Structural properties of ultra-small thorium and uranium dioxide nanoparticles embedded in a covalent organic framework

We report the structural properties of ultra-small ThO₂ and UO₂ nanoparticles (NPs), which were synthesized without strong binding surface ligands by employing a covalent organic framework (COF-5) as an inert template. The resultant NPs were used to observe how structural properties are affected by decreasing grain size within bulk actinide oxides, which has implications for understanding the behavior of nuclear fuel materials. Through a comprehensive characterization strategy, we gain insight regarding how structure at the NP surface differs from the interior. Characterization using electron microscopy and small-angle X-ray scattering indicates that growth of the ThO₂ and UO₂ NPs was confined by the pores of the COF template, resulting in sub-3 nm particles. X-ray absorption fine structure spectroscopy results indicate that the NPs are best described as ThO₂ and UO₂ materials with unpassivated surfaces. The surface layers of these particles compensate for high surface energy by exhibiting a broader distribution of Th–O and U–O bond distances despite retaining average bond lengths that are characteristic of bulk ThO₂ and UO₂. The combined synthesis and physical characterization efforts provide a detailed picture of actinide oxide structure at the nanoscale, which remains highly underexplored compared to transition metal counterparts.

ThO₂ and UO₂ nanoparticles synthesized using a COF-5 template exhibit unpassivated surfaces and provide insight into nanoscale properties of actinides.     

Evaluation of Pd→B Interactions in Diphosphinoborane Complexes and Impact on Inner-Sphere Reductive Elimination

The dative Pd→B interaction in a series of ^RDPB^R’ Pd⁰ and Pd^II complexes (^RDPB^R’=(o-PR₂C₆H₄)₂BR’, diphosphinoborane) was analyzed using XRD, ¹¹B NMR spectroscopy and NBO/NLMO calculations. The borane acceptor discriminates between the oxidation state Pd^II and Pd⁰, stabilizing the latter. Reaction of lithium amides with [(^RDPB^R’)Pd^II(4-NO₂C₆H₄)I] chemoselectively yields the C−N coupling product. DFT modelling indicates no significant impact of Pd^II→B coordination on the inner-sphere reductive elimination rate.     

Synthesis and optical characterization of new ketocyanine dyes with extended polymethine chaines

New symmetrical and unsymmetrical near-infrared absorbing ketocyanine-type dyes are synthesized and their optical characterization is done. The relationships between their structure and optical properties are discussed. The synthesized ketocyanine dyes show a positive solvatochromism and are poorly fluorescent. A new procedure for the preparation of N,N,N′,N′-tetramethylvinamidinium perchlorate, which was a very useful reagent for the synthesis of various heterocyclic compounds as well as for the preparation of cyanine dyes is developed.     

Surfacial carbonized palygorskite as support for high-performance Pt-based electrocatalysts

Pt nanoparticles deposited on a low-cost, surfacial, carbonized palygorskite (Pt/C-PLS) prepared by carbonizing sucrose were evaluated as a methanol oxidation catalyst for direct methanol fuel cells. Transmission electron microscopy and Fourier transfrom infrared spectrophotometry analyses revealed that carbon was formed on the surface of PLS and that free silica presented in the C-PLS support. The catalytic activity of methanol oxidation of Pt/C-PLS was higher than that of Pt/C, and the former catalyst had better CO tolerance.     

Multiparticle simulation of ion motion in the ion trap mass spectrometer: Resonant and direct current pulse excitation

A PC-based program that simulates the behavior of a collection of ions is extended to include the effects of collisions with the buffer gas and enhanced visualization methods. The simulations are based on the quadrupole field associated with the actual ion trap electrode structure. Ionization is simulated in such a way as to distribute ionization events randomly over rf phase angles and yield a realistic collection of stored ions. The effects of buffer gas collisions on ion motion during both mass-selective instability and resonance ejection scans are found to include the expected dampening of spatial excursions as well as limitation of the kinetic energy of trapped ions. In both experiments, ion ejection occurs over a number of secular cycles in the vicinity of the theoretical instability point. Activation via a resonant ac signal or a short dc pulse is shown to result in phase-locking of the ions as well as the expected increase in the size of the excursions in the z direction and in ion kinetic energy. Collisions cause dephasing and loss of kinetic energy. Radial dc activation is compared with activation in the axial direction. Experimental data for dc pulse activation of the n-butylbenzene molecular ion are analyzed in phase space and the onset of surface-induced dissociation is correlated with changes in the experimental m/z 91 to m/z 92 fragment ion ratio. Poincaré sections are shown for resonantly excited ions and their value in demonstrating improvement of the resolution of these experiments over conventional mass-selective instability scans is shown.     

Tubular Structures of Calcium Carbonate: Formation,Characterization, and Implications in Natural Mineral Environments

Chemical gardens are self-assembled tubular precipitates formed by a combination of osmosis, buoyancy, and chemical reaction, and thought to be capable of catalyzing prebiotic condensation reactions. In many cases, the tube wall is a bilayer structure with the properties of a diaphragm and/or a membrane. The interest in silica gardens as microreactors for materials science has increased over the past decade because of their ability to create long-lasting electrochemical potential. In this study, we have grown single macroscopic tubes based on calcium carbonate and monitored their time-dependent behavior by in situ measurements of pH, ionic concentrations inside and outside the tubular membranes, and electrochemical potential differences. Furthermore, we have characterized the composition and structure of the tubular membranes by using ex situ X-ray diffraction, infrared and Raman spectroscopy, as well as scanning electron microscopy. Based on the collected data, we propose a physicochemical mechanism for the formation and ripening of these peculiar CaCO₃ structures and compare the results to those of other chemical garden systems. We find that the wall of the macroscopic calcium carbonate tubes is a bilayer of texturally distinct but compositionally similar calcite showing high crystallinity. The resulting high density of the material prevents macroscopic calcium carbonate gardens from developing significant electrochemical potential differences. In the light of these observations, possible implications in materials science and prebiotic (geo)chemistry are discussed.     

DOSE-RESPONSE OF CHRONIC ULTRAVIOLET EXPOSURE ON EPIDERMAL FORWARD SCATTERING-ABSORPTION IN SK-1 HAIRLESS MOUSE SKIN

This work provides a dose-response model of UV-induced epidermal-stratum corneum thickening induced by irradiation at wavelength lambda. This model assumes that photobiochemical reaction(s) can give rise to hyperplasia in a manner which is predictable from a simple photochemical kinetic scheme. In this work, we derive an equation which predicts an approximately linear relationship between the logarithm of the increase in optical skin thickening measured at 320 nm (delta OD320) and total cumulative dose (DT) seen by the target cells in or near the basal layer. For each excitation wavelength lambda, the slope R(lambda) of the log delta OD320 vs DT plot is proportional to epsilon(lambda) phi rx, where epsilon(lambda) is the extinction coefficient for the target chromophore at excitation wavelength, and phi rx is the quantum yield for the photochemical reaction(s) leading to hyperplasia. Our data previously obtained from irradiation of SK-1 hairless mice with "monochromatic" UV wavebands at 280, 290, 300, 307 and 313 nm (Menter et al., 1988, Photochem. Photobiol. 47, 225-260.) and data from Sterenborg and van der Leun at 254 and 313 nm (1988, Photodermatology 5, 71-82) are in good agreement with this model, except for 254 and 280 nm excitation, which are greatly attenuated by epidermis-stratum corneum. For excitation at the latter wavelengths, "dark" regressive processes successfully compete with the "light" reaction(s) which lead to (pre)cancerous lesion. This difficulty notwithstanding, the "intrinsic" action spectrum for hyperplasia derived from these measurements indicates that the target chromophore preferentially absorbs in the UV-C region.(ABSTRACT TRUNCATED AT 250 WORDS)     

Palladium-catalyzed amination of aryl nonaflates

The first detailed study of the palladium-catalyzed amination of aryl nonaflates is reported. Use of ligands 2-4 and 6 allows for the catalytic amination of electron-rich and -neutral aryl nonaflates with both primary and secondary amines. With use of Xantphos 5, the catalytic amination of a variety of functionalized aryl nonaflates resulted in excellent yields of anilines; even 2-carboxymethyl aryl nonaflate is effectively coupled with a primary alkylamine. Moderate yields were obtained when coupling halo-aryl nonaflates with a variety of amines, where in most cases the aryl nonaflate reacted in preference to the aryl halide. Overall, aryl nonaflates are an effective alternative to triflates in palladium-catalyzed C-N bond-forming processes due to their increased stability under the reaction conditions.     

Slurry sampling for determination of lead in marine plankton by electrothermal atomic absorption spectrometry

A slurry sampling method has been developed for the determination of Pb in marine plankton by ETAAS using a freshwater plankton certified reference material (CRM 414). Slurries were prepared in 1–3% m/v range with 1% v/v HNO₃ by ultrasonic agitation for 5 min. The effects of several chemical modifiers, including Ir(NO₃)₂, Mg(NO₃)₂, Pd(NO₃)₂, Pd(NO₃)₂ + Mg(NO₃)₂, and Mg(NO₃)₂ + NH₄H₂PO₄, were investigated for the stabilization of Pb during thermal pretreatment. Lead in slurries was effectively stabilized up to 1000 °C with Ir, Pd and Pd + Mg modifiers among which Pd + Mg provided the best results with complete atomization at 1850 °C. Firings in the presence of Ir were, problematic due to ash formation inside the atomizer. Water, dilute HNO₃ and HF were examined as suspension medium. Dilute HNO₃ (1–2% v/v) proved to be advantageous over water as it afforded extraction of Pb from plankton almost quantitatively in 5 min agitation. Hydrofluoric acid was the least suitable medium. Increasing HF concentration up to 5% v/v resulted in inaccuracy and substantial background absorption. Fast-heating furnace method provided comparable accuracy and precision to that of conventional-heating in slurries of CRM 414. Detection limits and characteristic masses were, respectively, 0.49 μg L^− 1 and 32 pg for the conventional method and 0.62 μg L^− 1 and 37 pg for the fast-heating method. However, fast-heating approach suffered from distorted peaks at high temperatures and incomplete pyrolysis of matrix at lower temperatures. Analysis of marine plankton samples for Pb was performed by using the conventional furnace program. The results showed a high correlation with those obtained by solution ICP-MS. Differences were statistically insignificant within 95% confidence interval.