1,720,963 research outputs found
Exploiting neutrons to probe the structures, and vibrational and rotational spectra, of inorganic and organometallic systems
No full textNew mode of coordination for the dinitrogen ligand: Formation, bonding, and reactivity of a tantalum complex with a bridging N-2 unit that is both side-on and end-on
No full textThe reaction of a mixture of 1 equiv of PhPH2 and 2 equiv of PhNHSiMe2CH2Cl with 4 equiv of (BuLi)-Li-n followed by the addition of THF generates the lithiated ligand precursor [NPN]Li-2. (TKF)2(where [NPN] = PhP(CH2SiMe2NPh)(2)). The reaction of [NPN]Li-2. (THF)(2) with TaMe3Cl2 produces [NPN]TaMe3, which reacts under HZ to yield the diamagnetic dinuclear Ta(IV! tetrahydride ([NPN]Ta)(2)(mu -H)4 This hydride reacts with N-2 with the loss of H-2 to produce ([NPN]Ta(mu -H))(2)(mu-eta (1):eta (2)-N-2), which was characterized both in solution and in the solid state, and contains strongly activated Nz bound in the unprecedented side-on end-on dinuclear bonding mode. A density functional theory calculation on the model complex [(H3P)(H2N)(2)Ta(mu -H)](2)-mu-eta (1):eta (2)-N-2) provides insight into the molecular orbital interactions involved in the side-on end-on bonding mode of dinitrogen. The reaction of ([NPN]Ta(mu -H))(2)(mu-eta (1):eta (2)-N-2) with propene generates the end-on bound dinitrogen complex ([NPN]Ta(CH2CH2CH3))(2)(mu-eta (1):eta (1)-N-2), and the reaction of [NPN]Li-2. (THF)(2) with NbCl3-(DME) generates the end-on bound dinitrogen complex ([NPN]NbCl)(2)(mu-eta (1):eta (1)-N-2). These two end-on bound dinitrogen complexes provide evidence that the bridging hydride ligands are responsible for the unusual bonding mode of dinitrogen in ([NPN]Ta(mu -H))(2)(mu-eta (1):eta (2)-N-2). The dinitrogen moiety in the side-on end-on mode is amenable to functionalization; the reaction of ([NPN]Ta(mu -H))(2)(mu-eta (1):eta (2)-N-2) with PhCH2Br results in C-N bond formation to yield [NPN]Ta(mu-eta (1):eta (2)-N2CH2Ph)(mu -H)(2)TaBr[NPN]. Nitrogen-15 NMR spectral data an provided for all the tantalum-dinitrogen complexes and derivatives described
Structures of furanosides : geometrical analysis of low-temperature X-ray and neutron crystal structures of five crystalline methyl pentofuranosides
Full text linkCrystal structures of all five crystalline methyl D-pentofuranosides, methyl alpha -D-arabinofuranoside (1), methyl beta -D-arabinofuranoside (2), methyl alpha -D-lyxofuranoside (3), methyl beta -D-ribofuranoside (4) and methyl alpha -D-xylofuranoside (5) have been determined by means of cryogenic X-ray and neutron crystallography. The neutron diffraction experiments provide accurate. unbiased H-atom positions which are especially important because of the critical role of hydrogen bonding in these systems. This paper summarizes the geometrical and conformational parameters of the structures of all five crystalline methyl pentofuranosides, several of them reported here for the first time. The methyl pentofuranoside structures are compared with the structures of the five crystalline methyl hexopyranosides for which accurate X-ray and neutron structures have been determined. Unlike the methyl hexopyranosides, which crystallize exclusively in the C-1 chair conformation, the five crystalline methyl pentofuranosides represent a very wide range of ring conformations
X-ray and neutron diffraction study of the new cationic bis(hydrido)-bridged platinum-iridium complex [(PEt3)2Pt(.mu.-H)2IrH2(PEt3)2 ]+[BPh4]
No full textThe structure of [(PEt3)2pt(μ-H)2IrH2(PEt3)2]+[BPh4]-has been studied by X-ray and neutron diffraction techniques, at room temperature and 22 (1) K, respectively. At both temperatures, crystals of the title compound are monoclinic, with space group P21/c and Z = 4. Unit cell parameters at 22 K are as follows: a = 18.989 (10) A, b = 15.314 (3) A, c = 19.792 (6) A, β = 118.73 (3)°, and V = 5047 (4) A3. The coordination around the Ir atom is distorted octahedral, while that around Pt is distorted square planar. The neutron analysis shows that the Pt-Ir distance is 2.677 (1) A and that the distances between the metals and bridging hydrido ligands differ by 0.15 A (Ir-H = 1.879 (3) and 1.882 (3) A; Pt-H = 1.726 (3) and 1.736 (3) A). The two terminal Ir-H bonds are equivalent (1.586 (3) and 1.591 (3) A) and comparable in length to those found in other neutron diffraction studies
No evidence for proton transfer along the N-H center dot center dot center dot O hydrogen bond in N-methylacetamide: Neutron single crystal structure at 250 and 276 K
No full textThe crystal structure of N-methylacetamide (C3H7NO), M-r = 73.095, has been determined from single-crystal neutron diffraction data at two temperatures, 250 and 276 K, above and below the previously reported phase transition located at 274 K in this work. Crystal data: 250 K [276 K]: space group Pn2la [Pn2(1)m], a = 9.671(2) [4.878(1)] Angstrom, b = 6.613(6) [6.567(1)] Angstrom, c = 7.218(1) [7.332(2)] Angstrom; V = 465.7(6) [234.9(5)] Angstrom (3); D-n = 1.043 [1.034] g.cm(-3), R(F-2) = 0.168 [0.134], wR(F-2) = 0.062 [0.051], S = 1.18 [1.11]. This new investigation of the structure of N-methylacetamide was undertaken in order to assess a recent suggestion based on inelastic neutron scattering spectroscopy that transfer of the amide proton along the peptide hydrogen bond may be responsible for the vibrational anomalies. While we found no evidence for proton transfer along the N-H...O hydrogen bond (d(NH) = 1.025(17) Angstrom, and d(H...O) 1.856(14) Angstrom for at T = 250 K) at either temperature evidence for some molecular disorder is present in accord with our previous C-13 NMR studies. In addition, we find short intramolecular contacts between the amide hydrogen atom and those on both neighboring methyls, which may well affect the vibrational properties of the respective molecular groups
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
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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