1,721,008 research outputs found
Solvation structure of the Hg(NO3)2 and Hg(TfO)2 salts in dilute aqueous and methanol solutions: An insight into the Hg2+ coordination chemistry
No full textA synergic approach combining X-ray absorption spectroscopy (XAS) and Molecular Dynamics (MD) has been used to investigate the solvation properties of dilute aqueous and methanol solutions of the Hg(NO3)2 and Hg(TfO)2 (where TfO−=trifluoromethanesulfonate or triflate) salts. A new Lennard-Jones potential has been developed to be used in classical MD simulations of Hg2+-containing systems by refining the force-field parameters to reproduce the Hg2+ ion coordination in water in agreement with the XAS data. A different behavior of the Hg2+ ion in water and methanol has been highlighted by the analysis of the MD simulations carried out with the newly developed interaction potential: in aqueous solutions of the Hg(NO3)2 and Hg(TfO)2 salts, the Hg2+ first coordination shell is composed only of water molecules, while in methanol solutions of the same salts the Hg2+ cation coordinates both counterions and methanol molecules in its first solvation sphere. The MD results have been confirmed by comparison with the Hg L3-edge XAS experimental data. Independently of the formation of ion pairs with the counterions, the Hg2+ ion in solution tends to form heptacoordinated first shell complexes, with an arrangement of the ligands depending on the nature of the solvent: in water a 7-fold cluster of water molecules in a C2 symmetry is found, while in methanol the Hg2+ solvation shell is composed of both counterions and methanol molecules, with oxygen atoms coordinating Hg2+ arranged in a distorted pentagonal bipyramid geometry. These findings represent a significant step forward in the understanding of the solvation chemistry of the Hg2+ ion, which is fundamental to improve the efficiency of mercury removal procedures that are crucial to the safety of water resources
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
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
Computational studies on mercury halides as ligands in organometallic transition complexes
No full textMercury is present in the structure of several heterometallic clusters, usually bridging three or four transition metal centres.1 Heterobimetallic compounds with a single M-Hg bond were prepared in the past, and the simpliest situation is represented by the {HgX} fragment (X = halide or pseudohalide) bonded to a transition metal fragment. Compounds of this type can be obtained following different synthetic approaches, such as the reaction of HgX2 with suitable precursors or the insertion of Hg in a M-X bond. The most common transition metals belong to the Groups 6 - 9, with CO, cyclopentadienide (Cp) and related species as ancillary ligands. Selected examples are the complexes [M(HgX)(Cp)(CO)3] (M = Cr, Mo, W), [M(HgX)(CO)5], (M = Mn, Re), [Fe(HgX)(Cp)(CO)2] and [Co(HgX)(CO)4].2
To the best of our knowledge, the nature of the M-Hg bond was never computationally investigated. In this communication we report the outcomes of DFT calculations on model compounds having general formulae [M(HgX)(Cp)(CO)n] (M = Cr, Mo, W, n = 3; M = Fe, Ru, Os, n = 2; X = Cl, Br, I) and [M(HgX)(CO)n] (M = Mn, Tc, Re, n = 5; M = Co, Rh, Ir, n = 4; X = Cl, Br, I). The nature of the M-Hg bond, the factors affecting the bond strength and the behaviour of {HgX} as Lewis acid or base were studied in detail.
1 E. Rosenberg, K. I. Hardcastle, Comprehensive Organometallic Chemistry II, vol. 10 (1995) 323.
2 S. V. Maslennikov et al., Russ. J. Gen. Chem. 72 (2002) 1495; J. Granifo and M. E. Vargas, J. Organomet. Chem. 408 (1991) 357; M. Cano and J. A. Campo, Polyhedron 10 (1991) 133; R. Kumar et al., J. Organomet. Chem. 323 (1987) 53; J. R. Chipperfield et al., J. Chem. Soc. Dalton Trans. (1977) 485 and references therein
Coordination of the Co2+and Ni2+Ions in Tf2N-Based Ionic Liquids: A Combined X-ray Absorption and Molecular Dynamics Study
Full text linkMolecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) have been combined to study the coordination of the Co2+ and Ni2+ ions in ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide ([Tf2N]-) anion and having different organic cations, namely, 1-butyl-3-methylimidazolium ([C4mim]+), 1,8-bis(3-methylimidazolium-1-yl)octane ([C8(mim)2]2+), N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium ([choline]+), and butyltrimethylammonium ([BTMA]+). Co and Ni K-edge XAS data have been collected on 0.1 mol L-1 Co(Tf2N)2 and Ni(Tf2N)2 solutions and on the metallic salts. MD simulations have been carried out to obtain structural information on the metal ion coordination. The analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the solutions has been carried out based on the atomistic description provided by MD, and the studied ILs have been found to be able to dissolve both the Co(Tf2N)2 and Ni(Tf2N)2 salts giving rise to a different structural arrangement around the metal ions as compared to the solid state. The combined EXAFS and MD results showed that the Co2+ and Ni2+ ions are surrounded by a first solvation shell formed by six [Tf2N]- anions, each coordinating in a monodentate fashion by means of the oxygen atoms. The nature of the IL organic cation has little or no influence on the overall spatial arrangement of the [Tf2N]- anions, so that stable octahedral complexes of the type [M(Tf2N)6]4- (M = Co, Ni) have been observed in all the investigated ILs
Solubilization and coordination of the HgCl2 molecule in water, methanol, acetone, and acetonitrile: an X-ray absorption investigation
No full textX-ray absorption spectroscopy (XAS) has been employed to carry out structural characterization of the local environment around mercury after the dissolution of the HgCl2 molecule. A combined EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) data analysis has been performed on the Hg L3-edge absorption spectra recorded on 0.1 M HgCl2 solutions in water, methanol (MeOH), acetone and acetonitrile. The Hg-Cl distance determined by EXAFS (2.29(2)-2.31(2) Å) is always comparable to that found in the HgCl2 crystal (2.31(2) Å), demonstrating that the HgCl2 molecule dissolves in these solvents without dissociating. A small sensitivity of EXAFS to the solvent molecules interacting with HgCl2 has been detected and indicates a high degree of configurational disorder associated with this contribution. XANES data analysis, which is less affected by the disorder, was therefore carried out for the first time on these systems to shed light into the still elusive structural arrangement of the solvent molecules around HgCl2. The obtained results show that, in aqueous and MeOH solutions, the XANES data are compatible with three solvent molecules arranged around the HgCl2 unit to form a trigonal bipyramidal structure. The determination of the three-body Cl-Hg-Cl distribution shows a certain degree of uncertainty around the average 180° bond angle value, suggesting that the HgCl2 molecule probably vibrates in the solution around a linear configuration
Structural evolution of the butylated hydroxytoluene/menthol hydrophobic eutectic solvent upon methanol and ethanol cosolvent addition
Full text linkThe changes upon methanol (MeOH) and ethanol (EtOH) addition in the structural arrangement of the hydrophobic eutectic solvent formed by butylated hydroxytoluene (BHT) and L-menthol (MEN) in 1:3 M ratio have been studied using molecular dynamics (MD) simulations integrated by small- and wide-angle X-ray scattering (SWAXS) measurements. Different mixtures containing the eutectic solvent and a variable amount of MeOH and EtOH have been investigated and the cosolvent introduction at any concentration has been found to break the hydrogen-bonds (H-bonds) among the MEN molecules, that are the most relevant interactions in the pristine eutectic, while the BHT component remains substantially non-interacting through all the explored composition range. This H-bond network is replaced by interactions between the MEN component and the cosolvent molecules, where the MEN species acts preferentially as H-bond donor towards the MeOH and EtOH molecules that behave mostly as H-bond acceptors. An increasing interplay between the excess cosolvent molecules is also observed upon increasing their concentration. SWAXS data show a contraction of the electron-dense regions corresponding to the hydroxyl groups upon cosolvent addition. This evidence is compatible with the intercalation of the MeOH and EtOH molecules in the MEN-MEN H-bond network to promote MEN-cosolvent and cosolvent cosolvent interactions, in agreement with the MD results
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