1,720,977 research outputs found
Decomposition of Bayer process organics: low-molecular-weight carboxylates
Full text linkThe degradation of twenty-one low-molecular-weight organic carboxylates has been studied at 90 and 180 °C, in a synthetic Bayer liquor consisting of 6 mol kg− 1 aqueous NaOH solution, for periods of up to 36 days. The reactions were monitored and the major degradation products identified by HPLC and NMR spectroscopy. The carboxylates chosen for the study were either possible intermediates or known products arising from the decomposition of organic matter in the Bayer process. Aliphatic carboxylates without hydroxyl substituents were stable at 90 °C but decomposed at 180 °C, except for formate, acetate, oxalate and succinate. The corresponding aromatic carboxylates were stable even at 180 °C. Both aliphatic and aromatic carboxylates with hydroxyl substituents were unstable at 90 °C except for lactate and 4-hydroxy-benzoate. The most frequently detected decomposition products for both aliphatic and aromatic carboxylates were formate, acetate, oxalate, succinate and lactate. Phenolate was also observed for some aromatic carboxylates. These products are briefly discussed with reference to possible mechanisms for the degradation reactions
Solid-State Conformational Flexibility at Work: Energetic Landscape of a Single Crystal-to-Single Crystal Transformation in a Cyclic Hexapeptoid
Full text linkWe describe the energetic landscape beyond the solid-state dynamic behavior of a cyclic hexapeptoid decorated with four propargyl and two methoxyethyl side chains, namely, cyclo-(Nme-Npa2)2, Nme = N-(methoxyethyl)glycine, Npa = N-(propargyl)glycine. By increasing the temperature above 40 °C, the acetonitrile solvate form 1A starts to release acetonitrile molecules and undergoes a reversible single crystal-to-single crystal transformation into crystal form 1B with a remarkable conformational change in the macrocycle: two propargyl side chains move by 113° to form an unprecedented "CH-πzipper". Then, upon acetonitrile adsorption, the "CH-πzipper"opens and the crystal form 1B transforms back to 1A. By conformational energy and lattice energy calculations, we demonstrate that the dramatic side-chain movement is a peculiar feature of the solid-state assembly and is determined by a backbone conformational change that leads to stabilizing CH···OC backbone-to-backbone interactions tightening the framework upon acetonitrile release. Weak interactions as CH···OC and CH-πbonds with the guest molecules are able to reverse the transformation, providing the energy contribution to unzip the framework. We believe that the underlined mechanism could be used as a model system to understand how external stimuli (as temperature, humidity, or volatile compounds) could determine conformational changes in the solid state
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
A novel 2,6-bis(benzoxazolyl)phenol macrocyclic chemosensor with enhanced fluorophore properties by photoinduced intramolecular proton transfer
Full text linkMacrocyclic ligand L, in which a 2,6-bis(2-benzoxazolyl)phenol (bis-HBO) group is incorporated in triethylenetetramine, was designed and synthesized with the aim of creating a chemosensor with high selectivity and specificity for metal cations in an aqueous environment. The availability of several proton acceptors and donors, and amine and phenol hydroxy groups, respectively, affects the keto-enol equilibrium in both the ground and excited states, and the ligand properties show dependence on the pH of the solution. L is fluorescent in the visible range, through an excited-state intramolecular proton transfer (ESIPT) mechanism. The results of an exhaustive characterization of L by spectroscopic techniques and DFT calculations, as well as of its Zn(II), Cd(II) and Pb(II) complexes, show promising properties of L as a ratiometric metal cation chemosensor, since metal coordination prevents the ESIPT and gives rise to a peculiar displacement of the fluorescence emission from green to blue with Zn(II) and Cd(II), while with Pb(II) the fluorescence is quenched
New branched macrocyclic ligand and its side-arm, two urea-based receptors for anions: Synthesis, binding studies and crystal structure
No full textThe synthesis and characterization of the two new hosting molecules for anions are reported. The first one(L1) is a branched tetraazamacrocycle bearing two p-nitrophenylureido groups as side-arms, whereas the second one(L2) has the same linear chain and binding moiety of L1 side-arm.
The binding properties of both ligands towards the halides series and acetate anions (G) were investigated by NMR and UV-Vis spectroscopy in a dimethyl sulfoxide–0.5% water solution. Both ligands interact with F-, Cl- and
AcO- while Br- and I- did not. The NMR experiments proved that the binding occurs viaH-bond to the ureido fragments. Fluoride anion is basic enough to deprotonate the ureido group of both ligands, thus preventing the determination of the addition constants to both ligands. L1 forms G–L species of 1 : 1 ([GL1]) and 2 : 1 ([G2L1]) stoichiometry while L2 forms only the 1 : 1 [GL2] species. The higher value of the formation constant of the [AcO1]- vs. the [AcOL2]- species (log K = 5.5 vs. 2.8 for the reaction AcO- + L = AcOL-) suggested that both side-arms of L1 cooperate in binding acetate; this does not occur with Cl-.The results confirmed that this tetraaza-macrocyclic base acts as a preorganizing scaffold for side-arms when they are linked to it via an amide function. The crystal structure of L2_H2O is also reported
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
A New Benzoxazole-Based Fluorescent Macrocyclic Chemosensor for Optical Detection of Zn2+ and Cd2+
No full textBackground: Benzoxazole-containing ligands find many applications both in medicinal chemistry, catalysis and fluorescence chemosensing. Benzoxazole-containing macrocycles could be therefore a good strategy to achieve stable and selective fluorescent complexes with suitable metal ions. In this work, the synthesis, binding, and photochemical properties of a new fluorescent ligand (L) are reported. L is a cyclophane macrocycle containing the 1,3-bis(benzo[d]oxazol-2-yl)phenyl (BBzB) fluorophore and an aliphatic tetra-amine chain to form the macrocyclic skeleton. Methods: Spectrophotometric and spectrofluorimetric measurements,1H NMR analysis, and DFT calculations were performed. Results: L behaves as a PET-mediated chemosensor, being emissive at 390 nm at acidic pH and non-emissive at basic pH. The chemosensor is able to detect Zn2+ and Cd2+ in an aqueous medium (acetonitrile–water, 4:1 v/v) at neutral pH through a CHEF effect upon metal ion coordination. Paramagnetic metal ions (Cu2+) and heavy atoms (Pb2+, Hg2+) resulted in a quenching of fluorescence or very low emission. Conclusions: The new cyclophane macrocycle L was revealed to be a selective PET-regulated chemosensor for Zn2+ and Cd2+ in an aqueous medium, being able to bind up to two and one metal cations, respectively. The molecule showed a shifted emission towards the visible region compared to similar systems, suggesting a co-planar conformation of the aromatic fragment upon metal coordination. All these data are supported by both experimental measurements and theoretical calculations
Solid-solid transition between hydrated racemic compound and anhydrous conglomerate in Na-ibuprofen: a combined XRD, Solid-State NMR, calorimetric and computational study
No full textAppropriate 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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