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Stereochemically non-rigid transition metal complexes of 2,6-bis(phenyliminomethyl)pyridine and derivatives. Crystal structure of fac- ReBr(CO)(3)(PMTFA) (PMTFA=2,6-pyridylene-bis-(methylene-2-trifluoromethylaniline)
No full textThe Schiff bases 2,6-bis(phenyliminomethyl)pyridine and its derivatives (L) form bidentate chelate complexes with transition metals Re-I, Pt-IV and Pd-II of general formulation fac-[ReBr(Co)(3)L], fac-[PtXMe3L] (X = Cl, Br or I), and cis-[Pd(p-CF3C6F4)(2)L]. In solution these complexes are stereochemically non-rigid, undergoing 1,4-metallotropic shifts and restricted rotations of the substituted aromatic rings attached to the metal-coordinated imino nitrogen. Rates and activation energies of these internal motions were measured by one-dimensional NMR bandshape analysis and two-dimensional EXSY NMR experiments over a range of solution temperatures. Free energy values, DeltaG(double dagger) (298.15 K), for the fluxional shifts were in the range 76-97 kJ mol(-1) with the Pd-II complexes having the lowest energies. An X-ray crystal structure of fac-[ReBr(CO)(3)PMTFA] (PMTFA = 2,6-pyridylene-bis(methylene-2-trifluoroaniline)) showed the pendant imine function to be in an E-conformation with its nitrogen trans to the pyridyl nitrogen
Dynamic NMR investigations of fluxionality of 2-(dimethoxymethyl)pyridine and 2,6-bis(dimethoxymethyl)pyridine in rhenium(I) and platinum(IV) complexes
No full textThe ligands 2-(dimethoxymethyl)pyridine (L-1) and 2,6-bis(dimethoxymethyl)pyridine (L-2) formed bidentate chelate complexes with the isoelectronic transition metal moieties (ReX)-X-I(CO)(3) and (PtXMe3)-X-IV (X=halide). The complexes [ReX(CO)(3)L-1] (X=Cl or Br), [PtXMe3L1] (X=Br or I) and [ReBr(CO)(3)L-2] in organic solvents were shown by NMR to undergo fluxional processes which interconvert co-ordinated and pendant OMe groups. Rates and activation energies of these fluxions were measured by NMR methods (1-dimensional bandshape analysis or 2-dimensional exchange spectroscopy). Magnitudes of DeltaG double dagger (;) (298.15 K) for the fluxions were in the range 59-85 kJ mol(-1) with the order being Re-I-L-1> Re-I-L-2> Pt-IV-L-1. A carbon-carbon bond rotation mechanism is proposed for the fluxions in the L-1 complexes and concerted C-C bond rotation/metallotropic shift processes for the L-2 complexes. Crystal structures of [ReCl(CO)(3)L-1] and [PtIMe3L1] revealed distorted octahedral metal centres with N-M-O 'bite' angles of 73-75 degrees and a trans relationship of the axial halide and pendant OMe group
Solution fluxionality of some pyridine-2,6-dialdehydes, -diketones and -diesters when acting as bidentate ligands in rhenium(I) and platinum(IV) complexes. Crystal structure of ReBr(CO)(3)L (L = methylethyldipicolinate)
No full text2,6-Disubstituted pyridines, where the substituents are aldehyde, ketone or ester functions, form bidentate chelate complexes with the transition metal moieties fac-(ReX)-X-I(CO)(3) (X = halogen). 2-Substituted pyridines, where the substituents are aldehyde or ester functions, form similar types of complexes with the isoelectronic transition metal moieties fac-(ReX)-X-I(CO), and (PtXMe3)-X-IV. The fac-(ReX)-X-I(CO)(3) complexes of the 2,6-disubstituted pyridine ligands were shown by H-1 NMR spectra to undergo metallotropic shifts whereby the Re coordination switches between adjacent ON pairs of the ONO ligand donor set. Rates and activation energies of these fluxional shifts were measured by dynamic NMR bandshape analysis. Magnitudes of DeltaG(double dagger) (298.15 K) were in the range 59-64 kJ mol(-1) for the diketone and diester ligands. The dialdehyde ligand, 2,6-pyridinedicarboxaldehyde, formed an appreciably less-stable Re-I complex that was highly fluxional and showed a tendency to dissociation at ambient solution temperatures. The unsymmetrical diester ligand, methylethyldipicolinate, formed two distinct Re-I complex species in solution, in the approximate abundance ratio of 2:1, the more abundant structure involving coordination to the carbonyl of the ethyl ester function. This particular complex forms exclusively in the solid state and an X-ray crystal structure of [ReBr(CO)(3)L] (L = methylethyldipicolinate) is reported
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
Coordination properties of unsymmetrical Schiff base ligands containing N3 donor sets when restricted to bidentate chelate bonding modes. Crystal structures of fac-[PtIMe3(PMAMP)] (PMAMP=2-{(2?-pyridylmethylene)aminomethyl}pyridine) and fac-[PtIMe3(PMAQ)] (PMAQ=8-{(2?-pyridylmethylene)amino}quinoline)
No full textThe N3 ligands 2-{(2?-pyridylmethylene)aminomethyl}pyridine, (PMAMP), 8-{(2?-pyridylmethylene)amino}quinoline, (PMAQ) and 2-[{(6?-methyl-2?-pyridyl)methylene}aminomethyl]pyridine (MPMAMP) are analogous to 2,2?:6?,2?-terpyridine in respect of their spatial arrangement of N donor atoms. When acting as bidentate chelates PMAMP and PMAQ bind exclusively to the PtIV moiety PtIMe3 such that the five-membered chelate rings incorporate the imine bond. This was deduced by detailed solution NMR studies and X-ray crystal structures. In contrast, the ligand MPMAMP, with 6-methyl substitution of the pyridyl ring attached to the carbon of the imine bond, preferentially, but not exclusively, coordinates to the nitrogen of the other pyridyl ring. The population ratio of the two complexes formed was 6:1 (303 K, CDCl3 solution), but there was no NMR evidence of exchange between these species, in the temperature range 303–413 K, resulting from 1,4-metallotropic shifts, as occurs in analogous complexes such as fac-[PtIMe3(2,2?:6?,2?-terpyridine)]
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