1,721,366 research outputs found
The influence of crystal structure on molecular conformation: a case study
No full textIt is generally assumed that molecular structures concentrate in low-energy regions of conformational space in the solid state as they do in the gas phase. On the other hand, conformations that do not correspond to an intramolecular energy minimum are observed from time to time in the solid state; their occurrence is attributed to crystal-packing effects. We have recently encountered an interesting case while studying the crystal structure of the complexes [Co(phen)3][Cd2Cl7].3H2O and [Co(phen)3] [Cd2Br7].2H2O, containing two new discrete anions [Cd2X7]3- where X = Cl/Br.These anions have a ‘boomerang’ shape.
In a preliminary attempt to investigate the factors effecting the molecular conformation, DFT calculations at the GGA-PW21/DNP level of theory on [Cd2X7]3- ions (X= Cl or Br) has been performed both in vacuum and in simulated water environment using the COSMO model. In vacuum, both molecules assume a linear geometry around the bridging atom, with Cd-X-Cd angles very close to 180°, while in simulated water the optimized geometries are similar to those found in crystals. A more detailed computational study has shown that while the molecules in vacuum are definitely more stable in the ‘linear’ conformation, in water they experience a very flat potential energy surface, which means that practically all conformations are allowed. Actually, the Metal-Halogen-Metal angle in analogous [Hg2X7]3- halomercurate ions of known structure (six entries in the Cambridge Structural Database) shows a great variability assuming values in the wide range 98.5 -180.0 degrees. The crystal environment seems therefore to be the ultimate factor determining the final conformation
Sodium dinitritobis(pyrazine-2-carboxylato-N,O)cobaltate(II)
No full textR. P. Sharma, K. K. Bhasin, E. R. T. Tiekin
Spectra-structure relationship: Synthesis, characterization of copper(II) complexes with ibuprofenate, o-methoxybenzoate, p-ethoxybenzoate and single crystal X-ray structure determinations of [trans-Cu(en)2(H2O)2](L)2 where en=ethylenediammine,L= o-methoxy benzoate/p-ethoxybenzoate
No full textTo investigate the spectra–structure relationship, three new copper(II) complex salts of composition
[trans-Cu(en)2(H2O)2](L1)2 2H2O(1), [trans-Cu(en)2(H2O)2](L2)2(2) and [trans-Cu(en)2(H2O)2](L3)2 (3)
(where L1 = ibuprofenate, L2 = o-methoxybenzoate and L3 = p-ethoxybenzoate) were obtained when ethylenediamine
was added to ‘Cu(L)2’ (L = L1/L2/L3) in methanol–water solution. The intermediate ‘Cu(L)2’ salts were prepared by reaction of copper sulphate with sodium salt of ibuprofen, o-methoxybenzoic acid or p-ethoxybenzoic acid, respectively, in aqueous medium in 1:2 molar ratio. The newly synthesized complex salts have been characterized by elemental analyses, spectroscopic techniques (UV/visible and IR), magnetic moment determinations and conductance measurements. X-ray structure determination
revealed an ionic structure consisting of one [trans-Cu(en)2(H2O)2]2+, two [C8H7O3]- ions in 2 and one [trans-Cu(en)2(H2O)2]2+, two (C9H8O3)- ions in 3. In these complex salts central metal ion copper(II) is coordinated by four nitrogen atoms, originating from two chelating ethylenediamine ligands and two oxygen atoms of two coordinated water molecules, showing distorted octahedral geometry around copper metal ion. Both of these structurally correlated octahedral [trans-Co(en)2(H2O)2]2+ species have shown absorption at kmax 540 nm in the UV/visible spectrum
Second sphere coordination in anion binding: Synthesis, characterization, X-ray structure of tris(1,10-phenanthroline)cobalt(III)2,4-dinitrophenolate tetrahydrate, [Co(phen)3](dnp)3·4H2O where dnp = 2,4-dinitrophenolate
No full textA new cobalt(III) complex salt, [Co(phen)3](dnp)3·4H2O has been synthesized in an effort to utilize [Co(phen)3]3+ cation as a new host for di-substituted phenolate (2,4-dinitrophenolate) anion. This cobalt (III) complex salt has been characterized by elemental analyses and spectroscopic techniques (UV/visible, IR, NMR and conductance measurements). Single crystal X-ray structure determination confirmed the composition [Co(phen)3](dnp)3·4H2O. A strong network of hydrogen bonding interactions C–H...O (phenolate), C–H...O (nitro) involving second sphere coordination and O–H...O hydrogen bonding interactions stabilize the crystal lattice. The formation of a complex salt of definite composition along with solubility product data suggests that [Co(phen)3]3+ present in [Co(phen)3]Cl3 may be used as an anion receptor for 2,4-dinitrophenolate ion
Anion binding through second sphere coordination
No full textAnion recognition plays very crucial role in chemistry and many biological processes. Despite their importance, little has been done in the field of design and synthesis of supramolecular anion receptors as compared to that of cations. In most of the anion receptor molecules, the basic approach utilized is to develop an organic framework with the suitable functionalities to bind the anion with the help of hydrogen bonding interactions. An alternate approach which may be employed is to utilize the second sphere interactions around the metal centre for the binding of anions, mainly utilizing the directional nature of the hydrogen bonds and charge transfer electrostatic interactions. In this regard we have undertaken extensive research program to explore cationic cobalt(III) metal complexes as anion receptors; in particular, we have focused our attention on complex salts of [Co(phen)3]3+ (phen = 1,10-phenantroline) with a series of anions differing in size, charge and nature (organic or inorganic).
In this communication we report the crystal structures of the above listed salts and a detailed analysis of their packing modes, with particular reference to those weak intermolecular interactions that, besides electrostatic forces, contribute to stabilize the crystalline architecture. Understanding of such network interactions would be rewarding as it can provide means of constructing intricate and novel molecular entities based on second sphere coordination
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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