1,721,003 research outputs found
Molecular structure and internal dynamics of the antioxidant 2,6-di-tert-butylphenol
Full text linkAntioxidants are a class of chemical compounds with particular chemico-physical properties that make them suitable for reducing oxidative stress. In this work we report the rotational spectroscopy analysis of the antioxidant 2,6-di-tert-butylphenol in a jet expansion. The rotational spectrum reveals both fine and hyperfine tunnelling components. The largest spectral doubling consists of two distinct groups of lines separated by ∼190 MHz, and is due to the torsional motion associated with the hydroxyl group. Each component of the doublet is further split into three fine components, with separations below 1 MHz. The spectrum was reproduced with a two-state torsion-rotation semirigid Hamiltonian for each pair of torsional states. Additional observation of all the singly-substituted 13C isotopologues allowed to determine the substitution structure by means of the Kraitchman equations. The comparison with the equilibrium structure obtained by computational calculations at B3LYP-D3BJ/def2-TZVP level validate the accurate determination of the carbon skeleton and tert‐butyl group positions. The investigation of intramolecular dynamics with a monodimensional flexible model demonstrates that the tunnelling phenomenon arises from the hydroxyl group's equivalent positions, with a double-minimum potential separated by a barrier of 1000(100) cm−1 allowing for this large amplitude motion. However, the three-fold fine structurte, while plausibly associated to internal motions within the tert‐butyl group, will require further exploration
The Structure of 2,6-Di-tert-butylphenol–Argon by Rotational Spectroscopy
Full text linkThe molecular structure of a van der Waals-bonded complex involving 2,6-di-tert-butylphenol and a single argon atom has been determined through rotational spectroscopy. The experimentally derived structural parameters were compared to the outcomes of quantum chemical calculations that can accurately account for dispersive interactions in the cluster. The findings revealed a π-bound configuration for the complex, with the argon atom engaging the aromatic ring. The microwave spectrum reveals both fine and hyperfine tunneling components. The main spectral doubling is evident as two distinct clusters of lines, with an approximate separation of 179 MHz, attributed to the torsional motion associated with the hydroxyl group. Additionally, each component of this doublet further splits into three components, each with separations measuring less than 1 MHz. Investigation into intramolecular dynamics using a one-dimensional flexible model suggests that the main tunneling phenomenon originates from equivalent positions of the hydroxyl group. A double-minimum potential function with a barrier of 1000 (100) cm−1 effectively describes this extensive amplitude motion. However, the three-fold fine structure, potentially linked to internal motions within the tert-butyl group, requires additional scrutiny for a comprehensive understanding
Bifurcated CH2•••O and (C-H)2•••F-C Weak Hydrogen Bonds: The Oxirane–Difluoromethane Complex
No full textThe oxirane-difluorometane molecular complex is stabilized by two bifurcated weak hydrogen bond, CH2×××O and CH2×××F. The binding energy of the complex has been estimated to be 9.6 kJ mol-1 from the DJ centrifugal distortion parameter
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
Weak C-H×××O and C-H×××F-C hydrogen bonds in the Oxirane-Trifluoromethane Dimer
No full textThe oxirane-trifluoromethane dimer generated in a supersonic expansion has been characterized by Fourier transform microwave spectroscopy. The rotational spectra of the parent species and of its two 13C isotopomers in combination with ab initio calculations have been used to establish a Cs geometry for the dimer with the two monomers bound by one C-HâââO and two C-HâââF-C hydrogen bonds. An overall bonding energy of about 6.7 kJ/mol has been derived from the centrifugal distortion analysis. The lengths of the C-HâââO and C-HâââF hydrogen bonds, r(OâââH) and r (FâââH), are 2.37 and 2.68 Å, respectively. The C-HâââF-C interactions give rise to the HCF3 internal rotation motion barrier of 0.55(1) kJ/mol, which causes the A-E splittings observed in the rotational spectra. The analysis of the structural and energetic features of the C-HâââO and C-HâââF-C interactions allows us to classify them as weak hydrogen bonds. Ab initio calculations predict these weak interactions to produce blue shifts in the C-H vibrational frequencies and shortenings of the C-H lengths
Hydrogen Bonding in the Dimer and Monohydrate of 2-Adamantanol: A Test Case for Dispersion-Corrected Density Functional Methods
Full text linkWeakly-bound intermolecular clusters constitute reductionist physical models for non-covalent interactions. Here we report the observation of the monomer, the dimer and the monohydrate of 2-adamantanol, a secondary alcohol with a bulky ten-carbon aliphatic skeleton. The molecular species were generated in a supersonic jet expansion and characterized using broadband chirped-pulse microwave spectroscopy in the 2–8 GHz frequency region. Two different gauche-gauche O-H···O hydrogen-bonded isomers were observed for the dimer of 2-adamantanol, while a single isomer was observed for the monomer and the monohydrate. The experimental rotational parameters were compared with molecular orbital calculations using density functional theory (B3LYP-D3(BJ), B2PLYP-D3(BJ), CAM-B3LYP-D3(BJ), ωB97XD), additionally providing energetic and electron density characterization. The shallow potential energy surface makes the dimer an interesting case study to benchmark dispersion-corrected computational methods and conformational search procedures
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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