Advances on the Determination of the Astatine Pourbaix Diagram: Predomination of AtO(OH)2− over At− in Basic Conditions

It is generally assumed that astatide (At^?) is the predominant astatine species in basic aqueous media. This assumption is questioned in non‐complexing and non‐reductive aqueous solutions by means of high‐pressure anion‐exchange chromatography. Contrary to what is usually believed, astatide is found to be a minor species at pH=11. A different species, which also bears a single negative charge, becomes predominant when the pH is increased beyond 7. Using competition experiments, an equilibrium constant value of 10^?6.9 has been determined for the formation of this species from AtO(OH) with the exchange of one proton. The identification of this species, AtO(OH)₂^?, is achieved through relativistic quantum mechanical calculations, which rule out the significant formation of the AtO₂^? species, while leading to a hydrolysis constant of AtO(OH) in excellent agreement with experiment when the AtO(OH)₂^? species is considered. Beyond the completion of the Pourbaix diagram of astatine, this new information is of interest for the development of ²¹¹At radiolabeling protocols.     

Alkoxyallene‐ynes: Selective Preparation of Bicyclo[5.3.0] Ring Systems Including a δ‐Alkoxy Cyclopentadienone

The development of an intramolecular rhodium(I)‐catalyzed Pauson–Khand reaction of alkoxyallene‐ynes with a proximal alkoxy group is reported. This reaction, in the presence of a [Rh(cycloocta‐1,5‐diene)Cl]₂/propane‐1,3‐diylbis(diphenylphosphane) system under a CO atmosphere, constitutes a powerful tool for selectively accessing carbo‐ and heterobicyclo[5.3.0] frameworks featuring an enol ether moiety. Through this procedure, a straightforward access to guaiane skeletons with a tertiary hydroxy group at the C10 position was achieved.     

Anion Recognition by Aliphatic Helical Oligoureas

Anion binding properties of neutral helical foldamers consisting of urea type units in their backbone have been investigated. ¹H NMR titration studies in various organic solvents including DMSO suggest that the interaction between aliphatic oligoureas and anions (CH₃COO^?, H₂PO₄^?, Cl^?) is site‐specific, as it largely involves the urea NHs located at the terminal end of the helix (positive pole of the helix), which do not participate to the helical intramolecular hydrogen‐bonding network. This mode of binding parallels that found in proteins in which anion‐binding sites are frequently found at the N‐terminus of an α‐helix. ¹H NMR studies suggest that the helix of oligoureas remains largely folded upon anion binding, even in the presence of a large excess of the anion. This study points to potentially useful applications of oligourea helices for the selective recognition of small guest molecules.     

Thermodynamic stability of boron: the role of defects and zero point motion

Its low weight, high melting point, and large degree of hardness make elemental boron a technologically interesting material. The large number of allotropes, mostly containing over a hundred atoms in the unit cell, and their difficult characterization challenge both experimentalists and theoreticians. Even the ground state of this element is still under discussion. For over 30 years, scientists have attempted to determine the relative stability of alpha- and beta-rhombohedral boron. We use density functional calculations in the generalized gradient approximation to study a broad range of possible beta-rhombohedral structures containing interstitial atoms and partially occupied sites within a 105 atoms framework. The two most stable structures are practically degenerate in energy and semiconducting. One contains the experimental 320 atoms in the hexagonal unit cell, and the other contains 106 atoms in the triclinic unit cell. When populated with the experimental 320 electrons, the 106 atom structure exhibits a band gap of 1.4 eV and an in-gap hole trap at 0.35 eV above the valence band, consistent with known experiments. The total energy of these two structures is 23 meV/B lower than the original 105 atom framework, but it is still 1 meV/B above the alpha phase. Adding zero point energies finally makes the beta phase the ground state of elemental boron by 3 meV/B. At finite temperatures, the difference becomes even larger.     

Effects of alkali metal halide salts on the hydrogen bond network of liquid water

Measurements of the oxygen K-edge X-ray absorption spectrum (XAS) of aqueous sodium halide solutions demonstrate that ions significantly perturb the electronic structure of adjacent water molecules. The addition of halide salts to water engenders an increase in the preedge intensity and a decrease in the postedge intensity of the XAS, analogous to those observed when increasing the temperature of pure water. The main-edge feature exhibits unique behavior and becomes more intense when salt is added. Density functional theory calculations of the XAS indicate that the observed red shift of the water transitions as a function of salt concentration arises from a strong, direct perturbation of the unoccupied molecular orbitals on water by anions, and does not require significant distortion of the hydrogen bond network beyond the first solvation shell. This contrasts the temperature-dependent spectral variations, which result primarily from intensity changes of specific transitions due to geometric rearrangement of the hydrogen bond network.     

Are defect models consistent with the entropy and specific heat of glass formers?

We show that pointlike defect model of glasses cannot explain the thermodynamic properties of glass formers, as for example, the excess specific heat close to the glass transition, contrary to the claim of Garrahan and Chandler [Proc. Natl. Acad. Sci. U.S.A. 100, 9710 (2003)]. More general models and approaches in terms of extended defects are also discussed.     

Staphylococcus aureus bicomponent gamma-hemolysins, HlgA, HlgB, and HlgC, can form mixed pores containing all components

Staphylococcal gamma-hemolysins are bicomponent toxins forming a protein family with leucocidins and alpha-toxin. Two active toxins (AB and CB) can be formed combining one of the class-S components, HlgA or HlgC, with the class-F component HlgB. These two gamma-hemolysins form pores with marked similarities to alpha-toxin in terms of conductance, nonlinearity of the current-voltage curve, and channel stability in the open state. AB and CB pores, however, are cation-selective, whereas alpha-toxin is anion-selective. gamma-Hemolysins' pores are hetero-oligomers formed by three or four copies of each component (indicated as 3A3B and 3C3B or 4A4B and 4C4B). Point mutants located on a beta-strand of the class-S component that forms part of the protomer-protomer contact region can prevent oligomer assembly. Interestingly, these mutants inhibit growth of pores formed not only by their natural components but also by nonstandard components. This lead to the hypothesis that mixed ABC pores could also be formed. By studying the conductance of pores, assembled in the presence of all three components (in different ratios), it was observed that the magnitudes expected for mixed pores were, indeed, present. We conclude that the gamma-hemolysin/leucocidin bicomponent toxin family may form a larger than expected number of active toxins by cross-combining various S and F components.     

Anisotropic Lithium Ion Conductivity in Single‐Ion Diblock Copolymer Electrolyte Thin Films

Well‐defined single‐ion diblock copolymers consisting of a Li‐ion conductive poly(styrenesulfonyllithium(trifluoromethylsulfonyl)imide) (PSLiTFSI) block associated with a glassy polystyrene (PS) block have been synthesized via reversible addition fragmentation chain transfer polymerization. Conductivity anisotropy ratio up to 1000 has been achieved from PS‐b‐PSLiTFSI thin films by comparing Li‐ion conductivities of out‐of‐plane (aligned) and in‐plane (antialigned) cylinder morphologies at 40 °C. Blending of PS‐b‐PSLiTFSI thin films with poly(ethylene oxide) homopolymer (hPEO) enables a substantial improvement of Li‐ion transport within aligned cylindrical domains, since hPEO, preferentially located in PSLiTFSI domains, is an excellent lithium‐solvating material. Results are also compared with unblended and blended PSLiTFSI homopolymer (hPSLiTFSI) homologues, which reveals that ionic conductivity is improved when thin films are nanostructured.

     

Behaviour of Pluronic P84 block copolymer micelles above the gelification temperature as probed by positron annihilation lifetime spectroscopy

The new method based on positron annihilation lifetime spectroscopy (PALS) to determine both the mean core radius, R(core), and aggregation number, N(ag), of micelles is applied to the study of aqueous solutions of the triblock Pluronic P84 copolymer as a function of temperature (T), beyond the gelification point (334 K). Two long-lived components appear in the PALS spectra, ascribed to triplet positronium in the water bulk (o-Ps(aq)) and in the organic core of the micelles (o-Ps(org)). Of the various fitting parameters, only the lifetime of the latter species, tau4, and the micellar parameters, R(core) and N(ag), disclose the occurrence of gelification by first increasing up to 334 K, then decreasing. By contrast to what is known in case of phase transition, none of the parameters shows any abrupt change at 334 K, whereas the macroscopic viscosity of the solutions suffers a drastic increase. This is attributed to the fact that positronium is sensitive to the microviscosity of the solutions. At the transition point, the properties of the polyoxipropylene aggregates forming the organic core of the P84 micelles are not greatly affected. Furthermore, the fact that the experimental N(ag) values coincide with those calculated for spheres, from the R(core) values, indicates that the shape of the P84 cores does not change significantly after gelification. The onset of gelification results from a decrease in the hydrogen bonding interactions in the solution with an ensuing relative increase in the interactions between the polyoxipropylene (PPO) groups, initially forming the corona of the P84 micelles, in an intermicellar mode. This increased solicitation of the PPO groups outside their initial location would result in depletion in the number of surfactant molecules forming the micelles, viz. a decrease in both R(core) and N(ag) above 334 K. From the data, additional information can be gained regarding the local viscosity and surface tension in the micellar cores.