1,721,021 research outputs found
A high-pressure polymorph of propionamide from in situ high-pressure crystallisation from solution
No full textThe structure of a high-pressure polymorph of propionamide is reported from synchrotron single-crystal X-ray diffraction data. The polymorph was obtained by in situ high-pressure crystallisation from solution in a diamond-anvil cell at 0.9 GPa; its structure is 23% denser than that of the ambient-pressure polymorph at STP. PIXEL lattice and dimer energies calculations indicate that the ambient-and high-pressure polymorphs are energetically competitive. Together with acetamide and formamide, this is the third high-pressure polymorph of an aliphatic amide crystallising as a Z'=2 structure; the H-bonded network is however significantly different from the ones reported for the other two compounds. This study highlights the power of in situ high-pressure crystallisation techniques for accessing novel polymorphic forms of molecular compounds
Combined theoretical and experimental studies of proton migration and transfer in the solid state
Get PDFHydrogen bonds are of great interest in the solid state due to their importance
in structural, functional and dynamical properties of chemical systems. Moderate
hydrogen bonds have been linked with proton transfer, whereas the short, strong
hydrogen bonds enable proton migration.
Previous work in our group on relatively simple hydrogen bonded adducts
relied on the combination of ab initio computational modelling (molecular dynamics)
with variable temperature diffraction results (X-ray and neutron). These
demonstrated that the interplay of these techniques was successful in studying the
phenomena of proton transfer and migration.
The present work follows on from that, and focuses on the effects of
temperature and pressure on proton transfer and migration using both experimental
and computational methods. The systems studied continue to encapsulate adducts
with N…O and O…O hydrogen bonds.
The study of the adduct formed between squaric acid and 4,4’-bipyridine was
found to exhibit proton transfer associated with a single-crystal to single-crystal
phase transition at 450 K that is coupled to a colour change (yellow to red). X-ray
and neutron diffraction initially revealed the heavy atom structure and secondly the
location of the hydrogen atoms along the moderate N…O hydrogen bond (ca. 2.6 Å).
Computational modelling supported this and deduced the reason for the striking
colour change. Pressure studies also determined that the adduct underwent two
phase-transitions with a similar colour change, indicating that proton transfer is also a factor here, but with powder patterns different from the high temperature form,
indicating that further polymorphs for this interesting system must exist.
In an attempt to lower the temperature at which proton transfer would occur
the base was changed to one of a more basic nature, i.e. co-crystallisation of squaric
acid and 2,2’-dimethyl–4,4’-bipyridine was pursued. This lead to the formation of
two red crystals that were found to posses the base doubly protonated at all
temperatures studied (from 300 K to 100 K).
The adduct of N,N-dimethylurea with phosphoric acid was obtained from a
systematic study designed to follow the success of a previously reported system that
showed proton migration (the adduct of urea and phosphoric acid). The new material
was found to crystallise as the 2:1 adduct and maintained the short, strong hydrogen
bonds characteristic of the parent structure.
As part of the systematic approach undertaken throughout the research
presented here, co-crystallisation of a combination of acids and bases were attempted
in order to synthesise new materials containing short, strong hydrogen bonds. These
yielded the adducts between oxalic acid and 2,2’-dimethyl-4,4’-bipyridine, and
oxamic acid and 4,4’-bipyridine. In addition to these adducts some compounds ended
up reacting to create new ones, e.g. the fusing of dimethyl urea and squaric acid to
give N-(2-hydroxycyclobutene-3,4-dione)-N’,N’-dimethylurea and N-(2-
hydroxycyclobutene-3,4-dione)-N,N'-dimethylurea, while a new polymorphs of one
of the precursors on its own was also obtained (N,N’-dimethylurea). The resulting
co-crystallisations did not all follow the design quite as intended. They did, however,
yield interesting new structures, some of which have the potential to be proton
migration and transfer systems
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
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
1,1-Diamino-2,2-dinitroethylene
No full textInelastic Neutron Scattering spectrum of 1,1-Diamino-2,2-dinitroethylene, C2H4N4O4, measured on the TOSCA instrument.
DADNE, FOX-7, (NH2)2C-C(NO2)
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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