Underground sources of radioactive noble gas

It is well known that radon is present in relatively high concentrations below the surface of the Earth due to natural decay of uranium and thorium. However, less information is available on the background levels of other isotopes such as ¹³³Xe and ^131mXe produced via spontaneous fission of either manmade or naturally occurring elements. The background concentrations of radioxenon in the subsurface are important to understand because these isotopes potentially can be used to confirm violations of the comprehensive nuclear-test-ban treaty during an on-site inspection. Recently, Pacific Northwest National Laboratory measured radioxenon concentrations from the subsurface at the Nevada Nuclear Security Site (NNSS—formerly known as the Nevada Test Site) to determine whether xenon isotope background levels could be detected from spontaneous fission of naturally occurring uranium or legacy ²⁴⁰Pu as a result of historic nuclear testing. In this paper, we discuss the results of those measurements and review the sources of xenon background that must be taken into account during OSI noble gas measurements.     

Beryllium bonding with noble gas atoms

Quantum chemical calculations were carried out to investigate the nature of the bonding between a neutral Be₃ ring and noble gas atom. Electronic structure calculation for these complexes was carried out at different computational levels in association with natural bond orbital, quantum theory of atoms in molecules, electron localization function, symmetry adapted perturbation theory, and molecular electrostatic potential surface analysis of Be₃ complexes. The Be atoms in the Be₃ moiety are chemically bonded to one another, with the Be Be bond dissociation energy being ~125 kJ mol⁻¹. The Be₃ ring interacts with the noble gases through non-covalent interactions. The binding energies of the noble gas atoms with the Be₃ ring increases with increase in their atomic number. The non-covalent interaction index, density overlap region indicator and independent gradient model analyses reveal the presence of non-covalent inter-fragment interactions in the complexes. Energy decomposition analysis reveals that dispersion plays the major role towards stabilizing these systems.     

Dipole polarizabilities of noble gas endohedral fullerenes

First-principles calculations were performed on the polarizabilities of noble gas endohedral complexes, X@C₆₀ (X = He–Kr). The polarizability of X@C₆₀ increases from He–Kr, but is distinctly smaller than the sum of the X atoms and C₆₀, unlike many other non-bonded systems for which an additive rule holds. Compared to C₆₀, the increased polarizabilities in X@C₆₀ are mainly from the enhanced charge transfer contribution of the cage and the local polarization of the interior X atoms, though the interior atoms are found less polarized than the corresponding free ones.     

Vibrational properties of noble gas endohedral fullerenes

Analysis of IR and Raman spectra of Ar@C(60) and Kr@C(60) shows that the incorporation of noble gas atoms causes a blue shift of low energy vibrations, which have radial character, and a red shift of higher energy ones which have a tangential character movement. The mechanism of these phenomena is explained on the basis of ab initio numerical experiments with DFT and MP2 procedures. Methodological discussions are advanced, altogether with a scheme for the estimation of the van der Waals interaction between fullerene and noble gas, based on the frequency shifts.     

The implementation of accreditation in a chemical laboratory

This article describes the current state of the art in the accreditation of chemical laboratories, both in Europe and world-wide, emphasising the most common discussion topics. The results of recent proficiency testing in chemical laboratories are summarised to draw conclusions on the performance of accredited laboratories.     

Excimer emission spectrum in noble gas systems

A simple expression for the time shape of excimer emission as a function of temperature is obtained for the case of a realistic (exponential) ground state potential. The excimer binding energy and its equilibrium geometry are obtained for the lowest ¹Σ_u states of Ar*₂, Kr*₂, Xe*₂ and for the A¹Σ_u and D¹Σ_u states of Hc*₂, by comparing the theoretical formula with the corresponding experamental results. Agreement with electronic calculations when available of these parameter is very good. The relation between our line-shape expression and those given by other authors is discussed.     

Defect-mediated melting in superheated noble gas crystals

Molecular dynamics simulations have been used to investigate the mechanisms governing the homogeneous melting of pure noble gases at the limit of superheating. For each chemical species considered, the heterogeneous melting point was estimated by monitoring the thermal behavior of crystalline systems containing a high-angle grain boundary. To determine the limit to superheating, calculations were instead carried out on a perfect crystalline bulk. The temperature was gradually increased to bring the systems within the metastable region above the equilibrium melting point. The static order parameter was employed to monitor the structural disordering during the slow temperature increase and to determine the temperature at which the crystalline lattice collapses to a liquid. Structural disorder was further characterized by studying the appearance of atoms with defective coordination. Their relative number and spatial correlation appeared to play a fundamental role in destabilizing the crystalline lattice bulk and triggering the homogeneous melting. The fraction of atoms with defective coordination and the total length of the stringlike clusters they form in the vicinity of the homogeneous melting point were found to be approximately the same for all of the chemical species considered. These findings have been compared with theoretical predictions.     

Heuristic overlap-exchange model of noble gas chemical shifts

It is now generally recognized that overlap-exchange interactions are the primary cause of the medium-dependent magnetic shielding (chemical shift) in all noble gases except helium, although the attractive electrostatic-dispersion (van der Waals) interactions play an indirect role in determining the penetration of the interacting species into the repulsive overlap-exchange region. The short-range nature of these overlap-exchange interactions, combined with the fact that they often can be approximated by simple functions of the overlap of the wave functions of the interacting species, suggests a useful semiempirical model of these chemical shifts. In it the total shielding is the sum of shieldings due to pairwise interactions of the noble gas atom with the individual atoms of the medium, with the "atomic" shielding terms either estimated by simple functions of the atomic overlap integrals averaged over their Boltzmann-weighted separations, or determined by fits to experimental data in systems whose complexity makes the former procedure impractical. Results for (129)Xe chemical shifts in the noble gases and in a variety of molecular and condensed systems, including families of n-alkanes, straight-chain alcohols, and the endohedral compounds Xe@C(60) and Xe@C(70) are encouraging for the applicability of the model to systems of technical and biomedical interest.     

On the interatomic potentials for noble gas mixtures

Recently, a relatively simple scheme for the construction of isotropic intermolecular potentials has been proposed and tested for the like species interactions involving He, Ne, Ar, Kr and H₂. The model potential has an adjustable parameter which controls the balance between its exchange and Coulomb energy components. The representation of the Coulomb energy contains a damped multipolar dispersion energy series (which is truncated through O(R^?10) and provides additional flexibility through adjustment of the dispersion energy coefficients, particularly C₈ and C₁₀, within conservative error estimates. In this paper the scheme is tested further by application to interactions involving unlike noble gas atoms where the parameters in the potential model are determined by fitting mixed second virial coefficient data as a function of temperature. Generally the approach leads to potential of accuracy comparable to the best available literature potentials which are usually determined using a large base of experimental and theoretical input data. Our results also strongly indicate the need of high quality virial data.     

The size factor in the temperature retardation of diffusion in noble gas crystals

The times t* needed for noble gas crystals to reach the equilibrium state as a result of thermal motion of atoms upon temperature lowering during a nearly equilibrium process were calculated. The t* value was determined by solving a differential equation describing equalization of vacancy concentrations in a spherical sample of radius R. Upon temperature decrease, the t* values exponentially increase. As the sample radius R decreases, the time t* exponentially decreases. However, the tendency of t* to increase upon temperature lowering overrides the effect of decreasing sample size; therefore, there always exists a temperature range in which the diffusion is almost completely frozen.     

Magnetic circular dichroism of matrix isolated noble gas monohalides

MCD measurements on XeF, XeCl, XeBr and XeI matrix isolated in Ar at low temperature are reported. Some relationships among the various spectroscopic parameters are established by combining these data existing absorption and ESR measurements.     

Decay rates of inner-valence excitations in noble gas atoms

A Fano - algebraic diagrammatic construction - Stieltjes method has been recently developed for ab initio calculations of nonradiative decay rates [V. Averbukh and L. S. Cederbaum, J. Chem. Phys. 123, 204107 (2005)] of singly ionized states. In the present work this method is generalized for the case of electronic decay of excited states. The decay widths of autoionizing inner-valence-excited states of Ne, Ar, and Kr are calculated. Apart from the lowest excitation of Kr, they are found to be in good to excellent agreement with the experimental values. Comparison with the other theoretical studies shows that in many cases the new method performs better than the previously available techniques.     

H2 adsorption by noble gas insertion compounds: A computational study

In order to find a solution of energy-related problems, sophisticated hydrogen storing materials are needed as hydrogen is an abundant and environment friendly fuel. We have investigated the hydrogen storage potential of Ng inserted metal acetylide and metal cyanide compounds (metal ​= ​Cu, Ag and Au) at the ωB97X-D/cc-pVTZ-PP level of theory. Due to the difference in electronegativity and formal charge on metal atoms in the insertion compounds, the interaction with the hydrogen molecule is expected to be different. The adsorption energies, the free energy of adsorption, natural charges on atomic centers/moieties are obtained through the natural population analysis, and energy decomposition analysis has also been carried out for nH₂···MNgCCH and nH₂···MNgCN (n ​= ​1–3). The hydrogen adsorption capacity of the strongest and the weakest cases has also been investigated. Both the insertion compounds, MNgCCH and MNgCN, are found to adsorb a maximum of three hydrogen molecules on the metal site. The single H₂ adsorbed minimum energy structures of studied compounds show a “T-shaped” orientation while double H₂ adsorbed minimum energy structures are of “Y- shaped” geometry and those of tricoordinated structures resemble “T_d-like” shape. The negative value of Gibbs free energy change suggests the thermodynamical spontaneity of the hydrogen adsorption process.     

A DIM model for homogeneous noble gas ionic clusters

The method of diatomics-in-molecules (DIM) is applied to the calculation of the energy of the homogeneous noble-gas ionic clusters Ar _n ⁺ and Xe _n ⁺ forn=3, 4, ..., 22. The trimers are stable symmetric linear molecules exhibiting chemical binding, a result in agreement both with ab initio calculations and with previous DIM work. The clusters up ton=13 are best described as a trimer ion surrounded by neutrals, whereby the charge distribution changes slightly with increasingn. Both noble gases exhibit a special stability associated with the completion of the first shell of neutral atoms atn=13. Asn increases from 13 to 22, there is a greater delocalization of the positive charge, the central ion tending to become a linear tetramer, symmetric for Xe and unsymmetric for Ar. Energies of the excited electronic states are reported and the possibility of developing simpler DIM models for the clusters and for mixed noble gases is discussed.     

In-silico exploration of noble gas dimer enforced by noncovalent interaction

Density functional calculations have been carried out to investigate the possibility of trapping of noble gas dimers by cyclo[18]carbon dimer. Parallel-displaced conformation of the cyclo[18]carbon dimer is found to be the minimum energy structure. Noncovalent interaction is found to hold the noble gas dimers. The lighter noble gases (He, Ne) posses weaker attractive interactions while the heavier one (Ar, Kr) are held by stronger attractive interactions forming genuine bonds. Each of the noble gas atoms in turn forms noncovalent interaction with the cyclo[18]carbon monomers. The bond dissociation energy of the noble gas dimers dramatically increases inside the cyclo[18]carbon dimer. Energy decomposition analysis reveals that dispersion plays the major role toward the stabilization energy.     

Theoretical prediction of the noble gas complexes HeAuF and NeAuF

Ab initio calculations were carried out to investigate the structures and the stability of the noble gas complexes HeAuF and NeAuF through MP2 and CCSD(T) methods. The HeAuF was predicted to have a linear structure with weak He—Au covalent bonding, the distance of which is closer to the covalent limit in comparison with the corresponding van der Waals limit. The dissociation energy with respect to He + AuF was found to be 24 and 26 kJ·mol⁻¹ at the CCSD(T)/basis set B and B′ levels, respectively. However, similar calculations for NeAuF indicate that NeAuF is not a stable species.     

The clinical chemistry laboratory: current status, problems and diagnostic prospects

Although increased automation, advanced analytical techniques and sophisticated information technology have greatly improved the performance and quality in medical laboratory testing, several studies show that significant amounts of errors occur. Detailed analysis revealed that most of the errors occur in the preanalytical phase, while fewer errors occur in the intra- and post-analytical phase. The majority of errors are caused by wrong sampling or occur during transport to the laboratory. This review focuses on the analytical procedures in a large central laboratory. Possible problems are described by following samples from the patient to the laboratory and back. Finally, the advantages and disadvantages of point-of-care testing versus central laboratory are compared.