1,721,057 research outputs found
A comparative structural and spectroscopic study of diiron and diruthenium isocyanide and aminocarbyne complexes
Full text linkThe chemistry of [Fe2Cp2(CO)4] (1-Fe; Cp = η5-C5H5) has been widely investigated, and straightforward carbonyl/isocyanide substitution, followed by isocyanide alkylation, provides access to a variety of aminocarbyne complexes, displaying a rich and versatile reactivity. On the other hand, the parallel chemistry of [Ru2Cp2(CO)4] (1-Ru) has been much less developed, and relevant structural/bonding aspects scarcely elucidated. We report an IR study on 1-Ru in different solvents and in the solid state. The IR data of [M2Cp2(CO)3(CNR)] (M = Ru, R = 2,6-C6H3Me2 = Xyl, 2a-Ru; M = Ru, R = CH2Ph = Bn, 2b-Ru; M = Ru, R = Me, 2c-Ru; M = Fe, R = Xyl, 2a-Fe; M = Fe, R = Bn, 2b-Fe; M = Fe, R = Me, 2c-Fe) and [M2Cp2(CO)3{μ-CNR(Me)}]CF3SO3 (M = Ru, R = Xyl, 4a-Ru; M = Ru, R = Bn, 4b-Ru; M = Ru, R = Me, 4c-Ru; M = Fe, R = Xyl, 4a-Fe; M = Fe, R = Bn, 4b-Fe; M = Fe, R = Me, 4c-Fe) are comparatively presented, and the back-donation from the dimetallic core to π-acceptor ligands in complexes 4 is discussed. A new synthetic procedure to access 2a-Ru is reported, affording [Ru2Cp2(CO)2(CNXyl)2] (3) as by-product, as well as the single crystal X-ray structures of 2b-Ru and 4b-Ru. DFT calculations were carried out to elucidate the relative stability of isomeric forms within the series of isocyanide adducts 2-Ru and
Ion pairing in cationic NHC and NAC gold(I) η2-alkyne complexes
No full textGold(I) cationic complexes of general formula [LAu+ X-] [L = NAC (Nitrogen-Acyclic-Carbene) or NHCs (N-Heterocyclic Carbenes), X- = weakly coordinating anion] are successfully employed as catalysts in a large variety of organic reactions involving the activation of unsaturated carbon-carbon bonds. A key role in such reactions is played by the counterion. We have studied the ion pair structure in solution of [NHC-Au(η2-3-hexyne)]BF4 and [NAC-Au(η2-3-hexyne)]BF4 by NOE NMR spectroscopy and relativistic DFT calculation. In our previous work, two main orientations were observed for unsaturated NHC ligands: one with the anion close to the carbene backbone (A, most populated) and another with the anion close to the 3-hexyne (B). Here we focus on the effect of different carbenes on the ion pair structure, comparing the aromatic NHC (1BF4) with 5 different ligands: a non aromatic NHC (2BF4), a polycyclic ligand with an extended aromatic system (3BF4) and three different NAC (4BF4, 5BF4 and 6BF4). For 2BF4, the A:B ratio (observed with NOE NMR spectroscopy) remains almost the same as for 1BF4, whilst the ion pair structure of the 3BF4 becomes mainly non-specific, with a slight preference for the orientation B. Both cases can be explained analyzing the DFT Coulomb potential map, that shows an attractive region on the backbone of 2BF4 and a flat weak potential around the whole 3BF4. An interesting result was obtained for 4BF4 that shows a strong attractive region close to the two N-H that makes the IP 100% specific for the orientation A (figure beside). 5BF4 shows a less specific ion pair due to the presence of only one N-H on the back. For the 6BF4 the -OH group on the arm of the carbene introduce a third ion pair configuration, strongly stabilized by the hydrogen bond between the hydroxyl and the anion
Discriminating Halogen-Bonding from Other Noncovalent Interactions by a Combined NOE NMR/DFT Approach
Full text linkHerein a combined NOE NMR/DFT methodology to discriminate between adducts held together by halogen bonding (XB) and other noncovalent interactions (non-XB, such as lone pair/π), based on the determination of the XB donors′ and acceptors′ relative orientation, is proposed. In particular, 19F,1H HOESY NMR spectroscopy experiments and DFT calculations on different XB donors, such as perfluorohexyl iodide (I1), iodopentafluorobenzene (I2) and bromopentafluorobenzene (Br), combined with different Lewis bases, such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and 2,4,6-trimethylpyridine (Me3Py), were performed. The results clearly show that in the case DABCO/I1 the XB adduct is practically the only one present in solution, whereas for the other pairs a certain amount of non-XB adduct is present. Combining DFT and HOESY results, the amount of non-XB adducts can be roughly quantified under our experimental conditions as 4 % for DABCO/I2, between 10 and 20 % for Me3Py/I1 and Me3Py/I2, and 44 % for DABCO/Br
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
Lewis Base Activation of Lewis Acid: A Detailed Bond Analysis
Full text linkThe effect of a Lewis base (LB) on the nucleophilic attack on chalcogeniranium (chalcogen = sulfur and selenium) cations, the so-called LB activation of a Lewis acid, has been studied coupling natural orbital for chemical valence decomposition of the orbital interaction energy with charge displacement analysis. This methodology provides a detailed and accurate description of all the interactions (LB···chalcogen, chalcogen···olefin and olefin···ammonia) present in the system and leads to a deeper understanding of how they influence each other at all stages of the reaction: reactant complex, transition state, and product complex. In particular, the bond between the chalcogen and the olefin has been decomposed in terms of σ donation/π back-donation and the bond components quantified. This allowed determination of a linear relationship between the activation barrier of the nucleophilic attack and the net amount of charge donated by the olefin to the chalcogen
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
Cooperativity between hydrogen- and halogen bonds: the case of selenourea
Full text linkA combined experimental/computational study of cooperativity between halogen-(XB) and hydrogen bonding (HB) is presented. Selenourea (SeU) has been chosen due to its ability to act at the same time as an XB acceptor toward I(CF2)5CF3 (I1) through the two lone pairs on the selenium atom, and as a HB donor to the benzoate anion through its two amino moieties. All equilibrium constants have been estimated using either diffusion NMR and NMR titration techniques. Experimental results demonstrate that the –NH2⋯anion interaction strongly enhances the Se⋯I one of about one order of magnitude (in terms of formation constant of the adduct), whereas DFT results rationalize such results revealing that the presence of a HB between the benzoate and SeU strongly polarizes the latter, enhancing the negative partial charge on selenium and, consequently, its Lewis basicity and its XB acceptor properties
Characterization of Halogen Bonded Adducts in Solution by Advanced NMR Techniques
No full textIn the last 20 years, a huge amount of experimental results about halogen bonding (XB) has been produced. Most of the systems have been characterized by solid state X-ray crystallography, whereas in solution the only routine technique is the titration (by using 1H and 19F NMR, IR, UV-Vis or Raman spectroscopies, depending on the nature of the system), with the aim of characterizing the strength of the XB interaction. Unfortunately, the titration techniques have many intrinsic limitations and they should be coupled with other, more sophisticated techniques to have an accurate and detailed description of the geometry and stoichiometry of the XB adduct in solution. In this review, it will be shown how crucial information about XB adducts can be obtained by advanced NMR techniques, as Nuclear Overhauser Effect-based Spectroscopies (NOESY, ROESY, HOESY&hellip;) and diffusion NMR techniques (PGSE or DOSY).</jats:p
- …
