Complex salts of [ReII(NO)Br4(pyz)]−: synthesis,crystal structures,and DFT studies

Two nitrosyl Re(II) complexes formulated as [Ni(bipy)₃][Re(NO)Br₄(pyz)]₂ and [Cu(bipy)₂Br][ReNOBr₄(pyz)] (pyz = pyrazine, bipy = 2,2′-bipyridine) were synthesized and characterized by single-crystal X-ray diffraction. The pyrazine in [Re(NO)Br₄(pyz)]^? was not able to act as bridge toward a second metal ion, and the two salts were obtained. Computational studies at the density functional level of theory show that the charge on the nitrogen, which could be available for bridging, is dramatically reduced to less than half, decreasing its capability to bind a second metal ion.     

Single-molecule junctions based on nitrile-terminated biphenyls: a promising new anchoring group

We present a combined experimental and theoretical study of the electronic transport through single-molecule junctions based on nitrile-terminated biphenyl derivatives. Using a scanning tunneling microscope-based break-junction technique, we show that the nitrile-terminated compounds give rise to well-defined peaks in the conductance histograms resulting from the high selectivity of the N-Au binding. Ab initio calculations have revealed that the transport takes place through the tail of the LUMO. Furthermore, we have found both theoretically and experimentally that the conductance of the molecular junctions is roughly proportional to the square of the cosine of the torsion angle between the two benzene rings of the biphenyl core, which demonstrates the robustness of this structure-conductance relationship.     

A theoretical study of the electronic properties of hydrogenated spherical-like SiC quantum dots with C-rich and Si-rich compositions

Quantum dots have many potential applications in opto-electronics, energy storage, catalysis, and medical diagnostics, silicon carbide quantum dots could be very attractive for many biological and technological applications due to their chemical inertness and biocompatibility, however, there are seldom theoretical studies that could boost the development of these applications. In this work, the electronic properties of hydrogenated spherical-like SiC quantum dots with C-rich and Si-rich compositions are investigated using density functional theory calculations. The quantum dots are modeled by removing atoms outside a sphere from an otherwise perfect SiC crystal, the surface dangling bonds are passivated with H atoms. Our results exhibit that the electronic properties of the SiC-QD are strongly influenced by their composition and diameter size. The energy gap is always higher than that of the crystalline SiC, making these SiC QD's suitable for applications at harsh temperatures. The density of states and the energy levels show that the Si-rich quantum dots had a higher density of states near the conduction band minimum, which indicates better conductivity. These results could be used to tune the electronicproperties of SiC quantum dots for optoelectronic applications.     

Network planning under uncertainty with an application to hydropower generation

Summary We present a general modeling framework for therobust optimization of linear network problems with uncertainty in the values of the right-hand side. In contrast to traditional approaches in mathematical programming, we use scenarios to characterize the uncertainty. Solutions are obtained for each scenario and these individual scenarios are aggregated to yield a nonanticipative or implementable policy that minimizes the regret of wrong decisions. A given solution is termed robust if it minimizes the sum over the scenarios of the weighted upper difference between the objective function value for the solution and the objective function value for the optimal solution for each scenario, while satisfying certain nonanticipativity constraints. This approach results in a huge model with a network submodel per scenario plus coupling constraints. Several decomposition approaches are considered, namely Dantzig-Wolfe decomposition, various types of Benders decomposition and different quadratic network approaches for approximating Augmented Lagrangian decomposition. We present computational results for these methods, including two implementation versions of the Lagrangian based method: a sequential implementation and a parallel implementation on a network of three workstations.     

Decanedioic Acid (C10H18O4)/Dodecanedioic Acid (C12H22O4) System: Polymorphism of the Components and Experimental Phase Diagram

The experimental temperature/composition phase diagram of the binary system decanedioic acid (C₁₀H₁₈O₄)/dodecanedioic acid (C₁₂H₂₂O₄) was established by combining X‐ray powder diffraction (XRD), differential‐scanning calorimetry (DSC), infrared spectroscopy (IR), scanning electron microscopy (SEM), and thermo‐optical microscopy (TOM). Both compounds crystallize in the same ordered form, C (P2₁/c), which is the phase that melts in both cases. The C form melts in C₁₂H₂₂O₄ earlier than in C₁₀H₁₈O₄, in contrast to other unbranched‐chain compounds (alkanes, alkanols, and alkanoic acids) in which the melting temperatures increase as the C‐atom number rises. Contrary to what might be expected, total solid‐state miscibility is not observed. The C₁₀H₁₈O₄/C₁₂H₂₂O₄ binary system shows a complex phase diagram. At low temperatures, a new monoclinic form, C_i (P2₁/c), stabilizes as a result of the disorder of composition in the mixed samples; two [C+C_i] domains appear. Upon heating, four solid–solid and seven solid–liquid domains appear related by eutectic and peritectic invariants. All the crystallographic forms observed are isostructural.     

Calorimetric measurement of the CH/pi interaction involved in the molecular recognition of saccharides by aromatic compounds

Can a benzene molecule differentiate between two isomeric carbohydrates? It is generally accepted that two factors govern molecular recognition: complementarity and preorganization. Preorganization requires the presence of cavities for positioning the host's groups of complementary nature to those of the guest. This study shows that, in fact, groups should be complementary to recognize each other (for the case presented here, it is controlled by the CH/pi interaction) but preorganization is not essential. Since weak interactions have their origin in dispersion forces, they also have impact on the enthalpic term of the free energy, so it was considered that their participation can be demonstrated by measuring the energy involved. For recognition to happen, two conditions must be satisfied: specificity and associated stabilizing energy. In this study we evaluated the heat of dissolution of different carbohydrates such as methyl 2,3,4,6-tetra-O-methyl-alpha-d-mannopyranoside and methyl 2,3,4,6-tetra-O-methyl-beta-d-galactopyranoside using different aromatic solvents. The solvation enthalpies in benzene were -78.8 +/- 3.9 and -88.7 +/- 5.5 kJ mol(-1) for each carbohydrate, respectively; and these values yielded a CH/pi energy of interaction of 9.9 kJ mol(-1). In addition, NMR studies of the effect of the addition of benzene to chloroform solutions of the two carbohydrates showed that benzene specifically interacts with the hydrogen atoms of the pyranose ring at positions 3, 4, and 5 located on the alpha face of the methyl-beta-galactoside, so it is, in fact, able to recognize it. Thus, the interactions between carbohydrates and the aromatic residues of proteins occur in the absence of the confinement generated by the protein structure. By experimentally measuring the energy associated with this interaction and comparing it to theoretical calculations, it was also possible to unequivocally determine the existence of CH/pi interactions between carbohydrates and proteins.     

In situ neutron diffraction study on Pd-doped Mg0.65Sc0.35 electrode material

The behavior of a negative composite electrode made of Pd-doped Mg_0.65Sc_0.35 active material has been studied dynamically by in situ neutron diffraction during a complete charge-discharge electrochemical cycle starting from a virgin alloy. From the analysis of the collected diffraction patterns, phase identification, phase amounts, structural changes and cell volume evolutions have been determined as a function of the electrochemical state of (dis)charge. For the first charge, the active material shows a structural transformation from body-centered cubic (bcc) to face-centered cubic (fcc). During the following discharge, a two-phase behavior with equilibrium between a hydrogen-poor fcc phase and a hydrogen-rich one is observed. Complete discharge beyond the hydrogen-poor phase was not reached and a good electrochemical reversibility is expected between the hydrogen-poor and the hydrogen-rich fcc phases for this electrode material.     

Synthesis of IB-01211, a cyclic peptide containing 2,4-concatenated thia- and oxazoles, via Hantzsch macrocyclization

[structure: see text] An efficient and versatile convergent synthesis of IB-01211 based on a combination of peptide and heterocyclic chemistry is described. The key step in the synthesis is macrocyclization through intramolecular Hantzsch formation of the thiazole ring. Dehydration of a free primary alcohol to furnish the exocyclic methylidene present in the natural product was applied during the macrocyclization.     

Even-odd effect in Andreev transport through a carbon nanotube quantum dot

We have measured the current (I)-voltage (V) characteristics of a single-wall carbon nanotube quantum dot coupled to superconducting source and drain contacts in the intermediate coupling regime. Whereas the enhanced differential conductance dI/dV due to the Kondo resonance is observed in the normal state, this feature around zero-bias voltage is absent in the superconducting state. Nonetheless, a pronounced even-odd effect appears at finite bias in the dI/dV subgap structure caused by Andreev reflection. The first-order Andreev peak appearing around V=Delta/e is markedly enhanced in gate-voltage regions, in which the charge state of the quantum dot is odd. This enhancement is explained by a "hidden" Kondo resonance, pinned to one contact only. A comparison with a single-impurity Anderson model, which is solved numerically in a slave-boson mean-field approach, yields good agreement with the experiment.