The "non-nucleophilic" anion [Tf2N]- competes with the nucleophilic Br-: an unexpected trapping in the dediazoniation reaction in ionic liquids

Imidazolium ionic liquids containing [Tf2N]- anion are not as innocent as they are often considered; [Tf2N]- is more reactive than Br- in heterolytic dediazoniation reactions.     

Synthesis, characterization and olefin/CO exchange reactions of pyrazolylborato complexes of copper(I)

New bis(pyrazolyl)borato olefin complexes of copper(I) of general formula Cu[BH2(3,5-(CF3)2Pz)2](olefin) have been prepared (olefins: coe = cyclooctene, van = 4-vinylanisole, clsty = 4-chlorostyrene, tevs = triethylvinylsilane, fn = fumaronitrile). The structures of Cu[BH2(3,5-(CF3)2Pz)2](L), L = coe, van, tevs, fn, have been determined by X-ray diffraction methods. Considering the two N atoms of the bis(pyrazolyl)borate ligand and the midpoint of the C-C double bond of the coordinated olefin, the compounds with L = coe, van and tevs contain a copper atom in a trigonal planar coordination. A coordination polymer with N-coordinated fumaronitrile and tetrahedral coordination of copper atoms is obtained in the case of L = fn. The carbonylation reactions of Cu[BH(2)(3,5-(CF3)2Pz)2](olefin) (olefin = coe, clsty, van, tevs), Cu[BH2(3,5-(CF3)2Pz)2](olefin) + CO<==>Cu[BH2(3,5-(CF3)2Pz)2](CO) + olefin, have been studied gas volumetrically and the thermodynamical parameters of the equilibria for the displacement of the coordinated olefin by carbon monoxide have been determined. These data for copper(I) are compared with those reported in the literature.     

Planar bilayer studies reveal multiple conductance states for synthetic anion transporters

Compounds of the general type R(1)(2)NCOCH(2)OCH(2)CO-(Gly)(3)-Pro-(Gly)(3)-OCH(2)Ph insert in phospholipid bilayers and conduct ions. Different levels of activity were observed when R(1) was either decyl or octadecyl, as judged either by Cl(-) release, detected by ion selective electrodes, or carboxyfluorescein dequenching, detected by fluorescence. Either method reports average behavior for all ionophores over all liposomes. These methods also show that at least two ionophores are involved in the formation of each pore. Planar bilayer experiments reported here confirm pore formation by these compounds but identify more than one conductance state for each. The pseudo-dimer, in which two molecules of the type shown above are covalently linked, shows only two conductance states, of which one is dominant. This state has been characterized by use of a current-voltage plot.     

Electronic and structural shell closure in AgCu and AuCu nanoclusters

The structures of AgCu clusters containing 40 atoms are investigated. The most promising structural families (fcc clusters, capped decahedra, and two types of capped polyicosahedra) are singled out by means of global optimization techniques within an atom-atom potential model. Then, representative clusters of each family are relaxed by means of density-functional methods. It is shown that, for a large majority of compositions, a complex interplay of geometric and electronic shell-closure effects stabilizes a specific polyicosahedral family, whose clusters are much lower in energy and present large HOMO-LUMO gaps. Within this family, geometric and quantum effects concur to favor magic structures associated with core-shell chemical ordering and high symmetry, so that these clusters are very promising from the point of view of their optical properties. Our results also suggest a natural growth pathway of AgCu clusters through high-stability polyicosahedral structures. Results for AuCu clusters of the same size are reported for comparison, showing that the interplay of the different effects is highly material specific.     

On the interplay between CH...O and OH...O interactions in determining crystal packing and molecular conformation: an experimental and theoretical charge density study of the fungal secondary metabolite austdiol (C12H12O5)

The total experimental electron density rho(r), its Laplacian inverted delta(2)rho(r), the molecular dipole moment, the electrostatic potential phi(r), and the intermolecular interaction energies have been obtained from an extensive set of single-crystal X-ray diffracted intensities, collected at T = 70(1) K, for the fungal metabolite austdiol (1). The experimental results have been compared with theoretical densities from DFT calculations on the isolated molecule and with fully periodic calculations. The crystal structure of (1) consists of zigzag ribbons extended along one cell axis and formed by molecules connected by both OH...O and CH...O interactions, while in a perpendicular direction, adjacent molecules are linked by short CH...O intermolecular contacts. An extensive, quantitative study of all the intra- and intermolecular H...O interactions, based not only on geometrical criteria, but also on the topological analysis of rho(r), as well as on the evaluation of the pertinent energetics, allowed us (i) to assess the mutual role of OH...O and CH...O interactions in determining molecular conformation and crystal packing; (ii) to identify those CH...O contacts which are true hydrogen bonds (HBs); (iii) to determine the relative hydrogen bond strengths. An experimental, quantitative evidence is given that CH...O HBs are very similar to the conventional OH...O HBs, albeit generally weaker. The comparison between experimental and theoretical electric dipole moments indicates that a noticeable charge rearrangement occurs upon crystallization and shows the effects of the mutual cooperation of HBs in the crystal. The total intermolecular interaction energies and the electrostatic energy contribution obtained through different theoretical methods are reported and compared with the experimental results. It is found that the new approach proposed by Spackman, based on the use of the promolecular charge density to approximate the penetration contribution to intermolecular electrostatic energies, predicts the correct relative electrostatic interaction energies in most of the cases.     

Separation of molecular constituents from a humic acid by solid-phase extraction following a transesterification reaction

A selective removal of humic constituents involved in ester groups was conducted by a boron trifluoride-methanol transesterification reaction after removal of structurally unbound lipids. An analytical separation of subfractions containing specific classes of compound liberated from the humic matrix simplifies their identification by gas chromatography-mass spectrometry. We compared the traditional liquid-liquid separation into phenolic and aliphatic fractions with the modern and versatile fractionation technique using solid-phase extraction (SPE) on aminopropyl-bonded phases. Our results showed that both methods ensured separation of the same compounds, such as di- and tri-hydroxyalkanoic acids, α-, β- and ω-hydroxy fatty acids, alkanoic acids, α,ω-alkanedioic acids, n-alkanols, phenolic acids and sterols. Moreover, the SPE method not only provided a larger recovery of compounds, but involved smaller sample and solvent requirements, and larger ease and rapidity of sample handling than the traditional liquid-liquid separation. The SPE method should be thus recommended in structural studies of natural organic matter.     

Order parameters of alpha,omega-diphenylpolyenes in a nematic liquid crystal from an integrated computational and 13C NMR spectroscopic approach

The orientational order parameters and conformational behavior of five relatively large rodlike molecules, biphenyl, trans-stilbene, 1,3-diphenyl-butadiene, 1,3,5-diphenyl-hexatriene, and 1,3,5,7-diphenyl-octatetraene, dissolved in the thermotropic liquid crystal ZLI-1167, have been studied using an integrated approach combining (13)C NMR measurements and quantum mechanical computations of carbon chemical shift tensors. Besides biphenyl, the phenyl moiety of all structures has been found to have a high rotational mobility in the temperature range of the present experiments. The rank-two order parameter in the nematic phase is found to increase steadily from the shortest to the longest term of the series at any temperature within the nematic range. The molecular biaxiality order parameter is found to be small and essentially constant with temperature, giving further support to the common assumption of effective uniaxiality for these probes.     

POSITIVE SOLUTIONS FOR A NONLINEAR ELLIPTIC PROBLEM WITH STRONG LACK OF COMPACTNESS

This paper deals with the lack of compactness in the nonlinearelliptic problem – u + u = |u|^p–2u in , u > 0in , u = 0 on , when is un unbounded domain in ⁿ and 2 <p < 2n/(n–2). In particular, a domain is provided where non-converging Palais–Smale sequences existat every energy level. Nevertheless, it is proved that the problemhas infinitely many solutions on .     

Rigidity and moduli space in Real Algebraic Geometry

Let R be a real closed field and let X be an affine algebraic variety over R. We say that X is universally map rigid (UMR for short) if, for each irreducible affine algebraic variety Z over R, the set of nonconstant rational maps from Z to X is finite. A bijective map from an affine algebraic variety over R to X is called weak change of the algebraic structure of X if it is regular and φ⁻¹ is a Nash map, which preserves nonsingular points. We prove the following rigidity theorem: every affine algebraic variety over R is UMR up to a weak change of its algebraic structure. Let us introduce another notion. Let Y be an affine algebraic variety over R. We say that X and Y are algebraically unfriendly if all the rational maps from X to Y and from Y to X are trivial, i.e., Zariski locally constant. From the preceding theorem, we infer that, if dim (X)≥1, then there exists a set of weak changes of the algebraic structure of X such that, for each t,s ∈ R with t≠s, and are algebraically unfriendly. This result implies the following expected fact: For each (nonsingular) affine algebraic variety X over R of positive dimension, the natural Nash structure of X does not determine the algebraic structure of X. In fact, the moduli space of birationally nonisomorphic (nonsingular) affine algebraic varieties over R, which are Nash isomorphic to X, has the same cardinality of R. This result was already known under the special assumption that R is the field of real numbers and X is compact and nonsingular. The author is a member of GNSAGA of CNR, partially supported by MURST and European Research Training Network RAAG 2002–2006 (HPRN–CT–00271).     

Synthesis of gem-difluorinated 1,6-naphthyridine-5,7-diones

A synthetic route toward new 8,8-difluoro-1,6-naphthyridine-5,7-diones, which are of interest as new building blocks in pharmaceutical chemistry, is described. The key steps include a copper-mediated cross-coupling of ethyl bromodifluoroacetate and 2-bromo-3-cyanopyridine, followed by hydrolysis of the nitrile function and subsequent cyclization.