1,720,965 research outputs found
The Chalcogen Bond in Crystalline Solids: A World Parallel to Halogen Bond
No full textConspectusThe distribution of the electron density around covalently bonded atoms is anisotropic, and this determines the presence, on atoms surface, of areas of higher and lower electron density where the electrostatic potential is frequently negative and positive, respectively. The ability of positive areas on atoms to form attractive interactions with electron rich sites became recently the subject of a flurry of papers. The halogen bond (HaB), the attractive interaction formed by halogens with nucleophiles, emerged as a quite common and dependable tool for controlling phenomena as diverse as the binding of small molecules to proteinaceous targets or the organization of molecular functional materials. The mindset developed in relation to the halogen bond prompted the interest in the tendency of elements of groups 13-16 of the periodic table to form analogous attractive interactions with nucleophiles.This Account addresses the chalcogen bond (ChB), the attractive interaction formed by group 16 elements with nucleophiles, by adopting a crystallographic point of view. Structures of organic derivatives are considered where chalcogen atoms form close contacts with nucleophiles in the geometry typical for chalcogen bonds. It is shown how sulfur, selenium, and tellurium can all form chalcogen bonds, the tendency to give rise to close contacts with nucleophiles increasing with the polarizability of the element. Also oxygen, when conveniently substituted, can form ChBs in crystalline solids. Chalcogen bonds can be strong enough to allow for the interaction to function as an effective and robust tool in crystal engineering. It is presented how chalcogen containing heteroaromatics, sulfides, disulfides, and selenium and tellurium analogues as well as some other molecular moieties can afford dependable chalcogen bond based supramolecular synthons. Particular attention is given to chalcogen containing azoles and their derivatives due to the relevance of these moieties in biosystems and molecular materials. It is shown how the interaction pattern around electrophilic chalcogen atoms frequently recalls the pattern around analogous halogen, pnictogen, and tetrel derivatives. For instance, directionalities of chalcogen bonds around sulfur and selenium in some thiazolium and selenazolium derivatives are similar to directionalities of halogen bonds around bromine and iodine in bromonium and iodonium compounds. This gives experimental evidence that similarities in the anisotropic distribution of the electron density in covalently bonded atoms translates in similarities in their recognition and self-assembly behavior. For instance, the analogies in interaction patterns of carbonitrile substituted elements of groups 17, 16, 15, and 14 will be presented. While the extensive experimental and theoretical data available in the literature prove that HaB and ChB form twin supramolecular synthons in the solid, more experimental information has to become available before such a statement can be safely extended to interactions wherein elements of groups 14 and 15 are the electrophiles. It will nevertheless be possible to develop some general heuristic principles for crystal engineering. Being based on the groups of the periodic table, these principles offer the advantage of being systematic
Chalcogen Bonds in Crystals of Bis(o-anilinium)diselenide Salts
No full textThe diselenide moiety is labeled as a novel and robust chalcogen bond (ChB) donor group. The molecular electrostatic potential of two prototype diselenide derivatives shows the presence of two σ-holes along both the covalent bonds in which each selenium atom is involved. The propensity of selenium atoms of diselenides to work as electrophilic sites is confirmed by computational studies on the bis(o-anilinium)diselenide cation, and single crystal X-ray analysis of salts of this cation reveals the presence of close selenium⋯anion contacts. Comparison with halogen bonds in crystal structures of ionic δ 3 -iodane derivatives supports the rationalization of these close contacts as charge-assisted ChBs. Discrete adducts or two-dimensional networks are formed, suggesting the profitable use of the diselenide moiety in ChB based crystal engineering
C(sp3) atoms as tetrel bond donors: A crystallographic survey
No full textThe σ-hole and π-hole interactions allow for a systematic understanding of some features of the attractive interactions involving elements of groups 13–18 of the periodic table and of some other groups. Areas of depleted electron density, where the electrostatic potential can be positive, exist on these atoms and these areas can form attractive interactions with electron rich sites (nucleophiles). When the electrophilic atom belongs to groups 14, 15, 16, or 17, the resulting interactions are named tetrel, pnictogen, chalcogen, and halogen bond, respectively. Here we discuss the tetrel bonds (TtBs) formed in crystalline solids on interaction of sp3 hybridized carbon atoms with lone pair possessing atoms and anions. A mapping of the specific short contacts formed in the solid by C(sp3) atoms is realized by discussing selected structures from the Cambridge Structural Database. This mapping led to the identification of some functional groups particularly tailored to form TtBs which can affect or control the packing in crystalline solids. Specifically, it is shown that methyl and methylene groups bound to ammonium, pyridinium, and sulfonium residues can give rise to particularly short and directional TtBs. Topologically, the formed adducts can exist as discrete species or one, two, or three dimensional networks. Fluorine atoms and perfluorinated residues as well as nitro and cyano substituents can also lead to the formation of TtBs which can control molecular conformation and packing in the solid
The Relevance of Size Matching in Self-assembly: Impact on Regio- and Chemoselective Cocrystallizations
No full textDecamethonium diiodide is reported to perform the chemo- and regioselective encapsulation of para-dihalobenzenes through the competitive formation of halogen-bonded cocrystals starting from solutions that also contain ortho and meta isomers. Selective caging in the solid occurs even when an excess ortho or meta isomers, or even a mixture of them, is present in the solution. A prime matching between the size and shape of the dication and the formed dianions plays a key role in enabling the selective self-assembly, as proven by successful encapsulation of halogen-bond donors as weak as 1,4-dichlorobenzene and by the results of cocrystallization trials involving mismatching tectons. Encapsulated para-dihalobenzenes guest molecules can be removed quantitatively by heating the cocrystals under reduced pressure and be recovered as pure materials. The residual decamethonium diiodide can be recycled with no reduction in selectivity
Binding motif of ebselen in solution: Chalcogen and hydrogen bonds team up
Full text linkEbselen (2-phenyl-1,2-benzoselenazol-3(2H)one), a glutathione peroxidase mimic, is active against several RNA viruses, among others the retrovirus responsible for the COVID-19 pandemic. In this paper 77Se and 1H NMR studies of ebselen are reported and they identify the chalcogen bond (ChB) and hydrogen bond (HB) that are central in the landscape of interactions formed by the compound in solution. The selenium atom and the hydrogen atom at the C7 carbon act as ChB and HB donors and the O and N atoms of neutral molecules function as acceptors. The ChB and HB give rise to a bifurcated supramolecular synthon, which fastens the interaction acceptor opposite to the N-Se covalent bond of the selenazole ring. It is known that the biologically important reaction of ebselen with cysteine thiol groups is favoured when selenium acts as a chalcogen bond donor. This journal i
Cyanine dyes: Synergistic action of hydrogen, halogen and chalcogen bonds allows discrete I42- anions in crystals
No full textIn crystals of a benzoselenazole dye, a network of hydrogen bonds with the dye cations locks two iodide anions at a distance allowing for an iodine molecule to be pinned at either ends via two halogen bonds. I42- supramolecular anions are formed as discrete species that lie in the confined space defined by cation molecules
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
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