A look at bonds and bonding

Structural Chemistry - Even after roughly a century of quantum theory, there is still debate, sometimes rather contentious, as to the nature of the chemical bond—or is it bonds, or is it...     

Ruthenium (II) complexes bearing thioether-appended α-iminopyridine ligands: Arene precursors permit access to κ2-N,N and κ3-N,N,S complexes

Herein we report the preparation of a series of Ru(II) complexes featuring α-iminopyridine ligands bearing thioether functionality (NNS^R, where R = Me, CH₂Ph, Ph). Metallation using [(p-cymene)RuCl]₂ permits access to Ru complexes with a κ²-N,N donor set in which the thioether moiety remains uncoordinated. In the presence of a strong field ligand such as acetonitrile or triphenylphosphine, the p-cymene moiety is displaced, and the ligand adopts a κ³-N,N,S binding mode. These complexes are characterized using a combination of solution and solid state methods, including the crystal structure of [(NNS^Me)Ru (NCMe)₂Cl]Cl. The κ²-N,N-Ru(II) complexes are shown to serve as efficient precatalysts for the oxidation of sec-phenethyl alcohol at modest loadings (alcohol: Ru = 20:1), using a variety of external oxidants and solvents. The complex bearing an S-Ph donor was found to be the most active oxidation catalyst of those surveyed, suggesting that the thioether donor plays an active role in the catalytic cycle.     

Electrostatics and Polarization in σ- and π-Hole Noncovalent Interactions: An Overview

The energetics of σ- and π-hole interactions can be described very well in terms of electrostatics and polarization, consistent with their Coulombic natures. When both of these components are taken into account, very good correlations with quantum-chemically computed interaction energies are obtained. If polarization is only minor, as when the interactions are quite weak, then electrostatics can suffice, as represented by the most positive electrostatic potential associated with the σ- or π-hole. For stronger interactions, the combination of electrostatics plus polarization is very effective even for interaction energies considerably greater in magnitude than what is normally considered noncovalent bonding. Several procedures for treating polarization are summarized, including the use of point charges and the direct inclusion of electric fields.     

Laser ablated bismuth cuprate thin films preparation and effect of oxygen nonstoichiometry upon superconductivity

Thin films of Bi₂Sr₂CaCu₂O₈ and (Bi, Pb)₂Sr₂Ca₂Cu₃O₁₀ have been prepared on monocrystalline (100) MgO substrates, using a laser ablation method with post annealing treatment. The influence of substrate temperature and oxygen pressure during deposition were investigated. SEM observations, EDS analysis, electric and magnetic measurements have been used to characterize the films. Superconducting “2212” films, with T_c(R = 0) at 80–83 K and J_c (50 K) up to 5 × 10⁵ A/cm², have been currently achieved, while Pb-doped “2223” films exhibit T_c as high as 110 K with J_c = 5 × 10⁴ A/cm² at 77 K. The effect of annealing at low temperature (350°C) in an argon flow has been studied for the 2212 phase, it shows the influence of the oxygen non-stoichiometry, i.e. of the hole carrier density upon T_c's which can be measured up to 89 K (zero resistance).     

Rapidity dependence of charged antihadron to hadron ratios in Au+Au collisions at sqrt[s(NN)]=200 GeV

We present ratios of the numbers of charged antihadrons to hadrons (pions, kaons, and protons) in Au+Au collisions at sqrt[s(NN)]=200 GeV as a function of rapidity in the range y=0-3. While the ratios at midrapidity are approaching unity, the K(-)/K(+) and p;/p ratios decrease significantly at forward rapidities. An interpretation of the results within the statistical model indicates a reduction of the baryon chemical potential from mu(B) approximately 130 MeV at y=3 to mu(B) approximately 25 MeV at y=0.     

Ternary analogues of Lie and Malcev algebras

We consider two analogues of associativity for ternary algebras: total and partial associativity. Using the corresponding ternary associators, we define ternary analogues of alternative and assosymmetric algebras. On any ternary algebra the alternating sum [a, b, c] = abc − acb − bac + bca + cab − cba (the ternary analogue of the Lie bracket) defines a structure of an anticommutative ternary algebra. We determine the polynomial identities of degree ?7 satisfied by this operation in totally and partially associative, alternative, and assosymmetric ternary algebras. These identities define varieties of ternary algebras which can be regarded as ternary analogues of Lie and Malcev algebras. Our methods involve computational linear algebra based on the representation theory of the symmetric group.     

Entropy-driven hydrogen bonding: stereodynamics of a protonated, N,N-chiral "proton sponge"

The C2-symmetric ("[DL]") and achiral ("[meso]") diastereoisomers of the hydrogen iodide salt of 1,8-bis-(N-benzyl-N-methylamino)naphthalene ([2H]-[I] ) interconvert in solution. Direct interconversion of the diastereoisomers of [2H]+ must involve hydrogen bond fission (to give "[nonHB-2H+]") and rotation-inversion of the non-protonated nitrogen centre. The global activation parameters (deltaH++ and deltaS++) for diastereoisomer interconversion in [D7]DMF have been determined from rate data obtained by temperature-drop and magnetisation-transfer 13C NMR spectroscopy over a temperature range of 170 degrees C. The process is found to have a high entropy of activation in both directions (deltaS++=163(+/-4) and 169(+/-4) JK(-1)mol(-1)) and this is suggested to arise through hydrogen bonding of the ammonium centre in [nonHB-2H+] with the solvent ([D7]DMF). Comparison of the enthalpy of activation (deltaH++) with that earlier found for diastereoisomer interconversion of the free-base form 2 suggests that the intramolecular hydrogen bond in [2H]+ is roughly equal in enthalpic strength (deltaH) with that made with the solvent ([D7]DMF) in the non-hydrogen-bonded intermediate [nonHB-2H+]. As such, the hydrogen bonding in [2H]+ may be considered as predominantly an entropically driven process, without any unusual enthalpic strength.     

Hydrophobic bacteria at the hexadecane-water interface: examination of micrometre-scale interfacial properties

Hydrophobic bacteria, like colloidal solids, can spontaneously adsorb onto fluid-fluid interfaces and modify their mechanical properties. In this study, two strains of bacteria--Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c--were prepared in their stationary (i.e. non-dividing) phase in the absence of biosurfactants; the cells were then used as emulsifiers to stabilize n-hexadecane droplets in aqueous environments. Using the micropipette technique, colloidal stability of the bacteria-coated droplets was examined through direct-contact experiments. Both types of bacteria were seen to function as effective stabilizers, although the Acinetobacter venetianus RAG-1 film provided stronger resistance to droplet-droplet coalescence. In addition to creating steric barriers, the adsorbed bacteria also interacted with one another at the interface, giving rise to higher order rheological properties. A technique of directly probing the mechanical properties of the emulsion drop surfaces (i.e. the adsorbed films) on the micrometre-scale revealed that (a) the films behaved as purely elastic sheets, and (b) with a reduction in cell concentration in the aqueous phase, less oil was emulsified, but the elastic moduli of the adsorbed films remained unchanged (suggesting an "all or none" adsorption process). These results are in contrast to a previous macroscopic (i.e. millimetre-scale) study, which showed that the absorbed films were viscoelastic, with the apparent elastic moduli depending strongly on cell concentration. The rheological properties of these bacteria-adsorbed interfaces appeared therefore to be length scale-dependent.     

NIR luminescence intensities increase linearly with proportion of polar thiolate ligands in protecting monolayers of Au38 and Au140 quantum dots

The near-infrared photoluminescence of monolayer-protected Au38 and Au140 clusters (MPCs) is intensified with exchange of nonpolar ligands by more polar thiolate ligands. The effect is general and includes as more polar in-coming ligands: thiophenolates with a variety of p-substituents; alkanethiolates omega-terminated by alcohol, acid, or quaternary ammonium groups; and thio-amino acids. Remarkably, place exchanges of the initial phenylethanethiolates on Au38 MPCs by p-substituted thiophenolates and thio-amino acids and of hexanethiolates on Au140 MPCs by omega-quaternary ammonium terminated undecylthiolates result in increases in the near-infrared (NIR) luminescence intensities that are linear with the number of new polar ligands. The increased intensities are systematically larger for thiophenolate ligands having more electron-withdrawing substituents. Analogous effects on intensities are observed in the NIR emission of Au140 MPCs upon place exchange of alkanethiolates with thiolates having short connecting alkanethiolate chains to quaternary ammonium and to omega-carboxylic acid termini, and with oxidative charging of the Au cores. The observations are consistent with sensitivity of the luminescence mechanism to any factor that enhances the electronic polarization of the bonds between the Au core atoms and their thiolate ligands. The luminescence is discussed in terms of a surface electronic excitation, as opposed to a core volume excitation.     

Inhibition of solute crystallisation in aqueous H(+)-NH(4)(+)-SO4(2-)-H2O droplets

Ice clouds in the Earth's upper troposphere can form via homogeneous nucleation of ice in aqueous droplets. In this study we investigate the crystallisation, or lack of crystallisation, of the solute phase and ice in aqueous (NH(4))(3)H(SO(4))(2)/H(2)O and NH(4)HSO(4)/H(2)O droplets. This is done using in situ X-ray diffraction of emulsified solution droplets mounted on a cold stage. From the diffraction patterns we are able to identify the phases of crystalline solute and ice that form after homogeneous freezing in micrometer sized droplets. An important finding from this study is that crystallisation of the solute does not always occur, even when crystallisation is strongly thermodynamically favoured. The nucleation and growth of solute phase crystals becomes inhibited since the viscosity of the aqueous brine most likely increases dramatically as the brine concentration increases and temperature decreases. If ice nucleates below a threshold freezing temperature, the brine appears to rapidly become so viscous that solute crystallisation is inhibited. This threshold temperature is 192 K and 180 K, in (NH(4))(3)H(SO(4))(2) and NH(4)HSO(4), respectively. We also speculate that the formation of cubic ice within a highly viscous brine blocks the solvent mediated cubic to hexagonal phase transformation, thus stabilising the metastable cubic ice in the most concentrated solution droplets.