A systematic density functional theory study of the electronic structure of bulk and (001) surface of transition-metals carbides

A systematic study of the bulk and surface geometrical and electronic properties of a series of transition-metal carbides (TMC with TM = Ti, V, Zr, Nb, Mo, Hf, Ta, and W) by first-principles methods is presented. It is shown that in these materials the chemical bonding is strongly covalent, the cohesive energies being directly related to the bonding-antibonding gap although the shift of the center of the C(2s) band related peak in the density of states with respect to diamond indicates that some metal to carbon charge transfer does also take place. The (001) face of these metal carbides exhibits a noticeable surface rumpling which grows along the series. It is shown that neglecting surface relaxation results in very large errors on the surface energy and work function. The surface formation induces a significant shift of electronic energy levels with respect to the corresponding values in the bulk. The extent and nature of the shift can be understood from simple bonding-antibonding arguments and is enhanced by the structural rippling of this surface.     

Determination of the micelle-solute association constants of some benzene and naphthalene derivatives by micellar high performance liquid chromatograph

Summary The interaction of some benzene and naphthalene derivatives with sodium dodecyl sulphate, hexadecyltrimethylmmonium bromide and polyoxyethylene [23] dodecanol micelles has been evaluated by high operformance liquid chromatography using micellar mobile phases. The micelle-solute association constants have been obtained for the compounds investigated. Good correlation between free energy of transfer for water-micelles and for octanol-water has been observed.     

In situ time-resolved characterization of Au-CeO2 and AuOx-CeO2 catalysts during the water-gas shift reaction: presence of Au and O vacancies in the active phase

Synchrotron-based in situ time-resolved x-ray diffraction and x-ray absorption spectroscopies were used to study the behavior of nanostructured {Au+AuO(x)}-CeO(2) catalysts under the water-gas shift (WGS) reaction. At temperatures above 250 degrees C, a complete AuO(x)-->Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO(2)(111) single crystal corroborate that cationic Au(delta+) species cannot be the key sites responsible for the WGS activity at high temperatures. The rate determining steps for the WGS seem to occur at the gold-ceria interface, with the active sites involving small gold clusters (<2 nm) and O vacancies.     

Standardization of multi-nuclide mixtures by a new spectrum unfolding method

A new spectrum unfolding method has been applied to double-labeled mixtures with excellent results, while the CIEMAT/NIST method has been used for gels. The convenience of applying both methods has been demonstrated with mixtures containing more than three components.⁹⁰Sr+⁹⁰Y,⁸⁹Sr,²⁰⁴TI,⁴⁵Ca and³⁵S nuclides were combined as three, four and five components, and the different quench values and activity ratios were assayed. The discrepancies between computed and experimental activities were also obtained. Mixtures with some of their components below background have been prepared in order to test low-level activities.     

Absorptiometric determination of micro- and submicroamounts of iron using an extractive method with pyridine-2-acetaldehyde salicyloylhydrazone

The formation and extraction into chloroform of iron complex with PASH (λ_max = 640 nm) was studied. Beer's law is obeyed between 2.7 and 16.0 μg · ml⁻¹ of iron, in organic phase (10 ml). The method can be applied to volume ratios V_aq.:V_org. from 1:1 to 20:1. The minimum concentration determinable in aqueous phase is 135 ng ml⁻¹ of iron. The interferences of 73 species were evaluated and eliminated when it was possible. The extraction method of the green complex was applied for the spectrophotometric determination of iron(II) in several standard, geochemical, and bromatological samples. A procedure based on the standard addition method was applied satisfactorily to the determination of as little as 25 ng of iron(II) per milliliter.     

Evaluation of enzymatic digestion and liquid chromatography-mass spectrometry peptide mapping of the integral membrane protein bacteriorhodopsin

A method for the complete peptide mapping of the model integral membrane protein bacteri-orhodopsin is demonstrated. Utilizing more effective enzymatic digestion, procedures with capillary liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) and tandem mass spectrometry (MS/MS), all predicted tryptic digestion products were detected, as well as peptides from all previously reported post-translational modifications of bacteriorhodopsin. A significant contribution of chymotryptic-like digestion products was also observed. A characterization of the behavior of hydrophobic integral membrane peptides in a reversed-phase liquid chromatographic separation is also provided. The method reported here offers improved compatibility of the solubilizing reagents with both the chromatography and mass spectrometry, rendering it suitable for high-throughput proteomic applications.     

Vitamin D: a concise synthesis of the C19 hydroxylated enyne A-ring, an interesting precursor for the preparation of C19 substituted vitamin D analogues

A new C₁₉ hydroxylated enyne 15, as potential A-ring building block of vitamin D analogues, was synthesized in enantiomerically pure form in nine steps from (−)-(S)-limonene. This short synthesis involved ozonolyzis of 1,2-limonene oxide followed by a Criegee rearrangement, epoxide trans diaxial ring opening by lithium acetylide, elimination, epoxidation and syn β-elimination of the resulting homopropargylic oxirane.     

Oligomérisation anionique de l'ethylène. I. Etude cinétique

A kinetic study of ethylene oligomerization in hexane, in the presence of n-BuLi–TMEDA complexes, allowed us to suggest a new mechanism for anionic ethylene oligomerization. n-BuLi and n-Bu(CH₂CH₂)Li species have the same reactivity. The RLi–TMEDA complex in a 1-to-1 stoichiometry is the active species. The following kinetic equation has been established: It reflects the intervention of associated species (n-BuLi–TMEDA)₂ as well as the influence of the concentration of the complexing agent on the kinetics of oligomerization.     

Studies on Ziegler-Natta catalysts. Part II. Reactions between α- or β-TiCl3 and AlMe3, AlMe2Cl,or AlEt3 at various temperatures

The following reactions, carried out in the absence of solvents, has been studied: α-TiCl₃ + Al(CH₃)₃ at 20°C., β-TiCl₃ + Al(CH₃)₃ at 65°C., α-TiCl₃ + Al(CH₃)₂Cl at 20 and 65°C., and α-TiCl₃ + Al(C₂H₅)₃ between 30 and 65°C. It appears that a general reaction mechanism, such as discussed in the preceding paper of this series, applies to all these reactions between TiCl₃ and aluminum alkyls. The differences in overall stoichiometry between some of these systems may be linked to differences in stability of the intermediate Ti? C bonds. In the case of α-TiCl₃ + Al(CH₃)₂Cl, alkylation is probably accompanied by fixation of the AlCH₃Cl₂ on the nonvolatile product.     

First enantioselective synthesis of (−)-neplanocin F

(−)-Neplanocin F, the natural isomer of a component of the neplanocin family was enantioselectively synthesized starting from d-γ-ribonolactone. The synthetic approach was based on the preparation of a suitable carbocyclic precursor bearing three hydroxyl groups orthogonally protected. The key steps of the synthesis were the regioselective protection of a secondary allylic alcohol over a homoallylic one and the coupling of the nucleobase with a triflate intermediate.