1,721,023 research outputs found
Ring fluorination effects on molecular water clusters: the cases of 2-fluoropyridine, 3-fluoropyridine and penta-fluoropyridine. A rotational spectroscopy study.
No full textThe importance of organic fluorinated compounds in science and in everyday life is growing year after year.1 The effects of fluorination are related to the high electronegativity of this atom and its polarizability. In general the introduction of a fluorine atom is used in order to modulate physicochemical properties, like hydrophilicity and lipophilicity, of bio-organic molecules and functional materials.1,2 Additionally, structural changes can also be related to the electronic effect of the fluorine atom or to the molecule capability of creating new hydrogen bonds or non-covalent bonds, having fluorine as acceptor.
For this reason we investigate clusters where a molecule of water is used as a probe to reveal the changes on the electrostatic potential on the fluorinated compounds.
The experimental conditions are achieved in supersonic expansions using Molecular Beam Fourier Transform Microwave Spectroscopy technique (MBFTMW).3 The high resolution and sensitivity of rotational spectroscopy give direct access to the structural arrangement of the systems, allowing the measurement of bond lengths and angles. Moreover this gas phase technique allows unveiling subtle structural and dynamical effects usually related to changes in non-covalent interactions.
The series of clusters studied between different fluorinated pyridines and a molecule of water are: 2-fluoropyridine-water, 3-fluoropyridine-water and penta-fluoropyridine-water (see Figure 1) complexes. The results clearly show that the introduction of a single fluorine atom into a molecule already induces significant effects, but as the number of fluorine atoms increases, such as in the case of penta-fluoropyridine, the system starts to behave as a completely novel species.4 The perfluorination effect is clearly observable in the penta-fluoropyridine-water adduct where the water oxygen lone pairs point towards the aromatic ring.
Figure 1: the observed adducts of 2-fluoropyridine, 3-fluoropyridine and penta-fluoropyridine with a molecule of water
Weak hydrogen bonds in adducts between freons: The rotational study of CH2F2–CH2ClF
No full textThe rotational spectra of 2 isotopologues of the molecular adduct CH2F2–CH2ClF show that the two
subunits are held together through two weak C–H F–C and one C–H Cl–C linkages, rather than
through three weak C–H F–C connections. This suggests the weak Cl H interaction to be observed
preferentially to a weak F H one. Structural information on these weak hydrogen bonds has been
obtained. The dissociation energy has been estimated from centrifugal distortion to be 5.3 kJ mol1
State-of-the-art computation of the rotational and IR spectra of the methyl-cyclopropyl cation: Hints on its detection in space
Full text linkRecent measurements by the Cassini Ion Neutral Mass Spectrometer demonstrated the presence of numerous carbocations in Titan's upper atmosphere. In [Ali et al., Planet. Space Sci., 2013, 87, 96], an analysis of these measurements revealed the formation of the three-membered cyclopropenyl cation and its methyl derivatives. As a starting point of a future coordinated effort of laboratory experiments, quantum-chemical calculations, and astronomical observations, in the present work the molecular structure and spectroscopic properties of the methyl-cyclopropenyl cation have been investigated by means of state-of-the-art computational approaches in order to simulate its rotational and infrared spectra. Rotational parameters have been predicted with an expected accuracy better than 0.1% for rotational constants and on the order of 1-2% for centrifugal-distortion terms. As for the infrared spectrum, despite the challenge of a large amplitude motion, fundamental transitions have been computed to a good accuracy, i.e., the uncertainties are expected to be smaller than 5-10 wavenumbers
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
On the competition between weak O H⋯F and C H⋯F hydrogen bonds, in cooperation with C H⋯O contacts, in the difluoromethane – tert -butyl alcohol cluster
Full text linkThe 1:1 complex of tert-butyl alcohol with difluoromethane has been characterized by means of a joint experimental-computational investigation. Its rotational spectrum has been recorded by using a pulsed-jet Fourier-Transform microwave spectrometer. The experimental work has been guided and supported by accurate quantum-chemical calculations. In particular, the computed potential energy landscape pointed out the formation of three stable isomers. However, the very low interconversion barriers explain why only one isomer, showing one O. H⋯F and two C. H⋯O weak hydrogen bonds, has been experimentally characterized. The effect of the H. →. tert-butyl-group substitution has been analyzed from the comparison to the difluoromethane-water adduct
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
Conformational analysis of 1,4-butanediol: A microwave spectroscopy study
No full textThe rotational spectra of two conformers of 1,4-butanediol have been assigned by pulsed-jet Fouriertransform microwave spectroscopy. Both species display an intra-molecular OH O hydrogen bond. For the most stable conformer, the rotational spectra of four isotopologues (OH–OH, OD–OH, OH–OD and OD–OD) have been measured. The la-type transition of its two symmetric species, OH–OH and
OD–OD, are split into two evenly spaced component lines separated by 6.292(1) and 0.0137(5) MHz, respectively. These splittings are due to the concerted internal rotation of the two OH (or OD) alcoholic groups, which connects two equivalent minima. A partial rs structure has been obtained
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
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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