1,720,996 research outputs found
Crystal structure and packing energy calculations of (+)-6-aminopenicillanic acid
No full textThe X-ray single-crystal structure of (2S,5R,6R)-6-amino-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid, commonly known as (+)-6-aminopenicillanic acid (C8H12N2O3S) and a precursor of a variety of semi-synthetic penicillins, has been determined from synchrotron data at 150K. The structure represents an ordered zwitterion and the crystals are nonmerohedrally twinned. The crystal structure is composed of a three-dimensional network built by three charge-assisted hydrogen bonds between the ammonium and carboxylate groups. The complementary analysis of the crystal packing by the PIXEL method brings to light the nature and ranking of the energetically most stabilizing intermolecular interaction energies. In accordance with the zwitterionic nature of the structure, PIXEL lattice energy calculations confirm the predominance of the Coulombic term (-379.1kJmol-1) ahead of the polarization (-141.4kJmol-1), dispersion (-133.7kJmol-1) and repulsion (266.3kJmol-1) contributions
Pressure-induced structural changes in wet vitamin B12
No full textThe effects of pressure on the structure of cyanocobalamin (vitamin B12) have been investigated by compressing a single crystal grown from water together with its mother liquor to 1.0 GPa. The structure of a crystal grown from water in situ in a capillary has also been re-determined at room temperature and pressure. A conformational rearrangement of selected amide side chains, reduced displacement parameters and remarkable solvent ordering have been observed at high pressure.Alexander von Humboldt Foundation; University of Gottingen; KIT-ISS-Ank
Pharmaceutical hydrates under ambient conditions from high-pressure seeds:A case study of GABA monohydrate
No full textThe monohydrate form of the neurotransmitter γ-amino butyric acid (GABA) has been crystallised in the 0.4-0.8 GPa pressure range, recovered to ambient pressure and then used as a seed. Theoretical calculations indicate that this hydrate is only thermodynamically favoured over the two anhydrous forms at high pressures.</p
Searching for novel crystal forms by in situ high-pressure crystallisation: the example of gabapentin heptahydrate
No full textHigh-pressure crystallisation of an aqueous solution of the GABA analogue gabapentin at 0.8 GPa resulted in the formation of a previously unknown heptahydrate form, whose structure has been determined by in situ single-crystal X-ray diffraction. The structure and water framework of this unusually highly hydrated small molecule are described in detail.Alexander von Humboldt Foundation; University of Gottinge
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
High spin cycles: topping the spin record for a single molecule verging on quantum criticality
Get PDFThe cyclisation of a short chain into a ring provides fascinating scenarios in terms of transforming a finite array of spins into a quasiinfinite
structure. If frustration is present, theory predicts interesting quantum critical points, where the ground state and thus lowtemperature
properties of a material change drastically upon even a small variation of appropriate external parameters. This can be
visualised as achieving a very high and pointed summit where the way down has an infinity of possibilities, which by any parameter
change will be rapidly chosen, in order to reach the final ground state. Here we report a mixed 3d/4f cyclic coordination cluster that
turns out to be very near or even at such a quantum critical point. It has a ground state spin of S = 60, the largest ever observed for
a molecule (120 times that of a single electron). [Fe10Gd10(Me-tea)10(Me-teaH)10(NO3)10]·20MeCN forms a nano-torus with
alternating gadolinium and iron ions with a nearest neighbour Fe–Gd coupling and a frustrating next-nearest neighbour Fe–Fe
coupling. Such a spin arrangement corresponds to a cyclic delta or saw-tooth chain, which can exhibit unusual frustration effects. In
the present case, the quantum critical point bears a ‘flatland’ of tens of thousands of energetically degenerate states between
which transitions are possible at no energy costs with profound caloric consequences. Entropy-wise the energy flatland translates
into the pointed summit overlooking the entropy landscape. Going downhill several target states can be reached depending on the
applied physical procedure which offers new prospects for addressability
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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