1,720,954 research outputs found
The substitution behaviour of terpy polyglycoxyl and ruthenium-platinum complexes with biologically significant nucleophiles. A kinetic and mechanistic study.
No full textM. Sc. University of KwaZulu-Natal, Pietermaritzburg 2013.The kinetics of two sets of complexes was investigated under pseudo first order conditions using UV-Visible spectrophotometry as a function of concentration and temperature. The first set consisting 4’-substituted terpy polyglycoxyl complexes reacted with biological relevant N-donor nucleophiles showed that the oxygen on the polyglycoxyl group donates electrons to the Pt-metal resulting in a decrease in reactivity from Pt to Pt(eg). The first carbon-oxygen pendant bond plays a crucial role in regulating the electron density donated because there is insignificant change in the reactivity as the chain length is increased to Pt(deg) and then Pt(teg). The trend in the reactivity slightly increases in general from Pt(eg) to Pt(teg) due to reduction in steric hindrance imposed by the ethylene glycoxy unit on one side of the complex. The reactivity of the nucleophiles is pKa and steric dependent. It can be concluded that reactivity of the metal complexes is mostly electronically controlled while that of the nucleophiles is dependent on the basicity and steric bulk. The obtained kinetic data is supported by the DFT calculations that reveal a less electrophilic Pt(II) metal centre for complexes bearing the 4’-substituent. The temperature dependent studies support an associative mode of activation.
The second set of complexes studied included Ru(III)-Pt(II) complexes with a semi-rigid linker 4’-pyridyl-2,2’:6’,2’’terpyridine (qpy) whose results indicated that increase in the overall charges of the respective complexes is the key reason for the observed increase in the reactivity. Additionally, replacing the cis pyridyl group in Pt1 by Ru(III) polypyridyl to give Pt2 and Pt3 respectively lowers the energy of the LUMO (ԉ*) orbitals. The two qpy groups in the tri-nuclear complex Pt3 only slightly increases the reactivity from that of Pt2 because the qpy groups are in orthogonal positions preventing π-electron communication hence the two Pt(II) centres act independently of each other.
The observed activation parameters for both sets of complexes support an associative mode of substitution. The results of this project elucidate intrinsic, electronic and steric properties of the complexes that might be exploited for medicinal, photophysical or other applications
The rates and mechanisms of substitution from Ru(II) complexes with different non-leaving ligand environments.
No full textDoctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.Results from multiple studies have confirmed that the nature of ligands on the metal centre determines the properties of an anti-cancer agent in a biological environment. Ligands affect solubility, substitution reactivity, stability of compound and product after substitution and the type of interaction between the agent and DNA among others properties. Due to competition for sulphur biomolecules by anti-cancer agents in the cells, substitution reactions of potential anti-cancer metal complexes with biologically relevant ligands sheds light on the possible interaction modes of the Ru(II) complexes and stability of the resulting products. This helps in the design, synthesis and administration of new pharmacological agents and in the concept of chemo-protection. On this basis, the study of rate of substitutions from the Ru(II) complexes by thiourea nucleophiles under pseudo-first order conditions was undertaken. The reactions were studied as a function of concentration and temperature using standard Stopped Flow technique for ultrafast reactions or UV-Visible Spectrophotometer.
The first series of the complexes investigated the role of arene ligands on the rate of substitutions in (aqua)(η6-arene)Ru(II) complexes. The rate of substitution for the tri-aqua Ru(II) complexes was controlled by the π-acceptor ability of the arene ligands. For the complexes bearing 2,2’-bipyridyl co-ligand, the leaving aqua ligands are located trans to the arene ligands. For these complexes, the reactivity increase in accordance to the number and type of alkyl substituents on the η6-arene ligands which donate inductively into the -molecular orbitals, causing increased trans labialisation of the coordinated aquo co-ligand. Compared to the reactivity of tri-aquo complexes, the auxiliary bipyridyl ligands lower the rate of substitution for the later complex by a factor of about 100, due to its steric hindrance at the Ru(II) metal centre. The significantly negative activation entropies and positive activation enthalpies suggest that the activation process is dominated by bond making. In the second study, the role of arene and phosphino ligands on the rate of chloride substitution from Ru (II) complexes containing arene and phosphino co-ligands was investigated. It was observed that the coordinated arene ligand donates electrons towards the Ru(II) metal centre and its -electron cloud presents an electrostatic repulsive effect onto and around the Ru centre as measured by the projected cone angle. The bidentate bis(diphenylphosphino)-methane ligand hinders the approach of nucleophiles during the substitution process. When the bis(diphenylphosphino)methane chelate is expanded through the introduction of a methylene carbon within the bridge, the steric hindrance to the approach of nucleophiles is reduced and
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the ligand assumes a trough like conformation which traps the nucleophile within the coordination sphere. This enhances the reactivity by a factor of 103.
The rate of chloride substitution from 2,4,6-tris-(2-pyridyl)-1,3,5-triazineRu(PPh3)(Cl) and analogous complexes was done in the third study. The study showed that higher π-acceptor ability of cis ligands increase the electrophilicity of the metal centre resulting in enhanced reactivity. Electron donating substituents on the ligands at the cis position lower the π-acceptor ability of the ligand hence lower electrophilicity of the Ru(II) metal centre leading to slower rate of substitution. The investigation on the effect of 2-(2-Pyridyl)azole-based ancillary ligands (L) on the chloride substitution from [RuII(tpy)(L)(Cl)]+ in the fourth study revealed that strong electronegative atoms (O or S) in the auxiliary ligands enhance their π-back-donation capacity thereby increasing the electrophilicity of the metal centre and hence the reactivity. On the other hand, the –NH group donate electron density to the metal centre by outer sphere proton donation causing trans-effect thereby which increases the reactivity more than in the former case. The fifth study sought to understanding the effect of substituents on rate of chloride substitution from Ru(II)tpy complexes. Ru(II)tpy complexes with the tpy having electron donating substituents trans to the labile ligand were dominated by trans-effect while those where the tpy bears electron accepting substituents had enhanced π-back-bonding controlling the reactivity. The rate of substitution from the Ru(II) complexes was more strongly affected by electron donating substituents. Electron donating ligands at the cis position slow down the rate of substitution from the Ru(II) metal centre.
Data from DFT calculations performed using Gaussian09 suite of programmes was used to support the observed rates of substitution from the Ru(II) complexes. Large negative values of entropies of activation and positive enthalpies of activation indicate associative mode of activation. On the other hand small positive values of entropies of activation indicate dissociatively activate interchange mode of substitutions. The studies were explored on model Ru(II) complexes with bio-relevant thiourea nucleophiles to predict possible interaction with biomolecules which has become part of the methods used in the endeavour to search for alternative anti-cancer agents with improved efficacy and higher spectrum of activity
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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