1,721,236 research outputs found
Crystal growth of coordination polymers by gel diffusion
Full text linkThe use of gels or viscous materials as growth media for a wide range of compounds, including proteins, inorganic and organic compounds has already been reported in the literature. In the presence of a gel, sedimentation and convection currents are greatly suppressed and the mass transport of the molecules occurs mainly by diffusion. As a result, a lower nucleation density and a better crystal quality are usually observed.[1a] We report, here, new interesting phenomena observed in crystallization experiments ofcoordination compounds by using gels as diffusion media.[1b].
We have tested a variety of gel and resin-like systems as dispersion matrixes to crystallize a special class of polymeric and porous inorganic compound: the “coordination polymers”. We have used the gel technique to increase the size and quality of the crystals and as a mean to modify the crystal habit and get new crystalline species. [1b] In our experiments, we have observed some new phenomena of gel inclusion and morphological alteration of crystal surfaces.
Diffraction, optical observations and interferometric measurements have been used for in situ monitoring the mass transport in the gelled/fluid phase during the crystallization process and for studying the crystallization kinetic.
Funding from the Cariplo Foundation grant ‘‘2012‐0921” is gratefully acknowledged.
[1] a) K.H. Henisch, Crystals in Gels and Liesegang Rings, Cambridge University Press, Cambridge, 1998; b) L.
Carlucci, G. Ciani, J. M. Garcìa-Ruiz, M. Moret, D. M. Proserpio and S. Rizzato Cryst. Growth Des., 2009, 9(12),
5024-5034.
[2] S.Rizzato et al., unpublished results
Morphological and structural effects of gels on coordination polymers crystallization
No full textThe use of gels or viscous materials as growth media for a wide range of compounds, including proteins and inorganic and organic compounds has been reported in the literature. In the presence of gel, sedimentation and convection currents are greatly suppressed and the mass transport of the molecules mainly occurs by diffusion, resulting, usually, in a lower nucleation density and a better crystal quality.[1a]
Here we report some new interesting phenomena observed in crystallization experiments of coordination compounds, such as, “coordination polymers”, by using gels as diffusion media.[1b]
By using a semi-liquid silica-gel system as a dispersion matrix, we were able to increase the quality of the crystals but also slow down the solvent loss, stabilizing the partially dehydrated structure of a very flexible porous framework. This has enabled an accurate determination of the dynamic behavior of the system during the desolvation process by using single crystal X-ray diffraction. Unit cells and structure solution has been obtained at any point during the process.
The improved stability of the crystals and the large flexibility of the network have also made possible to carry out in-situ high-pressure diffraction measurements.
Other important results concern the effects of gels on the macroscopic crystal shape and on the micromorphology of the crystal surfaces (Figure), and the capacity of gel media to promote the concomitant crystallization of topological isomers with a monotropic relationship.[2]
Funding from the Cariplo Foundation under the grant ‘‘2012‐0921” is gratefully acknowledged.
[1] a) K.H. Henisch, Crystals in Gels and Liesegang Rings, Cambridge University Press, Cambridge, 1998; b) L. Carlucci, G. Ciani, J. M. Garcìa-Ruiz, M. Moret, D. M. Proserpio and S. Rizzato Cryst. Growth Des., 2009, 9(12), 5024-5034.
[2] S.Rizzato et al., unpublished results
Crystallization Behavior of Coordination Polymers. 2. Surface Micro-Morphology and Growth Mechanisms of [Cu(bpp)3Cl2] 3 2H2O by in Situ Atomic Force Microscopy
No full textCrystal growth of the one-dimensional coordination polymer [Cu(1,3-bis(4-pyridyl)propane)3Cl2]·2H2O was investigated in aqueous solutions by in situ atomic force microscopy. Details about the growth mechanisms of the {001} form at low supersaturation were obtained. In particular, growth hillocks due to simple and complex dislocation sources were observed as the only active mechanism. The thinnest steps observed on {001} faces and delimiting growth hillocks were d002 layers, in accordance with the elementary growth layer expected from the systematic extinction conditions of space group I2/a and hence from Bravais−Friedel−Donnay−Harker rules and Hartman and Perdok periodic bond chains theory. This feature of the {001} faces suggests that small oligomeric species can be involved in the crystal growth processes. The effects of unknown impurities in the organic ligand upon the growth of crystals of the title compound were also studied
Gaston Maspero, A propos d’un article de M. Moret sur l’ÉgyptoIogie en France. [Extrait du Bulletin de l’Institut français d’Archéologie orientale, t. VIII]. Le Caire 1910.
No full textAndersson-Akmar Ernst. Gaston Maspero, A propos d’un article de M. Moret sur l’ÉgyptoIogie en France. [Extrait du Bulletin de l’Institut français d’Archéologie orientale, t. VIII]. Le Caire 1910. In: Sphinx : revue critique embrassant le domaine entier de l'égyptologie, vol. 15, 1911. pp. 119-124
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
Molecular mechanics in crystalline media
No full textA general, computationally easy method for minimizing the steric energy of a molecule, polymer, surface, or net within the field of a fixed, or periodically updated, crystal lattice has been devised. The goal has been obtained by coupling molecular mechanics (MM) to Kitaigorodsky’s atom atom pairwise potential (AAPP). The primary outcomes of such MMAAPP computations are the “solid state” conformation of the molecule, its intramolecular steric energy and its interaction energy with the surrounding lattice. These computations are ideally suited for studying relative stabilities of different polymorphs, plastic deformations of a whole crystal lattice, and molecular motions of flexible guest molecules in host crystal lattices. The proposed approach can help in understanding solid-state dynamics, factors controlling reactivity in crystal lattices, and crystals as “supramolecular entities”.
In addition, the capability of building “hypothetical” crystals with sterically reasonable geometries can be exploited in the process of solving crystal structures from partial diffraction data
Molecular resolution images of the surfaces of natural zeolites by atomic force microscopy
No full textAtomic force microscopy (AFM) was used to characterize the morphological and cleavage surfaces in a number of natural zeolites. The investigated zeolites (stilbite, heulandite, thomsonite, yugawaralite, laumontite, and a few others) show rather interesting and sample-dependent microtopographical features related to the mechanisms involved in the surface growth processes at the molecular level. The results obtained by AFM on stilbite, heulandite, and yugawaralite during the preliminary surface characterization are presented, and the images show that molecular resolution can be achieved and crystallographically interpreted by careful preparation of the sample. (C) 2003 Elsevier Inc. All rights reserved
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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