1,721,138 research outputs found
Addressing the stereochemistry of complex organic molecules by density functional theory-NMR
No full textDetermining the stereochemistry of organic molecules is a long-standing problem, with implications for stereoselective reactions, catalysis, and the structure of natural products. NMR spectroscopy is widely employed to this purpose; however, the interpretation of results is often entrusted to empirical methods or expert systems. Quantum chemical calculations, generally based on density functional theory, provide a reliable framework to predict the relevant parameters (chemical shifts and coupling constants, generally involving 1H and 13C) of diastereoisomers with useful accuracy. While the approach virtually spans all molecules of interest in organic chemistry, including those that contain heavy atoms, the analysis of results still poses significant challenges
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
A Novel Algorithm for the Coupling Control in Solid Phase Peptide Synthesis
No full textThis paper describes a new method for the evaluation of conductimetric data collected during the in-line monitoring of the coupling step in solid-phase peptide synthesis. The control scheme relies on a feed-forward artificial neural network algorithm which can predict the final yield of the reaction within its initial 5 min by analyzing the conductivity signal profile. The yield values predicted by the artificial neural network algorithm result in good accordance with the data obtained by the commonly used ninhydrin test
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
Insights on the Isotropic-to-Smectic A Transition in Ionic Liquid Crystals from Coarse-Grained Molecular Dynamics Simulations: The Role of Microphase Segregation
No full textWe have investigated the role of microphase segregation as the driving force in the stabilization of thermotropic ionic liquid crystals of smectic type. To this end we have applied the heterogeneity order parameter, initially proposed for ionic liquids, to the coarse-grained molecular dynamics simulation results for a model system of an imidazolium nitrate ionic liquid crystal, [C16mim][NO3], whose phase diagram was recently studied. We have found that the heterogeneity order parameters become larger when the system goes through the transition from the isotropic phase to the smectic A phase as the temperature decreases. This can be understood by considering that, in the smectic A phase, the layered structure allows the tail groups to have a degree of aggregation larger than that in the isotropic phase. Our results highlight the role of microsegregation in the stabilization of ionic liquid crystals, which cannot be revealed by the commonly used translational and orientational order parameters used to describe liquid crystal phases
Relativistic DFT calculation of Sn-119 chemical shifts and coupling constants in tin compounds
No full textThe nuclear shielding and spin−spin coupling constants of 119Sn in stannane, tetramethylstannane, methyltin halides Me4-nSnXn (X = Cl, Br, I; n = 1−3), tin halides, and some stannyl cations have been investigated computationally by DFT methods and Slater all-electron basis sets, including relativistic effects by means of the zeroth order regular approximation (ZORA) method up to spin−orbit coupling. Calculated 119Sn chemical shifts generally correlate well with experimental values, except when several heavy halogen atoms, especially iodine, are bound to tin. In such cases, calculated chemical shifts are almost constant at the scalar (spin-free) ZORA level; only at the spin−orbit level is a good correlation, which holds for all compounds examined, attained. A remarkable “heavy-atom effect”, analogous to that observed for analogous alkyl halides, is evident. The chemical shift of the putative stannyl cation (SnH3+) has also been examined, and it is concluded that the spectrum of the species obtained in superacids is inconsistent with a simple SnH3+ structure; strong coordination to even weak nucleophiles such as FSO3H leads to a very satisfactory agreement. On the contrary, the calculated 119Sn chemical shift of the trimesitylstannyl cation is in very good agreement with the experimental value. Coupling constants between 119Sn and halogen nuclei are also well-modeled in general (taking into account the large uncertainties in the experimental values); relativistic spin−orbit effects are again quite evident. Couplings to 13C and 1H also fall, on the average, on the same correlation line, but individual values show a significant deviation from the expected unit slope
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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