1,724,292 research outputs found
Direct experimental evidence that the X-H∙∙∙Cl bond is stronger than X-H∙∙∙F bond (X=O, C)
No full textWe have observed and assigned the rotational spectra of the molecular adducts CH2FCl∙∙∙H2O and (CH2FCl)2 with Fourier Transform Microwave Spectroscopy, obtaining straightforward information on their structural arrangement and internal dynamics.
a) O-H∙∙∙Cl vs O-H∙∙∙F
From the spectroscopic constants of various isotopomers of the water moiety we have extracted the structural information on the CH2FCl∙∙∙H2O complex and the structural data is compared to high level ab initio calculations.
In the case of CH2FCl∙∙∙H2O there is the possibility of forming two different adducts: the O-H∙∙∙Cl one and the O-H∙∙∙F one. The ab initio energies for the two conformations are very similar with the O-H∙∙∙F bound complex slightly more stable (ca. 20 cm-1). The rotational spectrum shows only the presence of the dimer which is hydrogen bound to the chorine atom although both conformations should have sizeable dipole moment components and should thus be observable.
So it seems that the chlorine bound complex is more stable than the fluorine bound one. In this case it is the water molecule that chooses, and we show experimentally that it prefers chorine.
The internal motions effects observed for CH2FCl∙∙∙H2O were used to extract information on the potential energy surface associated with the motion, which in the case of weak non covalent bonds is usually shallow and allows tunneling across low barriers.
b) C-H∙∙∙Cl vs C-H∙∙∙F
The dimer (CH2FCl)2 shows unambiguously the presence of weak hydrogen bonds of the type C-H∙∙∙Cl, C-H∙∙∙F and is stabilized by multiple bonds. Between all possible conformations, the observed one is the one bearing the highest number of C-H∙∙∙Cl interactions.
The structural information is extracted from the spectroscopic constants and compared to high level ab initio calculations which help to justify and understand the observations in this case the observed geometrical arrangement corresponds to the lowest energy conformational minimum.
In both complexes we observed fine structures of the rotational transitions due to nuclear quadrupole spin interaction or to the presence of large amplitude motions occurring in the dimer. The nuclear quadrupole spin interaction constants are used as additional structural information, since the effect depends on the orientation of the electric field gradient at the nucleus involved (in our case the chlorine nucleus)
Computational Insight into the Nature and Strength of the π-Hole Type Chalcogen∙∙∙Chalcogen Interactions in the XO2∙∙∙CH3YCH3 Complexes (X = S, Se, Te; Y = O, S, Se, Te)
No full textIn recent years, the non-covalent interactions between chalcogen centers have aroused substantial research interest because of their potential applications in organocatalysis, materials science, drug design, biological systems, crystal engineering, and molecular recognition. However, studies on π-hole-type chalcogen∙∙∙chalcogen interactions are scarcely reported in the literature. Herein, the π-hole-type intermolecular chalcogen∙∙∙chalcogen interactions in the model complexes formed between XO2 (X = S, Se, Te) and CH3YCH3 (Y = O, S, Se, Te) were systematically studied by using quantum chemical computations. The model complexes are stabilized via one primary X∙∙∙Y chalcogen bond (ChB) and the secondary C−H∙∙∙O hydrogen bonds. The binding energies of the studied complexes are in the range of −21.6~−60.4 kJ/mol. The X∙∙∙Y distances are significantly smaller than the sum of the van der Waals radii of the corresponding two atoms. The X∙∙∙Y ChBs in all the studied complexes except for the SO2∙∙∙CH3OCH3 complex are strong in strength and display a partial covalent character revealed by conducting the quantum theory of atoms in molecules (QTAIM), a non-covalent interaction plot (NCIplot), and natural bond orbital (NBO) analyses. The symmetry-adapted perturbation theory (SAPT) analysis discloses that the X∙∙∙Y ChBs are primarily dominated by the electrostatic component
Enhancing Effects of the Cyano Group on the C-X∙∙∙N Hydrogen or Halogen Bond in Complexes of X-Cyanomethanes with Trimethyl Amine: CH3−n(CN)nX∙∙∙NMe3, (n = 0–3; X = H, Cl, Br, I)
No full textIn this paper, density functional theory and wave function theory calculations are carried out to investigate the strength and nature of the intermolecular C-X∙∙∙N bond interaction as a function of the number of cyano groups, CN, in the X-bond donor while maintaining the X-bond acceptor as fixed. Specifically, complexes of X-cyanomethanes with trimethyl amine CH3−n(CN)nX∙∙∙NMe3 (n = 0–3; X = H, Cl, Br, I) are used as model systems. Geometrical parameters and vibrational C-X-stretching frequencies as well as interaction energies are used as relevant indicators to gauge hydrogen or halogen bond strength in the complexes. Additional characteristics of interactions that link these complexes, i.e., hydrogen or halogen bonds, are calculated with the use of the following theoretical tools: the atoms in molecules (AIM) approach, the natural bond orbital (NBO) method, and energy decomposition analysis (EDA). The results show that, for the specified X-center, the strength of C-X∙∙∙N interaction increases significantly and in a non-additive fashion with the number of CN groups. Moreover, the nature (noncovalent or partly covalent) of the interactions is revealed via the AIM approach
Unraveling the Strength and Nature of Se∙∙∙O Chalcogen Bonds: A Comparative Study of SeF2 and SeF4 Interactions with Oxygen-Bearing Lewis Bases
No full textChalcogen bonds (ChBs) involving selenium have attracted substantial scholarly interest in past years owing to their fundamental roles in various chemical and biological fields. However, the effect of the valency state of the electron-deficient selenium atom on the characteristics of such ChBs remains unexplored. Herein, we comparatively studied the σ-hole-type Se∙∙∙O ChBs between SeF2/SeF4 and a series of oxygen-bearing Lewis bases, including water, methanol, dimethyl ether, ethylene oxide, formaldehyde, acetaldehyde, acetone, and formic acid, using ab initio computations. The interaction energies of these chalcogen-bonded heterodimers vary from −5.25 to −11.16 kcal/mol. SeF2 participates in a shorter and stronger ChB than SeF4 for all the examined heterodimers. Such Se∙∙∙O ChBs are closed-shell interactions, exhibiting some covalent character for all the examined heterodimers, except for SeF4∙∙∙water. Most of these chalcogen-bonded heterodimers are predominantly stabilized through orbital interactions between the lone pair of the O atom in Lewis bases and the σ*(Se–F) antibonding orbitals of Lewis acids. The back-transfer of charge from the lone pair of selenium into the σ* or π* antibonding orbitals of Lewis bases is also observed for all systems. Energy decomposition analysis reveals that the electrostatic component significantly stabilizes the targeted heterodimers, while the induction and dispersion contributions cannot be ignored
Strong External Electric Fields Reduce Explosive Sensitivity: A Theoretical Investigation into the Reaction Selectivity in NH2NO2∙∙∙NH3
No full textControlling the selectivity of a detonation initiation reaction of explosive is essential to reduce sensitivity, and it seems impossible to reduce it by strengthening the external electric field. To verify this, the effects of external electric fields on the initiation reactions in NH2NO2∙∙∙NH3, a model system of the nitroamine explosive with alkaline additive, were investigated at the MP2/6-311++G(2d,p) and CCSD(T)/6-311++G(2d,p) levels. The concerted effect in the intermolecular hydrogen exchange is characterized by an index of the imaginary vibrations. Due to the weakened concerted effects by the electric field along the −x-direction opposite to the “reaction axis”, the dominant reaction changes from the intermolecular hydrogen exchange to 1,3-intramolecular hydrogen transference with the increase in the field strengths. Furthermore, the stronger the field strengths, the higher the barrier heights become, indicating the lower sensitivities. Therefore, by increasing the field strength and adjusting the orientation between the field and “reaction axis”, not only can the reaction selectivity be controlled, but the sensitivity can also be reduced, in particular under a super-strong field. Thus, a traditional concept, in which the explosive is dangerous under the super-strong external electric field, is theoretically broken. Compared to the neutral medium, a low sensitivity of the explosive with alkaline can be achieved under the stronger field. Employing atoms in molecules, reduced density gradient, and surface electrostatic potentials, the origin of the reaction selectivity and sensitivity change is revealed. This work provides a new idea for the technical improvement regarding adding the external electric field into the explosive system
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
Linked collectors and determiners for: Distribution and Occurrences of Laevilitorina across the Southern Hemisphere.
No full textNatural history specimen data linked to collectors and determiners held within, "Distribution and Occurrences of Laevilitorina across the Southern Hemisphere". Claims or attributions were made on Bionomia by volunteer Scribes, <a href="http://bionomia.net/dataset/cd023c5e-8729-41b2-b9df-1419289c0e40">https://bionomia.net/dataset/cd023c5e-8729-41b2-b9df-1419289c0e40</a> using specimen data from the dataset aggregated by the Global Biodiversity Information Facility, <a href="https://gbif.org/dataset/cd023c5e-8729-41b2-b9df-1419289c0e40">https://gbif.org/dataset/cd023c5e-8729-41b2-b9df-1419289c0e40</a>. Formatted as a Frictionless Data package
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
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