1,720,995 research outputs found
Flexible porous molecular materials responsive to CO2, CH4and Xe stimuli
Full text linkIn the search for flexible molecular crystals endowed with porosity, we achieved the fabrication of expandable crystalline prototypal structures, which allow the absorption of gases, without modifying the crystal architecture. The design brings together highly symmetrical tetrahedral elements to construct swellable porous adamantoid frameworks through co-operation of eight surrounding hydrogen bonds mounted on conformationally flexible groups. The flexibility of the porous crystals manifests itself in response to stimuli of selected gases, which promote reversible conformational changes, inducing breathing in the molecular structure. The backbone of the reticular construction is based on the formation of the carboxylic dimers, which project outwards from the tetrahedral molecular core to consolidate the 3D framework. Contact with proper gases such as CO2, Xe and hexane triggers a 56–70% enlargement of the channel cross-section. The accommodation of CO2 and Xe in the channel chambers was revealed by synchrotron-light X-ray diffraction, combined with molecular dynamics and density functional theory (DFT) theoretical calculations. Rare experimental observations of xenon dynamics, in which Xe diffuses along the channels and experiences different chamber orientations in the crystal, were gathered by analysing 129Xe NMR chemical shift anisotropy profiles, which encode the shape and orientation of each visited cavity along the channel. The jump rate and activation energy experienced was uniquely established by exploring Xe atoms in their diffusional path. Nitrogen showed a low affinity to the matrix and was unable to enlarge the pores, thus it was excluded from the restrictive pores of the empty crystal. Given the properties of molecular crystals, it is possible to outline some advantageous aspects, such as simple design, easy self-assembly, solubility, reversible gas uptake and absence of metal ions, and they can thus be considered for eco-friendly gas capture and separation
Heteroleptic Co(III) bisdithiocarbamato-dithione complexes: Synthesis, structure and bonding of [Co(Et(2)dtc)(2)(R(2)pipdt)]BF4 (R = Me, 1; Ph, 2; pipdt = piperazin-2,3-dithione) complexes
No full textThe reaction between the binuclear cobalt complex, [Co-2(Et(2)dtc)(5)](+), and Me(2)pipdt and Ph(2)pipdt ligands has provided almost quantitatively the cobalt tris-chelate heteroleptic complexes [Co(Et(2)dtc)(2)(R(2)pipdt)]BF4 (1 and 2). The molecular structure of 2 shows the metal in a distorted octahedral geometry. The nature of the bonding in these complexes has been elucidated with the support of DFT TD-DFT calculations. Both chelating S,S donors work as weak-field ligands. The comparison of the chemical reactivity for the homoleptic dithiocarbamate complex [Co(Et(2)dtc)(3)] and the heteroleptic [Co(Et(2)dtc)(2)(Ph(2)pipdt)](+) derivative shows that the global softness sigma is significantly higher in [Co(Et(2)dtc)(2)(Ph(2)pipdt)](+) than in the homoleptic dithiocarbamate complex, due to a reduction of nephelauxetic effect induced by the dithioxamide ligand. The kinetics for the reaction between the reagents in CH2Cl2 has been followed spectrophotometrically as a function of temperature in pseudo-first order conditions with respect to R(2)pipdt ligands. Kinetic results further support a reaction mechanism involving a one-end reversible dissociation of the [Co-2(Et(2)dtc)(5)](+) dimer forming a reactive cobalt(III)dithiocarbamato center susceptible to attack by nucleophiles. The effectiveness and versatility of the above reaction is an easy and clean method to provide heteroleptic-dithiocarbamates with a variety of suitable ligands of interest for applicative purposes
A Combined Solution and Solid-State Study on the Tautomerism of an Azocalix[4]arene Chromoionophore
No full textAzocalixarenes functionalized with cation binding sites are popular chromoionophores due to the ease of synthesis and the large complexation-induced shifts of their absorption band that originate from an azo-phenol-quinone-hydrazone tautomerism. Despite their extensive use, however, a thorough investigation of the structure of their metal complexes has not been reported. We describe herein the synthesis of a new azocalixarene ligand (2) and the study of its complexation properties with the Ca2+ cation. Through a combination of solution (H-1 NMR and UV-vis spectroscopies) and solid-state (X-ray diffractometry) techniques, we demonstrate that metal complexation induces a shift of the tautomeric equilibration towards the quinone-hydrazone form, while deprotonation of the complex results in the reversion to the azo-phenol tautomer
Halogen-bonded architectures of multivalent calix[4]arenes
Full text linkA small family of novel halogen-bonded crystalline supramolecular architectures of calixarenes was obtained by the co-crystallization of cone (1) and 1,3-alternate tetrakis(3-iodopropargyloxy)calix[4]arene (4) as tetradentate halogen donors with different multidentate acceptors. Particularly interesting is the interpenetrated diamondoid network of 4 with DABCO, which represents the first example of a 2D network of calixarene macrocycles where halogen bonding is the key interaction for self-organization
New ligand behavior of 4-amino-3-R-1,2,4-Δ2-triazoline-5-thione. Crystal structures of silver(I) [R = H] and copper(I) [R = CH2CH3] complexes
No full textThe molecular structures of [Ag(HL1)2]NO3 and [CuCl(HL2)3] (HL1 = 4-amino-1,2,4-Δ2-triazoline-5-thione; HL2 = 4-amino-3-ethyl-1,2,4-Δ2-triazoline-5-thione) have been determined by single crystal X-ray analyses. Unusual dimeric and polymeric species are present in the silver compound due to the versatile coordination ability of the ligand. The tetrahedral copper(I) complex (C3 symmetry) is chiral owing to the conformational helicoidal disposition of the three S-coordinated triazoline molecules interacting via hydrogen bonds with the coordinated chlorine atom and, in a head-to-tail fashion, with other complexes forming chains running parallel to c. A spontaneous resolution is observed and in the crystal structure only A conformers are present. (C) 2000 Elsevier Science S.A
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
Synthesis, crystal structure, and optical properties of fluorinated poly(pyrazole) ligands and in silico assessment of their affinity for volatile organic compounds
Full text linkThree new fluorinated bis(pyrazoles), namely: 1,4-bis(1H-pyrazol-4-ylethynyl)-2-fluorobenzene (H2BPEFB), 1,4-bis(1H-pyrazol-4-ylethynyl)-2,3-difluorobenzene (H2BPEF2B) and 1,4-bis(1H-pyrazol-4-ylethynyl)-tetrafluorobenzene (H2BPEF4B), have been synthesized taking advantage of Sonogashira coupling reactions, and characterized as per their crystal and molecular structure, spectroscopic and dielectric properties, and hydrophobicity. In the crystal structures, the three molecules, whose deviation from planarity increases on increasing the fluorination degree, interact by means of hydrogen bonds, forming 2D supramolecular layers. Notably, the absorption and fluorescence emission properties are only slightly affected by the fluorination degree in both the solid state and solution. Furthermore, the spectral line-shapes are weakly dependent on the environment when dissolved in a number of solvents of different polarity and hydrogen-bonding affinity. On the other hand, the dielectric constant monotonically increases on increasing the number of fluorine atoms. In silico molecular modeling with time-dependent density functional theory has offered a valuable means to rationalize the above mentioned behaviors and has shed some light on the ligand affinity towards representative gases-H2O and CO2-and organic solvents-toluene
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
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
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