Electronic Structure of Pentagonal Carbon Nanocones: An ab Initio Study.

In this work, we investigate the electronic structure of a particular class of carbon nanocones having a pentagonal tip and C5v symmetry. The ground-state nature of the wave function for these structures can be predicted by the recently proposed generalized Hückel rule that extends the original Hückel rule for annulenes to this class of carbon nanocones. In particular, the structures here considered can be classified as closed-shell or anionic/cationic closed-shells, depending on the geometric characteristics of the cone. The goal of this work is to assess the relationship between the electronic configuration of these carbon nanocones and their ability to gain or lose an electron as well as their adsorption capability. For this, the geometry of these structures in the neutral or ionic forms, as well as systems containing either one lithium or fluorine atom, was optimized at the DFT/B3LYP level. It was found that the electron affinity, ionization potential, and the Li or F adsorption energy present an intimate connection to the ground-state wave function character predicted by the generalized Hückel rule. In fact, a peculiar oscillatory energy behavior was discovered, in which the electron affinity, ionization energy, and adsorption energies oscillate with an increase in the nanocone size. The reasoning behind this is that if the anion is closed-shell, then the neutral nanocone will turn out to be a good electron acceptor, increasing the electron affinity and lithium adsorption energy. On the other hand, in the case of a closed-shell cation, this means that the neutral nanocone will easily lose an electron, leading to a smaller ionization potential and higher fluorine adsorption energy.

Solution to the Thomson Problem for Clifford Tori with an Application to Wigner Crystals.

In its original version, the Thomson problem consists of the search for the minimum-energy configuration of a set of point-like electrons that are confined to the surface of a two-dimensional sphere (S2) that repel each other according to Coulomb's law, in which the distance is the Euclidean distance in the embedding space of the sphere, i.e., R3. In this work, we consider the analogous problem where the electrons are confined to an n-dimensional flat Clifford torus Tn with n = 1, 2, 3. Since the torus Tn can be embedded in the complex manifold Cn, we define the distance in the Coulomb law as the Euclidean distance in Cn, in analogy to what is done for the Thomson problem on the sphere. The Thomson problem on a Clifford torus is of interest because supercells with the topology of a Clifford torus can be used to describe periodic systems such as Wigner crystals. In this work, we numerically solve the Thomson problem on a square Clifford torus. To illustrate the usefulness of our approach, we apply it to Wigner crystals. We demonstrate that the equilibrium configurations we obtain for large numbers of electrons are consistent with the predicted structures of Wigner crystals. Finally, in the one-dimensional case, we analytically obtain the energy spectrum and the phonon dispersion law.

Mapping of Hückel zigzag carbon nanotubes onto independent polyene chains: Application to periodic nanotubes.

The electric polarizability and the spread of the total position tensors are used to characterize the metallic vs insulator nature of large (finite) systems. Finite clusters are usually treated within the open boundary condition formalism. This introduces border effects, which prevent a fast convergence to the thermodynamic limit and can be eliminated within the formalism of periodic boundary conditions. Recently, we introduced an original approach to periodic boundary conditions, named Clifford boundary conditions. It considers a finite fragment extracted from a periodic system and the modification of its topology into that of a Clifford torus. The quantity representing the position is modified in order to fulfill the system periodicity. In this work, we apply the formalism of Clifford boundary conditions to the case of carbon nanotubes, whose treatment results in a particularly simple zigzag geometry. Indeed, we demonstrate that at the Hückel level, these nanotubes, either finite or periodic, are formally equivalent to a collection of non-interacting dimerized linear chains, thus simplifying their treatment. This equivalence is used to describe some nanotube properties as the sum of the contributions of the independent chains and to identify the origin of peculiar behaviors (such as conductivity). Indeed, if the number of hexagons along the circumference is a multiple of three, a metallic behavior is found, namely a divergence of both the (per electron) polarizability and total position spread of at least one linear chain. These results are in agreement with those in the literature from tight-binding calculations.

Clifford Boundary Conditions: A Simple Direct-Sum Evaluation of Madelung Constants.

We propose a simple direct-sum method for the efficient evaluation of lattice sums in periodic solids. It consists of two main principles: (i) the creation of a supercell that has the topology of a Clifford torus, which is a flat, finite, and borderless manifold; (ii) the renormalization of the distance between two points on the Clifford torus by defining it as the Euclidean distance in the embedding space of the Clifford torus. Our approach does not require any integral transformations nor any renormalization of the charges. We illustrate our approach by applying it to the calculation of the Madelung constants of ionic crystals. We show that the convergence toward the system of infinite size is monotonic, which allows for a straightforward extrapolation of the Madelung constant. We are able to recover the Madelung constants with a remarkable accuracy, and at an almost negligible computational cost, i.e., a few seconds on a laptop computer.

3D optical detection in radiodosimetry: EasyDosit hydrogel characterization.

In this study the spectrophotometric behaviour of gelatin-based hydrogels, in the presence and absence of dyes, was studied. The aim was to formulate equivalent-tissue phantoms to be used as 3D-dosimeter, suitable for Optical Computed Tomography (OCT). The hydrogels show good transparency and good stability of baseline optical density and, in the presence of dye, the response of optical density as a function of concentration was higher than in aqueous solution. The hydrogels were formulated in order to reduce the diffusion of the image of the irradiated field over time and to have stable fantoms as a function of time. To this purpose, the diffusion coefficients of two dyes, Bromophenol Blue (BPB) and Bromocresol Green (BCG), were determined as a function of the hydrogel chemical composition. As comparison, in some experiments Xylenol Orange (XO) was used. In particular, the presence of sucrose, as thickener, can reduce to almost half BPB mobility. In conclusion, it was shown that optical properties and controlled dye diffusion in gelatin-based hydrogels could allow using them as 3D-dosimeter for optical detection.

The Electronic Structure of Beryllium Chains.

We present an ab initio theoretical study of quasi one-dimensional beryllium chains, Be N, from an electronic structure perspective for N = 3, 4,···, 12. In particular, linear and cyclic systems were compared by using high-quality coupled-cluster formalism. Both linear and cyclic species were found to be local minima on the corresponding potential energy surface, for all the considered values of N. The linear geometry is the most stable one only in the case of Be4. Several indicators (energy gap, position spread tensor, locality of the molecular orbitals) clearly show that both linear and cyclic one-dimensional structures, unlike three-dimensional bulk beryllium, have a covalent insulating nature.

Copper(II) interactions with non-steroidal anti-inflammatory agents. III--3-Methoxyanthranilic acid as a potential *OH-inactivating ligand: a quantitative investigation of its copper handling role in vivo.

Pharmacological activities of copper(II) complexes are a direct function of the nature of their ligands associated with the metal ion in vivo. Some of these, defined as *OH-inactivating ligands (G. Berthon, Agents Actions 39 (1993) 210-217), may act as specific "lures" for hydroxyl radicals at inflammatory sites and behave as pseudo-catalase-like agents. This property has been advanced for anthranilic acid (H. Miche, V. Brumas, G. Berthon, J. Inorg. Biochem. 68 (1997) 27-38). With a view to improve the chemical features required to render such inactive substances effective anti-inflammatory drugs through their association with copper(II), an in vitro investigation into copper(II) interactions with the anionic form of an anthranilic acid derivative, namely 3-methoxyanthranilate (Man), has been performed under experimental conditions pertaining in vivo. Copper(II)-Man complex equilibria have been determined using glass electrode potentiometry, then checked by UV-vis and mass spectrometries. Given the prime role of histidine as a copper(II) ligand in blood plasma, copper(II)-histidine-Man ternary equilibria have also been studied. Subsequent computer simulations of the distribution of copper(II) in the extracellular fluid revealed that Man can specifically mobilize Cu(II) ions under inflammatory conditions without affecting their distribution under normal physiological conditions. Thiobarbituric acid reactive substances (TBARS) tests conducted with respect to standardized copper-mediated Fenton-type reactions (P. Maestre, L. Lambs, J.P. Thouvenot, G. Berthon, Free Rad. Res. 20 (1994) 205-218) have shown that, like anthranilic acid, Man can effectively both increase the Fenton-like reactivity of copper and decrease the amount of TBARS detected in solution, i.e., act as a potential *OH-inactivating ligand.

Aluminum speciation studies in biological fluids. Part 9. A quantitative investigation of aluminum(III)-glutamate complex equilibria and their potential implications for aluminum metabolism and toxicity.

As a nonessential element, aluminum is likely to be toxic both at low usual dietary levels in the long run (chronic toxicity) and at high therapeutic levels in shorter periods of time (acute toxicity). In both situations, aluminum toxicity is a direct function of aluminum bioavailability, which is itself dependent on Al(3+) solubility and charge neutralization. Dietary acids, by their intrinsic acidity and coordinating capacity, can extend the pH range, thus the section of the gastrointestinal tract, within which the Al(3+) ion remains soluble, and also help Al(3+) diffusion across the intestinal epithelium through the formation of neutral complex species. The present work examines the impact of glutamic acid, an essential amino acid also widely used in industrial food and drinks, on aluminum speciation in the gastrointestinal tract and blood plasma. Complex formation between the Al(3+) ion and glutamate has first been investigated through potentiometric titrations, complex stoichiometries being then checked by ESI mass spectrometry and NMR measurements. A series of mono- and polynuclear species has been characterized, whose influence on aluminum distribution in vivo has been assessed by computer simulation. The capacity of glutamate to maintain Al(3+) ions in solution under normal dietary conditions is predicted to be intermediate between glycine-like amino acids and succinate on the one hand, and tartrate and malate on the other hand, its Al(3+) neutralization effect being similar to that of succinate, tartrate and malate. These results, which point to a potential aggravating role of glutamate on aluminum gastrointestinal absorption, substantiate recent observations made on rats. In spite of the moderate effect expected from glutamate on aluminum bioavailability under most aluminum-based therapies investigated, attention is therefore called to the risk of glutamic acid ingestion simultaneously to any aluminum therapeutic form. Incidentally, the former implication of 'the' aluminum glutamate complex in the transfer of aluminum through the blood-brain barrier of aluminum loaded rats may effectively be attributed to one of the species characterized here, but is of no significance at all to aluminum contamination in humans, even at most extreme levels.