1,720,986 research outputs found
Deep eutectic solvents: a structural point of view on the role of the cation
Full text linkIn this work we have developed an analytical procedure to identify metal ion coordination geometries in liquid media based on the calculation of Combined Distribution Functions (CDFs) starting from Molecular Dynamics (MD) simulations. CDFs provide a fingerprint which can be easily and unambiguously assigned to a reference polyhedron. The CDF analysis has been tested on five systems and has proven to reliably identify the correct geometries of several ion coordination complexes. This tool is simple and general and can be efficiently applied to different MD simulations of liquid systems
Unraveling the solvation geometries of the lanthanum(III) bistriflimide salt in ionic liquid/acetonitrile mixtures
No full textA synergic approach combining molecular dynamics (MD) and X-ray absorption spectroscopy (XAS) has been used to investigate the structural properties of the La(Tf2N)3 salt (where Tf2N = bistriflimide or bis(trifluoromethansulfonyl)imide) dissolved into several mixtures of acetonitrile and the 1,8-bis(3-methylimidazolium-1-yl)octane bistriflimide (C8(mim)2(Tf2N)2) ionic liquid (IL), with the IL molar fraction (χIL) ranging from 0 to 1. The XAS and MD results show that major changes take place in the La3+ first solvation shell when moving from pure acetonitrile to pure C8(mim)2(Tf2N)2. With increasing the IL concentration of the mixture, the La3+ first shell complex progressively loses acetonitrile molecules to accommodate more and more oxygen atoms of the Tf2N- anions. Except in pure C8(mim)2(Tf2N)2, La3+ is always able to coordinate both acetonitrile and Tf2N- anions, with a ratio between the two different ligands strongly dependent on the IL content. Moreover, the La3+ ion prefers to form a 10-coordinated first shell complex in all the investigated systems, with a slightly different geometry of the cluster depending on the composition of the La3+ first solvation shell. In particular, when moving from pure acetonitrile to pure C8(mim)2(Tf2N)2, the La3+ first solvation shell passes from a bicapped square antiprism geometry where all the Tf2N- anions act only as monodentate ligands, to a "1 + 5 + 4" structure in which the Tf2N- anion binds La3+ both in a monodentate and bidentate fashion. The great adaptability shown by the La3+ solvation structure allows it to reach the optimal balance among many different forces at play involving all of the different species present in the mixtures
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
Integrated experimental and theoretical approach for the structural characterization of Hg2+ aqueous solutions
No full textA Coupled Molecular Dynamics and XANES Data Analysis Investigation of Aqueous Cadmium(II)
No full textA coupled molecular dynamics and XANES data analysis investigation of aqueous cadmium(II)
No full textThe flexible nature of the first hydration shell of the cadmium(II) ion has been definitively assessed through an extensive study, combining X-ray absorption near-edge structure (XANES) spectroscopy and molecular dynamics (MD) simulations. The structural and dynamic properties of the cadmium(II) hydration shell have been determined from long-time MD simulations, and the influence of water−water interactions has been evaluated using the SPC/E and TIP5P water models. Comparison of the theoretical results with EXAFS data suggests that the TIP5P simulation provides a better description of the cadmium(II) hydration properties. XANES spectra have been computed starting from MD trajectories, without carrying out any minimization in the structural parameter space. The octahedral solvation of cadmium(II) in aqueous solution cannot be reconciled with the XANES results, while a flexible hydration shell is fully consistent with the experimental data, which unambiguously show the presence of a dominant percentage of heptahydrated species
Influence of the Second Coordination Shell on the XANES Spectra of the Zn2+ Ion in Water and Methanol
No full textA quantitative analysis of the X-ray absorption near-edge structure (XANES) spectra of the Zn2+ ion in methanol and aqueous solution has been carried out starting from the microscopic description of the systems derived from molecular dynamics (MD) simulations. The different sensitivity of XANES towards the second-shell solvent molecules in the two solvents has been highlighted: in methanol solution, the Zn2+ second solvation shell was found to provide a negligible contribution to the XANES spectrum, and it has been possible to reproduce the experimental data using a single coordination sphere in the calculation. Conversely, a reliable reproduction of the experimental spectrum of Zn2+ in aqueous solution could be obtained only by including the second-shell water molecules in the calculation. The combination of MD and XANES was found to be essential to evaluate the contribution of the ion second solvation shell to the XANES spectra of a metal ion in organic solvents
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
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