1,721,016 research outputs found
Synthesis, characterization and XRPD studies of metal-organic frameworks containing 1,4 bis(4-pyrazolyl) and 1,4 bis(5-tetrazolyl)benzene ligands
No full textIn the last decade we have been deeply exploring the coordination chemistry of polydentate ligands possessing multiple donor sites. Nowadays we decided to increased the complexity of the polytopic N-ligands aiming at the formation of oligomeric and polymeric species with different functional properties.
Inspires to the 1,4-benzene-dicarboxylate (BDC) the ligand of the highly porous and capable of H2 sorption MOF-5 proposed by Yaghi et al., and planning to employ polytopic longer spacers possessing nitrogen coordination sites (with the possibility of an exobidentate coordination), we turned our attention to the 1,4-bis(5-tetrazolyl)benzene species (C8H6N8, H2btb).
After having tailored high-yield, cheap and atoxic syntheses for H2btb [1], we started coupling this ligand with a transition metal ions aiming at the formation of microcrystalline materials of well defined stoichiometry, structure and functional properties: nanoporosity, cooperative magnetism and catalytic activity.
The isolated materials shows a variety of stoichiometries, each one with its own peculiar structure: homoleptic species [Ag2(btb), Cu(btb) and Co(btb)], hydrated (or solvated) compounds [Co(btb)•2H2O and Zn(btb)•nDMF], and even hydroxo complexes [Cu2(μ3-OH)2(btb)]. These compounds are either dense or moderately porous and, therefore, they are not prone to gas sorption or storage, as originally targeted [2].
In this context, we decided to develop a new strategy to synthesized the analogous 1,4-bis(4-pyrazolyl)benzene (C12H10N4, H2bpb) [1] with the hope to obtain microporous and non-dense species with the coordination of the metal transition elements. Specifically, in our research, we have coupled these ligands with Ni, Cu, Zn, Hg to afford the species Ni(bpb), Zn(bpb), Cu2(bpb) and Hg(bpb). These species are isolated by solvothermal reaction and fully characterized by elemental and thermogravimetric analysis, IR spectroscopy and we attained their full structural characterization resorting to state-of-the-art powder diffraction methods, occasionally coupled to thermodiffractometry analysis.
Adsorption capacities are also studied with preliminary gas sorption analysis, towards N2, and showing attractive adsorption capacities of the compound Ni(bpb) and Zn(bpb) in agreement with their structural characterization that shows a very low density and regulars networks.
[1] A. Maspero, S. Galli, N. Masciocchi, G. Palmisano, Chem. Lett., 2008, in press.
[2] A.Maspero, S. Galli, V.Colombo, G.Peli, N.Masciocchi, S.Stagni, E.Barea, J.A.R. Navarro, Inorg. Chim. ACTA, 2008, in press
Birkhoff coordinates of integrable Hamiltonian systems in asymptotic regimes
Full text linkIn this thesis we investigate two examples of infinite dimensional integrable Hamiltonian systems in -space dimension: the Toda chain with periodic boundary conditions and large number of particles, and the Korteweg-de Vries (KdV) equation on .
In the first part of the thesis we study the Birkhoff coordinates (Cartesian action angle coordinates) of the Toda lattice with periodic boundary condition in the limit where the number of the particles tends to infinity. We prove that the transformation introducing such coordinates maps analytically a complex ball of radius
(in discrete Sobolev-analytic norms) into a ball of radius (with independent of ) if and only if
. Then we consider the problem of equipartition of energy in the spirit of Fermi-Pasta-Ulam. We deduce that corresponding to initial data of size , , and with only the first Fourier mode excited, the energy remains forever in a packet of Fourier modes exponentially decreasing with the wave number. Finally we consider the original FPU model and prove that energy remains localized in a similar packet of Fourier modes for times one order of magnitude longer than those covered by previous results which is the time of formation of the packet. The proof of the theorem on Birkhoff coordinates is based on a new quantitative version of a Vey type theorem by Kuksin and Perelman which could be interesting in itself.
In the second part of the thesis we study the scattering map of the KdV on . We prove that in appropriate weighted Sobolev spaces of the form , with integers and in the case of no bound states, the scattering map is a perturbation of the Fourier transform by a regularizing operator. As an application of this result, we show that the difference of the KdV flow and the corresponding Airy flow is 1-smoothing
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Freezing of Energy of a Soliton in an External Potential
No full textIn this paper we study the dynamics of a soliton in the generalized NLS with a small external potential ϵV of Schwartz class. We prove that there exists an effective mechanical system describing the dynamics of the soliton and that, for any positive integer r, the energy of such a mechanical system is almost conserved up to times of order ϵ−r. In the rotational invariant case we deduce that the true orbit of the soliton remains close to the mechanical one up to times of order ϵ−r
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Birkhoff coordinates for the Toda lattice in the limit of infinitely many particles with an application to FPU
Full text linkIn this paper we study the Birkhoff coordinates (Cartesian action angle coordinates) of the Toda lattice with periodic boundary condition in the limit where the number N of the particles tends to infinity. We prove that the transformation introducing such coordinates maps analytically a complex ball of radius R/Nα (in discrete Sobolev-analytic norms) into a ball of radius R′/Nα (with R,R′>0 independent of N) if and only if α≥2. Then we consider the problem of equipartition of energy in the spirit of Fermi-Pasta-Ulam. We deduce that corresponding to initial data of size R/N2, 0<R≪1, and with only the first Fourier mode excited, the energy remains forever in a packet of Fourier modes exponentially decreasing with the wave number. Finally we consider the original FPU model and prove that energy remains localized in a similar packet of Fourier modes for times one order of magnitude longer than those covered by previous results which is the time of formation of the packet. The proof of the theorem on Birkhoff coordinates is based on a new quantitative version of a Vey type theorem by Kuksin and Perelman which could be interesting in itself
Highly stable isoreticular polypyrazolate based mofs
No full textAn important goal in devising technologically relevant metal-organic frameworks (MOFs) lies in the synthesis of robust networks capable of withstanding sustained use under extreme conditions. Together with this feature, another important challenge in this field is the development of stable structures including functional pending groups, with the aim of tuning pore properties (size and shape, volume and decoration), possibly enhancing uptake capacity and selectivity toward a targeted guest. Along this line, we have recently demonstrated that, using linear or triangular polytopic pyrazolate-based bridging ligands, some of the most stable porous metal-organic structures ever prepared can be isolated, as a result of the strong metal-pyrazolate framework bonds.1 We have now prepared two large series of isostructural pyrazolate-based MOFs (bearing NO2, NH2, OH, SO3H functional groups), reminiscent of NiBDP and ZnBDP parent structures [H2BDP = 1,4-bis(1H-pyrazol-4-yl)benzene]. In this contribution, our recent results upon these two families of MOFs will be presented, which include the description of i) the pre-synthesis functionalization of the four tagged-ligands, ii) the solvothermal synthesis of the related MOFs and iii) their extensive structural characterization by means of variable temperature X-ray powder diffraction analysis. Finally, the relationships between the nature of the organic residues and the structural and functional properties (gas sorption and separation) of the resulted materials will be highlighted
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