Growth and structure of water on SiO2 films on Si Investigated by Kelvin probe microscopy and in Situ X-ray spectroscopies

The growth of water on thin SiO2 films on Si wafers at vapor pressures between 1.5 and 4 Torr and temperatures between -10 and 21 degrees C has been studied in situ using Kelvin probe microscopy and X-ray photoemission and absorption spectroscopies. From 0 to 75% relative humidity (RH), water adsorbs forming a uniform film 4-5 layers thick. The surface potential increases in that RH range by about 400 mV and remains constant upon further increase of the RH. Above 75% RH, the water film grows rapidly, reaching 6-7 monolayers at around 90% RH and forming a macroscopic drop near 100%. The O K-edge near-edge X-ray absorption spectrum around 75% RH is similar to that of liquid water (imperfect H-bonding coordination) at temperatures above 0 degrees C and is ice-like below 0 degrees C.     

Highly enantioselective alpha-aminoxylation of aldehydes and ketones with a polymer-supported organocatalyst

The first catalytic enantioselective alpha-aminoxylation of aldehydes and ketones using an insoluble, polymer-supported organocatalyst (1) derived from trans-4-hydroxyproline is reported (ee: 96-99%). Reaction rates in the aminoxylation of cyclic ketones with 1 are higher than those reported with l-proline. The insoluble nature of 1 simplifies workup conditions and allows catalyst recycling without an apparent decrease in enantioselectivity or yield.     

Highly enantioselective Michael additions in water catalyzed by a PS-supported pyrrolidine

The development of a highly efficient, polymer-supported organocatalyst for the Michael addition of ketones to nitroolefins is described. A 1,2,3-triazole ring, constructed through a click 1,3-cycloaddition, plays the double role of grafting the chiral pyrrolidine monomer onto the polystyrene backbone and of providing a structural element, complementary to pyrrolidine, key to high catalytic activity and enantioselectivity. Optimal operation in water and full recyclability make the triazole linker attractive for the immobilization of organocatalysts.     

Carbon nanoparticles in lateral flow methods to detect genes encoding virulence factors of Shiga toxin-producing <Emphasis Type="Italic">Escherichia coli</Emphasis>

The use of carbon nanoparticles is shown for the detection and identification of different Shiga toxin-producing Escherichia coli virulence factors (vt1, vt2, eae and ehxA) and a 16S control (specific for E. coli) based on the use of lateral flow strips (nucleic acid lateral flow immunoassay, NALFIA). Prior to the detection with NALFIA, a rapid amplification method with tagged primers was applied. In the evaluation of the optimised NALFIA strips, no cross-reactivity was found for any of the antibodies used. The limit of detection was higher than for quantitative PCR (q-PCR), in most cases between 10⁴ and 10⁵ colony forming units/mL or 0.1–0.9 ng/μL DNA. NALFIA strips were applied to 48 isolates from cattle faeces, and results were compared to those achieved by q-PCR. E. coli virulence factors identified by NALFIA were in very good agreement with those observed in q-PCR, showing in most cases sensitivity and specificity values of 1.0 and an almost perfect agreement between both methods (kappa coefficient larger than 0.9). The results demonstrate that the screening method developed is reliable, cost-effective and user-friendly, and that the procedure is fast as the total time required is <1 h, which includes amplification.     

Measuring electron sharing between atoms in first-principle simulations

Calculations of large scale electronic structure within periodic boundary conditions, mostly based on solid state physics, allow the modeling of atomic forces and molecular dynamics for atomic assemblies of 100–1000 atoms, thus providing complementary information in material and macromolecular sciences. Nevertheless, these methods lack connections with the chemistry of simple molecules as isolated entities. In order to contribute to establish a conceptual connection between solid state physics and chemistry, the calculation of the extent of electron sharing between atoms, also known as delocalization index, is performed on simple molecules and on complexes with transition metal atoms, using density functional calculations where the Kohn–Sham molecular orbitals are represented in terms of plane waves and in periodic boundary conditions. These applications show that the useful measure of electron sharing between atomic pairs can be recovered from density functional calculations using the same set-up applied to large atomic assemblies in condensed phases, with no projections of molecular orbitals onto atomic orbitals.     

A multi-scale approach to spin crossover in Fe(II) compounds

We report here for the first time a multi-scale study on the concept of spin-crossover compounds, which integrates improved density functionals, a polarizable force field and hybrid QM/MM calculations. This multi-scale setup is applied to the temperature dependence of spin states of a Fe(II) compound with trispyrazolylborate ligands that exhibits spin-crossover. Our study shows a transition temperature of around 290 K, which is in perfect agreement with experimental results. Moreover, based on our data we provide the origin of why spin transition occurs in this iron-compound: it results directly from spin-state changes in the iron-compound that lead to more favourable electrostatic interactions for the high-spin state.     

All-metal aromatic clusters M4(2-) (M = B, Al, and Ga). Are π-electrons distortive or not?

The π-electrons in benzene, the quintessential aromatic molecule, were previously shown to be distortive, i.e., they prefer localized double bonds alternating with single bonds. It is the σ-electrons that force the double bonds to delocalize, leading to a regular, D(6h) geometry. Herein, we computationally investigate the double-bond localizing or delocalizing propensities of σ- and π-electrons in the archetypal all-metal aromatic cluster Al(4)(2-) and its second- and fourth-period analogs B(4)(2-) and Ga(4)(2-), using Kohn-Sham molecular orbital (MO) theory at BP86/TZ2P in combination with quantitative bond energy decomposition analyses (EDA). We compare the three all-metal aromatic clusters with the structurally related organic species C(4)H(4)(2+), C(4)H(4), and C(4)H(4)(2-). Our analyses reveal that the π-electrons in the group-13 M(4)(2-) molecules have a weak preference for localizing the double bonds. Instead, the σ-electrons enforce the regular D(4h) equilibrium geometry with delocalized double bonds.     

Aryl-O reductive elimination from reaction of well-defined aryl-Cu(III) species with phenolates: the importance of ligand reactivity

Well-defined aryl-Cu(III) species undergo rapid reductive elimination upon reaction with phenolates (PhO(-)), to form aryl-OPh cross-coupling products. Kinetic studies show that the reaction follows a different mechanistic pathway compared to the reaction with phenols. The pH active cyclized pincer-like ligand undergoes an initial amine deprotonation that triggers a faster reactivity at room temperature. A mechanistic proposal for the enhanced reactivity and the role of EPR-detected Cu(II) species will be discussed in detail.