides (monoamides) are known as extractants for U and Pu, and many studies have been carried out mainly by single-stage batch method. We have focused on two monoamides: N,N-di(2-ethylhexyl)-2,2-dimethylpropanamide (DEHDMPA) and N,N-di(2-ethylhexyl)butanamide (DEHBA), and proposed a multistage extraction process for recovering U and Pu by these monoamides. A continuous counter-current experiment was carried out to demonstrate the validity of this process. This process consisted of two cycles, and the 1st cycle and the 2nd cycle employed DEHDMPA and DEHBA as extractants, respectively. The feed solution for the 1st cycle was 5.1 mol/dm³ (M) nitric acid containing 0.92 M U, 1.6 mM Pu, and 0.6 mM Np. The raffinate collected in the 1st cycle was used as the feed for the 2nd cycle. The ratios of U recovered in the U fraction and U-Pu fraction were 99.1% and 0.8%, respectively, and the ratios of U in the used solvents were <0.04%. The ratio of Pu recovered in the U-Pu fraction was 99.7%, and the ratio of Pu in the used solvents was in the order of 10^–3–10^–4%. The concentration ratio of U with respect to Pu in the U-Pu fraction was 9, and this indicated that Pu was not isolated. The decontamination factor of U with respect to Pu in the U fraction was obtained as 4.5×10⁵. These results supported the validity of the proposed process.     

Electrochemical and spectroscopic studies of Pu(IV) and Pu(III) in nitric acid solutions

Electrochemical and absorption spectroscopic properties of Pu(IV) and Pu(III) in nitric acid have been investigated by using cyclic voltammetry (CV) and UV–Visible spectroscopy. CV using a glassy carbon electrode suggested that the electrochemical reaction of Pu(IV) nitrate complexes were found to be a quasi-reversible reduction to Pu(III) species. The formal redox potentials (E ⁰) for Pu(IV)/Pu(III) couples were +0.721, +0.712, +0.706, +0.705, +0.704, 0.694, and +0.696 V (vs. Ag/AgCl) when nitric acid concentrations are 1–7 M nitric acid solutions, respectively. These results indicate that the reduction product of Pu(IV) is only Pu(III). Further details for reaction mechanism of Pu(IV) were discussed on the basis of digital simulation of the experimental cyclic voltammograms. The absorption spectroscopic properties of Pu(III) and Pu(IV) in nitric acid solutions were investigated with UV–Visible spectrophotometry. As a result, it was founds that the intensities of the characteristic absorption peaks of Pu(III) and Pu(IV) tend to decrease with increasing nitric acid concentration for 1–8 M, and the peaks positions shifted longer or shorter wavelengths depending on the complex-forming abilities of Pu(III) and Pu(IV) with an increase in the nitric acid concentration.     

A preliminary study to assess the effect of some seawater components on the speciation of plutonium

Since Pu(IV) and Pu(V) exhibit very different sediment sorption behaviour, the transport of Pu in the aquatic environment is dependant upon oxidation state and the rate of interconversion between the species. A number of laboratory experiments have been carried out to determine possible parameters which influence the rate of Pu redox reactions and the extent of sorption by suspended particulate in the marine environment. Results suggest that, although the initial sorption of Pu(IV) did not appear to be dependant upon the major cations present in seawater, the sorption of Pu(V) was decreased in the presence of Ca²⁺ and Mg²⁺ ions. Both the rates of oxidation of dissolved Pu(IV) and reduction of dissolved Pu(V) increased with increasing suspended particulate concentration.     

Adsorption of plutonium oxide nanoparticles

Adsorption of monodisperse cubic plutonium oxide nanoparticles ("Pu-NP", [Pu(38)O(56)Cl(x)(H(2)O)(y)]((40-x)+), with a fluorite-related lattice, approximately 1 nm in edge size) to the muscovite (001) basal plane from aqueous solutions was observed in situ (in 100 mM NaCl background electrolyte at pH 2.6). Uptake capacity of the surface quantified by α-spectrometry was 0.92 μg Pu/cm(2), corresponding to 10.8 Pu per unit cell area (A(UC)). This amount is significantly larger than that of Pu(4+) needed for satisfying the negative surface charge (0.25 Pu(4+) for 1 e(-)/A(UC)). The adsorbed Pu-NPs cover 17% of the surface area, determined by X-ray reflectivity (XR). This correlates to one Pu-NP for every 14 unit cells of muscovite, suggesting that each particle compensates the charge of the unit cells onto which it adsorbs as well as those in its direct proximity. Structural investigation by resonant anomalous X-ray reflectivity distinguished two different sorption states of Pu-NPs on the surface at two different regimes of distance from the surface. A fraction of Pu is distributed within 11 ? from the surface. The distribution width matches the Pu-NP size, indicating that this species represents Pu-NPs adsorbed directly on the surface. Beyond the first layer, an additional fraction of sorbed Pu was observed to extend more broadly up to more than 100 ? from the surface. This distribution is interpreted as resulting from "stacking" or aggregation of the nanoparticles driven by sorption and accumulation of Pu-NPs at the interface although these Pu-NPs do not aggregate in the solution. These results are the first in situ observation of the interaction of nanoparticles with a charged mineral-water interface yielding information important to understanding the environmental transport of Pu and other nanophase inorganic species.     

Electrode processes of plutonium ions in phosphate media

Summary The electrolytic behaviour of plutonium ions in a mixture of phosphate and nitrate solutions was studied by flow-coulometry with column electrodes of glassy carbon(GC)-fibers and voltammetry with a GC disc electrode, and compared with that in phosphate-free media. The redox processes, PuO ₂ ²⁺ /PuO ₂ ⁺ and Pu⁴⁺/Pu³⁺, were demonstrated to be reversible even in the phosphate media and their half wave potentials shifted more negatively due to the formation of PuO₂(H₂PO₄)⁺ and Pu(HPO₄)₂. The rate of the irreversible reduction of PuO ₂ ⁺ to Pu³⁺ increased in the presence of phosphoric acid and the quantitative reduction was attained with the column electrode even at +0.35 V vs. saturated KCl-Ag/AgCl. The reduction process of PuO ₂ ⁺ was elucidated considering an intermediate Pu(IV)-species, PuO²⁺, which decomposed into Pu⁴⁺ by a post-chemical reaction. Analytical advantages of the use of phosphate media are discussed.     

Progress Towards the Development of a New GANEX Process

We report first results on the development of a new system for a GANEX 2^nd cycle developed by the “ACSEPT” project. A solvent system consisting of 0.2 M N,N,N’,N’-tetra-n-octyl-diglycolamide (TODGA) + 0.5 M N,N’-dimethyl-N,N’-dioctyl-2(2-hexyloxyethyl)malonamide (DMDOHEMA) in kerosene was formulated which co-extracts TRU elements and lanthanides with high distribution ratios and allows for Pu(IV) loading up to ∼40 gáL^–1 at 3 M HNO3. Neptunium redox chemistry and extraction in the TODGA/DMDOHEMA phase has been studied in order to identify optimum conditions for Np extraction. Further, the behaviour of non-lanthanide fission products was investigated to identify a substitute for oxalic acid (used as a Zr masking agent in DIAMEX processes where there is no Pu in the feed). 1,2-cyclohexanediaminetetraacetic acid (CDTA) efficiently suppresses the extraction of Zr and also masks Pd which would otherwise be co-extracted.     

Pyrrole-inverted isomer of 5,10,15,20-tetraaryl-21-selenaporphyrin

A novel 5,10,15,20-tetraaryl-21-selenaporphyrin isomer with an inverted pyrrole ring, i.e., 5,10,15, 20-tetraaryl-2-aza-21-carba-22-selenaporphyrin (SeC-TArPH) has been produced by a [3 + 1] condensation of 2, 5-bis(phenylhydroxymethyl)selenophene and 5,10-ditolyltripyrrin. The reaction yielded 5,20-diphenyl-10,15-bis(p-tolyl)-21-selenaporphyrin Se-DPDTPH (19%) and its isomer with an inverted pyrrole ring, i.e., 5,10-diphenyl-15,20-bis(p-tolyl)-2-aza-21-carba-22-selenaporphyrin, SeC-DPDTPH (1%). Mechanistically the synthesis of SeC-DPDTPH requires one beta-condensation at the pyrrole moiety of 5, 10-ditolyltripyrrin instead of the stereotypical alpha-condensation. The identity of inverted selenaporphyrin has been confirmed by high-resolution mass spectrometry and (1)H NMR spectroscopy. A saddle distortion mode for the inverted selenaporphyrin macrocycle SeC-DPDTPH has been determined by X-ray crystallography. NMR spectra are consistent with the existence of tautomeric equilibria that involve three tautomeric species of the neutral form of SeC-DPDTPH. The preference for the tautomer with the labile proton located at the peripheral N(2) nitrogen atom has been detected in pyridine-d(5) solution. The density functional theory (DFT) has been applied to determine the molecular and electronic structure of three tautomers of 2-aza-21-carba-22-selenaporphyrin: 2-N, 23-N, 24-NH, 2-N, 23-NH, 24-N, and 2-NH, 23-N, 24-N formally created from SeC-DPDTPH by a replacement of phenyl and tolyl groups with hydrogen. The total energies calculated using the B3LYP/6-311G//B3LYP/6-311G approach, demonstrate that relative stability of postulated tautomers decreases in the order 2-N, 23-NH, 24-N > 2-N, 23-N, 24-NH > 2-NH, 23-N, 24-N. The small energy differences between tautomeric species suggests their simultaneous presence in equilibrium.     

Determination of (236)U and transuranium elements in depleted uranium ammunition by alpha-spectrometry and ICP-MS

It is well known that ammunition containing depleted uranium (DU) was used by NATO during the Balkan conflict. To evaluate the origin of DU (the enrichment of natural uranium or the reprocessing of spent nuclear fuel) it is necessary to directly detect the presence of activation products ((236)U, (239)Pu, (240)Pu, (241)Am, and (237)Np) in the ammunition. In this work the analysis of actinides by alpha-spectrometry was compared with that by inductively coupled plasma mass spectrometry (ICP-MS) after selective separation of ultratraces of transuranium elements from the uranium matrix. (242)Pu and (243)Am were added to calculate the chemical yield. Plutonium was separated from uranium by extraction chromatography, using tri- n-octylamine (TNOA), with a decontamination factor higher than 10(6); after elution plutonium was determined by ICP-MS ((239)Pu and (240)Pu) and alpha-spectrometry ((239+240)Pu) after electroplating. The concentration of Pu in two DU penetrator samples was 7 x 10(-12) g g(-1) and 2 x 10(-11) g g(-1). The (240)Pu/(239)Pu isotope ratio in one penetrator sample (0.12+/-0.04) was significantly lower than the (240)Pu/(239)Pu ratios found in two soil samples from Kosovo (0.35+/-0.10 and 0.27+/-0.07). (241)Am was separated by extraction chromatography, using di(2-ethylhexyl)phosphoric acid (HDEHP), with a decontamination factor as high as 10(7). The concentration of (241)Am in the penetrator samples was 2.7 x 10(-14) g g(-1) and <9.4 x 10(-15) g g(-1). In addition (237)Np was detected at ultratrace levels. In general, ICP-MS and alpha-spectrometry results were in good agreement.The presence of anthropogenic radionuclides ((236)U, (239)Pu,(240)Pu, (241)Am, and (237)Np) in the penetrators indicates that at least part of the uranium originated from the reprocessing of nuclear fuel. Because the concentrations of radionuclides are very low, their radiotoxicological effect is negligible.     

Formation mechanism of Pt particles by photoreduction of Pt ions in polymer solutions

The formation mechanisms of metal particles (platinum (Pt) particles) in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by the photoreduction method have been studied by transmission electron microscopy (TEM) and in situ and ex situ X-ray absorption fine structure (XAFS) analysis. The average diameter of the dilute and concentrated Pt particles in the PVP solution is estimated from TEM to be 2.0 and 2.5 nm, respectively. XAFS analysis was performed for the reduction process of Pt4+ ions to metallic Pt particles for the Pt L3 edge of the colloidal dispersions of the concentrated Pt solutions. The photoreduction process proceeds by the following steps: (1) reduction of PtCl6(2-) to PtCl4(2-), (2) dissociation of Cl from PtCl4(2-), followed by reduction of Pt2+ ionic species to Pt0, (3) formation of a Pt0-Pt0 bond and particle growth by the association of Pt0-Pt0. The reduction of PtCl4(2-) to Pt0 is a slower process, compared with the reduction of PtCl6(2-) to PtCl4(2-). There is a delay between the disappearance of PtCl4(2-) and the formation of Pt0-Pt0 clusters.     

Controlled potential coulometry: the application of a secondary reaction to the determination of plutonium and uranium at a solid electrode

A method is described for the simultaneous determination of plutonium and uranium in mixed oxides by controlled potential coulometry at a gold working electrode in two stages: first a coulometric oxidation, at 0.73 V vs. a silver/silver chloride electrode, of Pu(III) and U(IV) to Pu(IV) and U(VI) by a combination of a direct electrode reaction and a secondary chemical reaction proceeding concurrently, and secondly, a coulometric reduction at 0.33 V of Pu(IV) to Pu(III), leaving uranium as U(VI). The determination is carried out in a mixture of sulphuric and nitric acids, and Ti(III) is used to reduce plutonium and uranium to Pu(III) and U(IV) before electrolysis. The precision (3sigma) of Pu:U ratio results obtained from mixtures containing about 30% and 2% plutonium was 0.5% and 1-5% respectively. The effect of experimental variables on the time taken to complete the coulometric determination is discussed.