A new method for quantifying 64Cu in nuclear debris samples.

Quantifying 64Cu in post-detonation nuclear debris samples can provide important diagnostic information regarding the structural materials used within a nuclear device. However, this task is challenging due to the weak gamma emissions associated with the decay of 64Cu, its short half-life (12.701 h), and the presence of interfering fission product radioisotopes. Large quantities of debris sample are generally needed to accurately quantify 64Cu, which can be problematic in sample-limited scenarios where other radiometric analyses are required. Herein, we present a new method for the separation of 64Cu from solutions of mixed fission products and demonstrate the quantification of its activity through use of gas-flow proportional beta counting. The new method was validated through a series of rigorous tests and was shown to improve the detection limit of 64Cu by over two orders of magnitude, from 2.5 × 106 to 1.3 × 104 atoms/sample for 100 min measurements.

N-Heterocyclic Carbene to Actinide d-Based π-bonding Correlates with Observed Metal-Carbene Bond Length Shortening Versus Lanthanide Congeners.

Comparison of bonding and electronic structural features between trivalent lanthanide (Ln) and actinide (An) complexes across homologous series' of molecules can provide insights into subtle and overt periodic trends. Of keen interest and debate is the extent to which the valence f- and d-orbitals of trivalent Ln/An ions engage in covalent interactions with different ligand donor functionalities and, crucially, how bonding differences change as both the Ln and An series are traversed. Synthesis and characterization (SC-XRD, NMR, UV-vis-NIR, and computational modeling) of the homologous lanthanide and actinide N-heterocyclic carbene (NHC) complexes [M(C5Me5)2(X)(IMe4)] {X = I, M = La, Ce, Pr, Nd, U, Np, Pu; X = Cl, M = Nd; X = I/Cl, M = Nd, Am; and IMe4 = [C(NMeCMe)2]} reveals consistently shorter An-C vs Ln-C distances that do not substantially converge upon reaching Am3+/Nd3+ comparison. Specifically, the difference of 0.064(6) Å observed in the La/U pair is comparable to the 0.062(4) Å difference observed in the Nd/Am pair. Computational analyses suggest that the cause of this unusual observation is rooted in the presence of π-bonding with the valence d-orbital manifold in actinide complexes that is not present in the lanthanide congeners. This is in contrast to other documented cases of shorter An-ligand vs Ln-ligand distances, which are often attributed to increased 5f vs 4f radial diffusivity leading to differences in 4f and 5f orbital bonding involvement. Moreover, in these traditional observations, as the 5f series is traversed, the 5f manifold contracts such that by americium structural studies often find no statistically significant Am3+vs Nd3+ metal-ligand bond length differences.

Trace Actinide Signatures of a Bulk Neptunium Sample.

In nuclear forensic analyses, measurements of actinide elements in a sample can assist with identifying interdicted or unknown materials. While these radiochemical signatures have been extensively investigated in uranium materials, less is known about bulk neptunium samples. This paper describes the measurement of trace actinide concentrations and isotopic profiles in a 237Np oxide sample. Uranium, plutonium, americium, and curium concentrations and isotopic profiles in the sample were determined and deemed potentially useful for distinguishing different sources of 237Np. Several different potential radiochronometry systems were also investigated; discordant results indicate that the Np sample was never completely purified of other actinide elements, or that subsequent contamination of the sample occurred. Few prior studies of neptunium materials have been reported, and these data suggest that trace actinide constituents could provide unique signatures to identify material out of regulatory control.

Chemical separation and measurement of platinum activation products.

A method has been developed to purify and measure platinum radioisotopes in the presence of fission products and environmental constituents. The method uses a combination of cation exchange and anion exchange chromatography and selective precipitation steps to remove other radioisotopes from the sample. The addition of stable platinum carrier allows for a gravimetric determination of the chemical yield of the procedure. Overall, the method is fast, simple, and potentially applicable for rapid turnaround of unknown samples. Using this method, multiple platinum radioisotopes were measured in two different irradiation experiments. The measured ratios of the platinum radioisotopes clearly reflect the neutron spectrum of the irradiation, suggesting that platinum radioisotopes could be valuable signatures in nuclear forensic analyses.

Nuclear Sample Provenance and Age Determination Using Ruthenium Isotopes.

Measurements of the ruthenium isotopic composition of nuclear samples could provide information about the method of sample production, sample irradiation history, and age. To investigate the feasibility and applicability of this idea, this study focuses on measurements of the ruthenium isotope composition of a nominally single-isotope 106Ru radioactivity standard, where the complications of environmental mixing are eliminated. The measurements of the 106Ru standards reveal unusual stable ruthenium isotopic compositions consistent with fissiogenic ruthenium. Three different lots of the material have been investigated, and the isotopic composition is found to be different for lot 1 as compared to lots 2 and 3, indicating a longer irradiation duration incurred during the production of lot 1. Through measurements of 106Ru and its 106Pd daughter, radiochronometry can be used to infer the ages of the samples. Lot 1 is older than lots 2 and 3 and was produced 4.91(5) years before the reference date of 1/1/21, approximately 2.7 years before lots 2 and 3. In an effort to better understand the sample production pathway, the isotopic measurements are compared with nuclear reactor simulations, which suggest that the material was generated by irradiation of a low-enriched uranium target material in a light water reactor. These findings have significant implications for nuclear treaty monitoring, providing an example of the power of ruthenium isotope measurements to discern details of sample origin and history.

Measurements of extinct fission products in nuclear bomb debris: Determination of the yield of the Trinity nuclear test 70 y later.

This paper describes an approach to measuring extinct fission products that would allow for the characterization of a nuclear test at any time. The isotopic composition of molybdenum in five samples of glassy debris from the 1945 Trinity nuclear test has been measured. Nonnatural molybdenum isotopic compositions were observed, reflecting an input from the decay of the short-lived fission products (95)Zr and (97)Zr. By measuring both the perturbation of the (95)Mo/(96)Mo and (97)Mo/(96)Mo isotopic ratios and the total amount of molybdenum in the Trinity nuclear debris samples, it is possible to calculate the original concentrations of the (95)Zr and (97)Zr isotopes formed in the nuclear detonation. Together with a determination of the amount of plutonium in the debris, these measurements of extinct fission products allow for new estimates of the efficiency and yield of the historic Trinity test.

Energetic Chromophores: Low-Energy Laser Initiation in Explosive Fe(II) Tetrazine Complexes.

The synthesis and characterization of air stable Fe(II) coordination complexes with tetrazine and triazolo-tetrazine ligands and perchlorate counteranions have been achieved. Time-dependent density functional theory (TD-DFT) was used to model the structural, electrochemical, and optical properties of these materials. These compounds are secondary explosives that can be initiated with Nd:YAG laser light at lower energy thresholds than those of PETN. Furthermore, these Fe(II) tetrazine complexes have significantly lower sensitivity than PETN toward mechanical stimuli such as impact and friction. The lower threshold for laser initiation was achieved by altering the electronic properties of the ligand scaffold to tune the metal ligand charge transfer (MLCT) bands of these materials from the visible into the near-infrared region of the electromagnetic spectrum. Unprecedented decrease in both the laser initiation threshold and the mechanical sensitivity makes these materials the first explosives that are both safer to handle and easier to initiate than PETN with NIR lasers.

Independent Control of Optical and Explosive Properties: Pyrazole-Tetrazine Complexes of First Row Transition Metals.

Complexes of 3-amino-6-(3,5-dimethylpyrazole)tetrazine) (NH2TzDMP, 1) and 3-(3,3'-dinitroazetidine)-6-(3,5-dimethylpyrazole)tetrazine) (DNAZTzDMP, 2) with first row transition metal centers were synthesized. Reactions of Fe(II)(H2O)6(BF4)2 and Fe(NO3)3·9H2O with 1 and 2 both led to complexes of the form [(RTzDMP)3Fe]X2 (X = BF4, R = NH2 (3), DNAZ (4); X = NO3, R = NH2 (5), DNAZ (6)), which showed intense MLCT bands in the visible region of the spectrum. Ligands 1 and 2 also reacted with Cu(II)(NO3)2·5/2H2O to form [(RTzDMP)2Cu(NO3)][NO3] (R = NH2 (7), DNAZ (8)) in addition to reacting with Cu(I)(CH3CN)4(PF6) to form [(RTzDMP)2Cu][PF6] (R = NH2 (9), DNAZ (10)). Lastly reactions of 1 and 2 with Co(NO3)2·6H2O and Ni(NO3)2·6H2O led to [(NH2TzDMP)2Co(H2O) (NO3)][NO3] (11), [(DNAZTzDMP)2Co(H2O)2][NO3]2 (12), [(NH2TzDMP)3Ni][NO3]2 (13), and [(DNAZTzDMP)2Ni(H2O)2][NO3]2 (14). The complexes display rich electrochemical and photophysical properties that are unaffected by derivation with explosive groups.

Knocking on wood: base metal complexes as catalysts for selective oxidation of lignin models and extracts.

This work began as part of a biomass conversion catalysis project with UC Santa Barbara funded by the first NSF Chemical Bonding Center, CATSB. Recognizing that catalytic aerobic oxidation of diol C-C bonds could potentially be used to break down lignocellulose, we began to synthesize oxovanadium complexes and explore their fundamental reactivity. Of course there were theories regarding the oxidation mechanism, but our mechanistic studies soon revealed a number of surprises of the type that keep all chemists coming back to the bench! We realized that these reactions were also exciting in that they actually used the oxygen-on-every-carbon property of biomass-derived molecules to control the selectivity of the oxidation. When we found that these oxovanadium complexes tended to convert sugars predominantly to formic acid and carbon dioxide, we replaced one of the OH groups with an ether and entered the dark world of lignin chemistry. In this Account, we summarize results from our collaboration and from our individual labs. In particular, we show that oxidation selectivity (C-C vs C-O bond cleavage) of lignin models using air and vanadium complexes depends on the ancillary ligands, the reaction solvent, and the substrate structure (i.e., phenolic vs non-phenolic). Selected vanadium complexes in the presence of added base serve as effective alcohol oxidation catalysts via a novel base-assisted dehydrogenation pathway. In contrast, copper catalysts effect direct C-C bond cleavage of these lignin models, presumably through a radical pathway. The most active vanadium catalyst exhibits unique activity for the depolymerization of organosolv lignin. After Weckhuysen's excellent 2010 review on lignin valorization, the number of catalysis studies and approaches on both lignin models and extracts has expanded rapidly. Today we are seeing new start-ups and lignin production facilities sprouting up across the globe as we all work to prove wrong the old pulp and paper chemist's adage: you can make anything from lignin except money!

Concurrent determination of 237Np and Pu isotopes using ICP-MS: analysis of NIST environmental matrix standard reference materials 4357, 1646a, and 2702.

An optimized method was developed to analyze environmental soil and sediment samples for (237)Np, (239)Pu, and (240)Pu by ICP-MS using a (242)Pu isotope dilution standard. The high yield, short time frame required for analysis, and the commercial availability of the (242)Pu tracer are significant advantages of the method. Control experiments designed to assess method uncertainty, including variation in inter-element fractionation that occurs during the purification protocol, suggest that the overall precision for measurements of (237)Np is typically on the order of ± 5%. Measurements of the (237)Np concentration in a Peruvian Soil blank (NIST SRM 4355) spiked with a known concentration of (237)Np tracer confirmed the accuracy of the method, agreeing well with the expected value. The method has been used to determine neptunium and plutonium concentrations in several environmental matrix standard reference materials available from NIST: SRM 4357 (Radioactivity Standard), SRM 1646a (Estuarine Sediment) and SRM 2702 (Inorganics in Marine Sediment).

Kläui ligand thin films for rapid plutonium analysis by alpha spectrometry.

As part of a nuclear forensics capability, rapid and effective methods to analyze for plutonium and other actinide metals are needed. A key requirement of these methods is that they afford a high chemical yield while still providing isotopic information necessary for forensic evaluation. Toward this objective, a new method for binding plutonium for analysis by alpha spectrometry has been developed. Thin films of Kläui-type tripodal oxygen donor ligands were prepared by spin-casting solutions onto glass substrates. Three different ligands were evaluated for plutonium binding, and the best results were obtained using the ethyl-substituted complex Na[Cp*Co(P(O)(OEt)2)3], which bound 80-88% of the dissolved Pu under equilibrium conditions. The thin films are simple and inexpensive to prepare and exhibit excellent alpha spectral resolution, having line widths of ~33 keV. The method has been successfully applied to analyze for plutonium in both an archived nuclear debris sample and a certified environmental soil sample. The results obtained from the soil analysis are in good agreement with the certified values, demonstrating the effectiveness of the method for rapid plutonium analysis.

Cobalt-catalyzed transfer hydrogenation of C=O and C=N bonds.

An earth-abundant metal cobalt catalyst has been developed for the transfer hydrogenation of ketones, aldehydes, and imines under mild conditions. Experiments are described which provide insights into the mechanism of the transfer hydrogenation reaction.

Understanding the mechanisms of cobalt-catalyzed hydrogenation and dehydrogenation reactions.

Cobalt(II) alkyl complexes of aliphatic PNP pincer ligands have been synthesized and characterized. The cationic cobalt(II) alkyl complex [(PNHP(Cy))Co(CH2SiMe3)]BAr(F)4 (4) (PNHP(Cy) = bis[(2-dicyclohexylphosphino)ethyl]amine) is an active precatalyst for the hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols. To elucidate the possible involvement of the N-H group on the pincer ligand in the catalysis via a metal-ligand cooperative interaction, the reactivities of 4 and [(PNMeP(Cy))Co(CH2SiMe3)]BAr(F)4 (7) were compared. Complex 7 was found to be an active precatalyst for the hydrogenation of olefins. In contrast, no catalytic activity was observed using 7 as a precatalyst for the hydrogenation of acetophenone under mild conditions. For the acceptorless dehydrogenation of 1-phenylethanol, complex 7 displayed similar activity to complex 4, affording acetophenone in high yield. When the acceptorless dehydrogenation of 1-phenylethanol with precatalyst 4 was monitored by NMR spectroscopy, the formation of the cobalt(III) acetylphenyl hydride complex [(PNHP(Cy))Co(III)(κ(2)-O,C-C6H4C(O)CH3)(H)]BAr(F)4 (13) was detected. Isolated complex 13 was found to be an effective catalyst for the acceptorless dehydrogenation of alcohols, implicating 13 as a catalyst resting state during the alcohol dehydrogenation reaction. Complex 13 catalyzed the hydrogenation of styrene but showed no catalytic activity for the room temperature hydrogenation of acetophenone. These results support the involvement of metal-ligand cooperativity in the room temperature hydrogenation of ketones but not the hydrogenation of olefins or the acceptorless dehydrogenation of alcohols. Mechanisms consistent with these observations are presented for the cobalt-catalyzed hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols.

Cobalt-catalyzed acceptorless alcohol dehydrogenation: synthesis of imines from alcohols and amines.

A cobalt catalyst has been developed for the acceptorless dehydrogenation of alcohols and applied to synthesize imines from alcohols and amines. Deuterium labeling studies suggest that the reaction proceeds by an initial reversible alcohol dehydrogenation step involving a cobalt hydride intermediate.

A biomimetic pathway for vanadium-catalyzed aerobic oxidation of alcohols: evidence for a base-assisted dehydrogenation mechanism.

The first step in the catalytic oxidation of alcohols by molecular O(2), mediated by homogeneous vanadium(V) complexes [LV(V)(O)(OR)], is ligand exchange. The unusual mechanism of the subsequent intramolecular oxidation of benzyl alcoholate ligands in the 8-hydroxyquinolinato (HQ) complexes [(HQ)(2)V(V)(O)(OCH(2)C(6)H(4)-p-X)] involves intermolecular deprotonation. In the presence of triethylamine, complex 3 (X = H) reacts within an hour at room temperature to generate, quantitatively, [(HQ)(2)V(IV)(O)], benzaldehyde (0.5 equivalents), and benzyl alcohol (0.5 equivalents). The base plays a key role in the reaction: in its absence, less than 12% conversion was observed after 72 hours. The reaction is first order in both 3 and NEt(3), with activation parameters ΔH(≠)=(28±4) kJ mol(-1) and ΔS(≠)=(-169±4) J K(-1) mol(-1). A large kinetic isotope effect, 10.2±0.6, was observed when the benzylic hydrogen atoms were replaced by deuterium atoms. The effect of the para substituent of the benzyl alcoholate ligand on the reaction rate was investigated using a Hammett plot, which was constructed using σ(p). From the slope of the Hammett plot, ρ=+(1.34±0.18), a significant buildup of negative charge on the benzylic carbon atom in the transition state is inferred. These experimental findings, in combination with computational studies, support an unusual bimolecular pathway for the intramolecular redox reaction, in which the rate-limiting step is deprotonation at the benzylic position. This mechanism, that is, base-assisted dehydrogenation (BAD), represents a biomimetic pathway for transition-metal-mediated alcohol oxidations, differing from the previously identified hydride-transfer and radical pathways. It suggests a new way to enhance the activity and selectivity of vanadium catalysts in a wide range of redox reactions, through control of the outer coordination sphere.

Pyro without fire: synthesis, structure, and reactivity of a dimeric vanadyl pyrophosphate coordination complex.

The complex {[(V(IV)O)bipy(H(2)O)](2)(µ-P(2)O(7))}·3H(2)O (2) was readily obtained as a nanocrystalline powder by one-pot synthesis under mild conditions. Single crystals of 2 were grown at room temperature over 2 months, and its structure was determined. Fundamental catalytic activity was proven for this species by testing for oxidation of benzyl alcohol in air.

Aerobic oxidation reactions catalyzed by vanadium complexes of bis(phenolate) ligands.

Vanadium(V) complexes of the tridentate bis(phenolate)pyridine ligand H(2)BPP (H(2)BPP = 2,6-(HOC(6)H(2)-2,4-(t)Bu(2))(2)NC(5)H(3)) and the bis(phenolate)amine ligand H(2)BPA (H(2)BPA = N,N-bis(2-hydroxy-4,5-dimethylbenzyl)propylamine) have been synthesized and characterized. The ability of the complexes to mediate the oxidative C-C bond cleavage of pinacol was tested. Reaction of the complex (BPP)V(V)(O)(O(i)Pr) (4) with pinacol afforded the monomeric vanadium(IV) product (BPP)V(IV)(O)(HO(i)Pr) (6) and acetone. Vanadium(IV) complex 6 was oxidized rapidly by air at room temperature in the presence of NEt(3), yielding the vanadium(V) cis-dioxo complex [(BPP)V(V)(O)(2)]HNEt(3). Complex (BPA)V(V)(O)(O(i)Pr) (5) reacted with pinacol at room temperature, to afford acetone and the vanadium(IV) dimer [(BPA)V(IV)(O)(HO(i)Pr)](2). Complexes 4 and 5 were evaluated as catalysts for the aerobic oxidation of 4-methoxybenzyl alcohol and arylglycerol ß-aryl ether lignin model compounds. Although both 4 and 5 catalyzed the aerobic oxidation of 4-methoxybenzyl alcohol, complex 4 was found to be a more active and robust catalyst for oxidation of the lignin model compounds. The catalytic activities and selectivities of the bis(phenolate) complexes are compared to previously reported catalysts.

Evidence for room temperature delignification of wood using hydrogen peroxide and manganese acetate as a catalyst.

Manganese acetate was found to catalyze the oxidative delignification of wood with hydrogen peroxide at room temperature. The delignification reaction was monitored by optical and Raman microscopy, and liquid chromatography/mass spectrometry. When exposed to H(2)O(2) and Mn(OAc)(3) in aqueous solution, poplar wood sections were converted into a fine powder-like material which consisted of individual wood cells within 4 days at room temperature and without agitation. Optical and Raman microscopy provided the spatial distribution of cellulose and lignin in the wood structure, and showed the preferential oxidation of lignin-rich middle lamellae. Raman spectra from the solid residue revealed a delignified and cellulose-rich material. Glucose yields following enzymatic hydrolysis were 20-40% higher in poplar sawdust pretreated with Mn(OAc)(3) for 2, 4, and 7 days at room temperature than those in sawdust exposed to water only for identical durations, suggesting the viability of this mild, inexpensive method for pretreatment of lignocellulosic biomass.