On the nature of noble gas - metal bond in silver aggregates.

The aim of this paper is to extend the study of the nature of the bond between noble gas to nano- and sub nanoscale silver aggregates. In the framework of DFT-PAW calculations implemented in the abinit package, we carried out a thorough investigation on the nature of the bond between the six noble gases NG (He, Ne, Ar, Kr, Xe and Rn) and numerous neutral silver aggregates Agn from the single atom Ag1 to the nanoparticle Ag147 using atoms-in-molecules (AIM) dual functional analysis,. We evaluated the impact of the silver aggregate size, the adsorption site and of the noble gas on the Ag-NG bond. Our study concluded on the favored adsorption of heavier noble gases (Kr, Xe and Rn) over that of lighter noble gases (He, Ne and Ar) on any aggregate size due to an enhanced chemical contribution in the bond. For these heavier noble gases, in accordance with studies carried out on surfaces, we noted their preferential adsorption on on-top sites rather than on hollow sites, which further evidences the chemical contribution to the bond. Moreover, the slight positive Bader charge on these heavier noble gases implies an electron transfer from the noble gas to the silver atom. Noble gas adsorption is favored on smaller, few-atom, two-dimensional clusters rather than on larger three-dimensional nanoparticles. Finally, we identified a universal power law with a unique exponent linking bond length and electronic density at the bond critical point for all aggregate sizes, noble gases and adsorption sites.

Accumulation and Speciation of Cobalt in Paracentrotus lividus.

Since the first human release of radionuclides on Earth at the end of the Second World War, impact assessments have been implemented. Radionuclides are now ubiquitous, and the impact of local accidental release on human activities, although of low probability, is of tremendous social and economic consequences. Although radionuclide inventories (at various scales) are essential as input data for impact assessment, crucial information on physicochemical speciation is lacking. Among the metallic radionuclides of interest, cobalt-60 is one of the most important activation products generated in the nuclear industry. In this work, a marine model ecosystem has been defined because seawater and more generally marine ecosystems are final receptacles of metal pollution. A multistep approach from quantitative uptake to understanding of the accumulation mechanism has been implemented with the sea urchin Paracentrotus lividus. In a well-controlled aquarium, the day-by-day uptake of cobalt and its quantification in different compartments of the sea urchin were monitored with various conditions of exposure by combining ICP-OES analysis and γ spectrometry. Cobalt is mainly distributed following the rating intestinal tract ≫ gonads > shell spines. Cobalt speciation in seawater and inside the gonads and the intestinal tract was determined using extended X-ray absorption fine structure (EXAFS). The cobalt inside the gonads and the intestinal tract is mainly complexed by the toposome, the main protein in the sea urchin P. lividus. Complexation with purified toposome was characterized and a complexation site combining EXAFS and AIMD (ab initio molecular dynamics) was proposed implying monodentate carboxylates.

Carboxylate- and Phosphonate-Modified Polyethylenimine: Toward the Design of Actinide Decorporation Agents.

Plutonium (Pu) is an anthropogenic element involved in the nuclear industry cycle. Located at the bottom of the periodic table within the actinide family, it is a chemical toxic but also a radiological toxic, regardless of isotopy. After nearly 80 years of Pu industrialization, it has become clear that inhalation and wounds represent the two main ways a person may become contaminated after an accident. In order to reduce the deleterious health effects of Pu, it is crucial to limit chronic exposure by removing it or preventing its incorporation into the body. Diethylenetriaminepentaacetic acid (DTPA) has emerged as the gold standard for Pu decorporation, although it suffers from very short retention time in serum. Other molecules like the hydroxypyridonate family with high chemical affinity have also been considered. We have been considering alternative polymeric chelates and, in particular, polyethylenimine (PEI) analogues of DTPA (the carbonate or phosphonate version), which may present a real breakthrough in Pu decorporation not only because of their higher loading capacity but also because of their indirect vectorization properties correlated with a specific biodistribution into the lungs, bone, kidney, or liver. In the first part of this Forum Article, new data on the structural characterization of the complexation of PuIV with polyethylenimine methylphosphonate (PEI-MP) were obtained using the combination of extended X-ray absorption fine structure spectroscopy and ab initio molecular dynamics (AIMD) calculations. The use of thorium (Th) as a Pu chemical surrogate is also discussed because its unique oxidation state is IV+ in solution. In the second part of the paper, we put this new set of data on PEI-MP-Pu into perspective with use of the PEI platform to complex ThIV and PuIV. Uptake curves of ThIV witth polyethylenimine methylcarboxylate (PEI-MC) are compared with those of PEI-MP and DTPA, and the AIMD data are discussed.

Bond-bending isomerism and metallophilicity in metal-halogen anions (Cu,Ag,Au)2X3 -, X = F, Cl, Br, I, At.

In the framework of DFT (ABINIT package), we have performed atomic relaxations on the (Cu,Ag,Au)2X3 -, X = F, Cl, Br, I, At anion series. Opposite to linear (MX2)- anions, all (M2X3)- systems are triangular (C 2v symmetry). According to the system, we classified these anions in three categories according to the relative strength of electronegativity, chemical hardness, metallophilicity and van der Waals interaction. We found two bond-bending isomers: (Au2I3)- and (Au2At3)-.

Xe Adsorption on Noble Metal Clusters: A Density Functional Theory Investigation.

The adsorption mechanism of xenon on three noble metal clusters (M = Ag, Au, and Cu) has been investigated in the framework of density functional theory (DFT) within generalized gradient approximation (GGA-PBE). The ab initio calculations were performed with the quantum molecular dynamics (QMD) package ABINIT using the projector augmented (PAW) formalism. The spin-orbit coupling (SOC) and dispersion effects (Van der Waals DFT-D3) have been taken into account. According to these calculations, the M-Xe bonds are partly covalent and electrostatic and their contribution depends on the cluster size and nature. This study underlines the importance of using the SOC and the Van der Waals (VdW) effects. Based on these results, copper nanoparticles have the highest affinity for interaction with xenon compared with silver and gold.

Polyethyleneimine methylenecarboxylate: a macromolecular DTPA analogue to chelate plutonium(iv).

Up until now, molecular chelating agents, such as diethylenetriamine pentaacetic acid (DTPA), have been the standard method for actinide human decorporation. Mainly active in blood serum, their distribution within the body is thus limited. To treat a wider range of organs affected by plutonium contamination, a potential new class of macromolecular decorporation agents is being studied. Polyethyleneimine methylenecarboxylate (PEI-MC) is one such example. It is being considered here because of its capacity for targeting the liver and bones.

A model for multiphoton absorption in dielectric materials induced by short laser pulses at moderate intensities.

We present a semi-analytical model for free electron production induced by multiphoton ionization in dielectric materials for short laser pulses at moderate intensities. Within this approach, the laser-induced absorption is described through the Bloch-Volkov formalism, and the electronic structure of materials is evaluated through first-principles calculations. Results obtained for NaCl and KDP (KH2PO4) materials show that significant deviations from the parabolic band approximation may occur. When the laser intensity increases, high multiphotonic orders may become the predominant mechanisms outside the centre of the Brillouin zone.

Properties of hot liquid cerium by LDA + U molecular dynamics.

We present ab initio simulations of liquid cerium in the framework of the LDA + U formulation. The liquid density has been determined self-consistently by searching for the zero pressure equilibrium state at 1320 K with the same set of parameters (U and J) and occupation matrices as those optimized for the γ phase. We have computed static and transport properties. The liquid produced by the simulations appears more structured than the available measurements. This raises questions regarding the ability of the theory to describe such a complex liquid. Conductivity calculations and temperature dependences are nevertheless in reasonable agreement with data.