1,720,973 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
REACTIVITY OF NITRIC OXIDE AND S-NITROSOTHIOLS WITH LOW-COORDINATE ZINC AND NICKEL CENTERS
No full textPh.D.Nitric oxide (NO) and biologically relevant sources of NO such as S-nitrosothiols (RSNOs) have become a topic of great interest due to their regulation of biological functions including vasorelaxation, neurotransmission, and bronchodilation. NO and RSNOs are also thought to interact with zinc-containing metalloproteins, such as metallothionein and zinc fingers, both of which contain Zn-S bonds due to coordination to cysteine residues within the proteins.In an effort to gain a better molecular level understanding of NO and RSNO reactivity with Zn-SR bonds, three families of zinc thiolate model complexes simulating His2Cys2Zn, His3Cys1Zn, and His1Cys3Zn sites are synthesized using beta-diketiminate, tris(pyrazolyl)borate, and N-heterocyclic carbene ligands. Nitric oxide itself is shown to be inert toward these complexes, however in the presence of an oxidant such as dioxygen, the Zn-S bonds are cleaved and an S-nitrosothiol is produced. S-nitrosothiols can interact with Zn-SR linkages to undergo transnitrosation. Thermodynamic preferences and kinetic parameters of this reaction are explored.The remediation of NO is also important in ecology as it is a component of photochemical smog. NO and NOx are produced as a byproduct of high temperature combustion in automobiles, and catalytic converters cleave the N-O bond to product dinitrogen and dioxygen. These catalysts require high temperatures and expensive precious metals such as Pt, Pd, and Rh to cleave the N-O bond, however.Activation of the N-O bond by Ni could prove useful for new more cost effective catalysts. Low-coordinate nickel nitrosyl complexes are explored using beta-diketiminate and N-heterocyclic carbene ligands. Calculations and preliminary experimental data are presented for the photochemical linkage isomerization of the Ni-N-O bond from linear to a metastable side-on binding mode
Synthetic Models for Copper Electron Transfer Sites and Their Reactivity with Nitric Oxide and Dinitrogen
No full textPh.D.Nitric oxide (NO) has been recently recognized as an important gaseous signal transmitter that participates in many biological processes, including vasorelaxation, neurotransmission, and immune responses. S-nitrosothiols (RSNO), such as such as S-nitrosocysteine and S-nitrosoglutathione, are important oxygen stable reservoirs for nitric oxide. Due to the therapeutical value of NO, interconversion between RSNO and NO as well as metalloenzymes that involving in these processes have become a new target of bioinorganic chemistry. This thesis centers on understanding the fundamental reactivity between copper thiolate and nitric oxide by using model complexes that emulate the common coordination environments of mono- or multi- copper electron transfer sites.To better understand the electronic structure of copper electron transfer sites, we developed several functional model complexes for the binuclear CuA site and type 1 copper site (T1Cu). While previous models were much harder to reduce than native sites, our model complexes are first to closely and reversibly reproduce their reduction potentials. More importantly, these electronic and steric modifications of copper model complexes allow for the isolation of a series of unprecedented copper S-nitrosothiol complexes in different binding modes (1-N(O)SR, 1-S(NO)R, 1-S(O)NR). These results demonstrate that the NO binding mode and binding affinity on copper thiolate are strongly dependent on the redox potential of copper and the steric bulk around the coordinate site.Along with the rich nitric oxide reactivity on copper sites, we also discovered that the prototypical tris(pyrazolyl)borate copper fragments that serve as model for the reversible O2 binding at hemocyanin, also reversibly binds dinitrogen. Isolation of the first example of end-on -N2 dicopper complex and its intriguing formation from a unique mixed-valence, dicopper hydride complex are presented. Preliminary mechanistic studies employing isolated dicopper hydride complex are suggestive of a highly reducing terminal copper(II) hydride species
Mechanistic Studies of Copper and Iron Catalyzed Ammonia Oxidation
No full textPh.D.Global energy demand is increasing because of industrialization and population growth. Fossil fuels (oil, natural gas, and coal) provide about 88% of the energy needed to meet current demand. The two major problems associated with fossil fuels are well established. First, fossil fuel resources are limited on our planet and will last approximately 120 (coal), 60 (natural gas), and 40 (oil) years, respectively. Second, burning fossil fuels releases greenhouse gases into the atmosphere which causes global warming. We are under tremendous pressure to find sustainable sources of energy. Hydrogen-enriched fuels such as ammonia (NH3), hydrogen (H2), ethanol (CH3CH2OH), and methanol (CH3OH) have recently gained attention as a hydrogen carrier fuels for storage of hydrogen. In contrast to other chemical fuels, hydrogen and ammonia are the only carbon-free renewable chemical fuels and can be produced from green sources. Green hydrogen is formed via water splitting powered by solar energy and ammonia is generated by the reaction of green hydrogen with nitrogen from the air.
Among all environmentally friendly energy sources, hydrogen has attracted perhaps the greatest interest, as water is the only byproduct. However, the large scale applications of flammable H2 technology has been strongly hindered by concerns about safety, effectiveness, economical and regulated storage of hydrogen especially for its application as a fuel in road transport.
Construction and operation costs for ammonia pipelines are more economical than the hydrogen pipelines. Due to its widespread use as a feedstock for nitrogen fertilizers, there is extensive infrastructure for ammonia storage and transport including rail, truck, oceangoing tankers, and pipeline. The transportation network has a key role in ammonia economy, more precisely, maritime trading is essential part of global distribution networks by having a comprehensive network of ports that delivers ammonia at large scale that makes ammonia promising energy vector. Analogous to C−C and C−H bonds in hydrocarbons, the N−H bonds in ammonia store chemical energy that can be converted to other forms of energy such as electricity. To increase efficiency of ammonia oxidation as a green fuel, cost-effective approaches for electrocatalytic oxidation of ammonia into electrical power are required. Homogeneous electrocatalysts are attractive methods to outline the fundamental mechanisms, spectroscopic, kinetic, and thermodynamic through which electrocatalytic ammonia oxidation may occur which is necessary to design catalysts with higher activity and selectivity.
Herein we describe mechanistic studies that uncover mechanism of ammonia oxidation by copper and iron catalysts. Research from our laboratory on some β-diketiminato copper(II) amides [CuII]-NHR suggests that they are unstable towards reductive coupling to [CuI] and hydrazines RNH-NHR which are easily oxidized to diazenes RN=NR. These findings demonstrate that related [CuII]-NH2 intermediates could undergo N-N coupling to give [CuI] and H2N-NH2, which may finally disproportionate to NH3 and N2. Electrochemical studies reveal that copper(I) β-diketiminate complexes may be used as electrocatalysts for ammonia oxidation. Detailed mechanistic studies suggest that cationic copper(II) ammine complexes {[CuII]-NH3}+ are unstable towards deprotonation in the presence of a base to form reactive [CuII]-NH2 intermediates. The unique electronic structure of the β-diketaminatato copper(II) amides leads to a low energy pathway for N-N bond formation to generate N2H4.
We also describe an electrocatalytic system for the oxidation of ammonia based on ferrocene (Cp2Fe), an inexpensive, robust catalyst utilizing Earth-abundant iron. Ferrocenium (Cp2Fe+), the 1-electron oxidized form of ferrocene, cleanly oxidizes ammonia to generate nitrogen gas (N2) and protons captured by excess ammonia as NH4+ with electrons reducing ferrocenium to ferrocene. This process occurs under electrocatalytic conditions to generate N2 with sustained current. Simple modification of ferrocene through sulfonation allows for solubility in liquid ammonia to enable electrocatalysis in highly concentrated, energy dense solutions of ammonia. Kinetic and computational analysis provides mechanistic insight into the oxidation of ammonia by ferrocenium
Investigating the Interconversion of Dinitrogen and Ammonia: An Interrogation of Secondary Coordination Sphere Influences on Critical Intermediates
No full textPh.D.The near thermoneutral interconversion of dinitrogen (N2) and ammonia (NH3) sustains life and society on Earth. Altogether, the N2 / NH3 interconversion involves a nexus of high energy intermediates connected through challenging N-H bond forming and breaking reactions that form high energy intermediates such as diazene (N2H2). Therefore, it is paramount to consider the influences governing N2H2 electronic structure and reactivity. Molecular N2H2 is highly unstable above -165 °C and is prone to bimolecular disproportionation to N2 and hydrazine (N2H4). Remarkably then, N2H2 formation from the initial 2 e- / 2 H+ reduction of N2 is considered in the mechanism of NH3 synthesis via nitrogenase enzymes. Controversial assignments of N2H2 binding modes at nitrogenase cofactors, however, highlights that different N2H2 coordination motifs lead to different mechanistic outcomes. Regardless, it is structurally evident that dynamic microenvironment H-bonding via proximal N-donor residues may govern transient N2H2 formation at cofactor sites.
The challenges of studying NxHy species at native nitrogenase require that such intermediates are interrogated at discrete model complexes. Therefore, we set out to examine the influence of H-bonding to N2H2 using a novel, modular [xHetTpCu]2(μ-N2H2) platform that hosts tunable, dual H-bonding interactions between a bridging trans-N2H2 ligand and two pendant N-heterocycles (xHet). Using stable [xHetTpCu]2(μ-OH)2 precursors, the influence of tunable redox innocent H-bonding was initially gauged. These findings accent characterization of less stable [xHetTpCu]2(μ-N2H2) complexes, which manifest redox non-innocent H-bonding via partial H-atom transfer between N2H2 and H- xHet pendants. These findings foreshadow that H-atom transfer via H-bonding xHet pendants may avail future low energy pathways to interconvert N2 and NH3.
Seeking to deliver efficient electrocatalysts that oxidize NH3 at low overpotentials based off of Fc+, we developed a [b-dik]CpFe platform to examine NH3 oxidation. Computational studies of this system outline key mechanistic paramters pertinent to inner- sphere NH3 oxidation. Furthermore, computational modelling of related [b-dik]xHetCpFe(NH3) species indicate that H-bonding between NH3 and xHet pendants favorably lowers NH3 oxidation overpotentials. Altogether these strategies underscore the importance of H-bonding in efficient N2 / NH3 interconversion
Mechanistic Studies of Copper and Nickel Catalyzed sp3 C−H Functionalization
No full textPh.D.sp3 C−H functionalization represents a paradigm shift from the conventional rationality of organic synthesis. Classical organic synthesis relies on the manipulation of functional groups while the new logic of C−H functionalization focuses on the direct installation of functional groups using supposedly “unreactive” C−H bonds even in the presence of more reactive functional groups. This paradigm has the potential to change the synthetic organic chemistry strategy for creating new molecules. This way, light aliphatic hydrocarbons can be converted into value−added chemicals such as amines. Alternatively, complex molecules with one or more functional groups may be converted into more functionalized chemicals for industrial purposes such as drug discovery.Recent advances in the field of organotransition metal chemistry have offered opportunities to achieve activation and functionalization of C−H bonds. Seminal work in this area focused on the reaction development for relatively simple hydrocarbons. Over the past few years, however, several C−H functionalization methods have been discovered and established for the synthesis of complex target molecules. Nowadays, the field has moved from an organometallic challenge with the goal of simply cleaving C−H bonds to form metal−carbon bonds to the development of new synthetic methods for efficient organic synthesis.Despite all the aforementioned advances, the development of site−selective C−H functionalization is still in its infancy. Although several catalyst sytems have been established and many mechanisms have been proposed for transition metal catalyzed C−H functionalization, only a handful of examples are well understood and are mechanistically deriven for resolving the site−selectivity issue. Therefore, more detailed mechanistic studies are needed towards the goal of creating a wide range of reactions suitable for the practical synthesis of both simple and complex molecules by C−H bond functionalization.Herein decribes a series of mechanistic studies that employ b−diketiminato copper(I) and nickel(I) complexes in catalytic intermolecular sp3 C−H functionaliztion. Initially, the successful isolation of a terminal copper−nitrene for nitrene group tranfer into nucleophiles such as isocyanides and phosphines as well as the benzylic C−H bonds is discussed (Chapter 1). Reactivity studies showed a dicopper(II) ketimide complex can be considered as a “masked” terminal copper−arylnitrene complex [Cu]=NAr (Ar = 2,6−iPr2C6H3) for successful nitrene transfer into isocyanides, phosphines and C−H bond of ethylbenzene.Learning from the isolation of the masked arylnitrene complex, several bulky −diketiminate ligands were designed and synthesized to stabilize transient copper−acylnitrene [Cu]=NC(O)Ar species for site−selective hydrogen atom abstraction (HAA) of R−H substrate to form primary and secondary organoradicals R• (chapter 2). Radical capture (RC) of this alkyl radical by the sterically congested [CuII]−NHC(O)Ar furnished primary and secondary amidated products R−NHC(O)Ar. This is the first example of the site−selective primary and secondary functionalization of aliphatic C−H bonds via transition−metal nitrenes. This primary site−selectivity completes that of the previously reported C−H functionalization by our laboratory for the tertiary selective C−H functionalization.Recent studies from our laboratory have discovered that C−H functionalization with unactivated amines, anilines or acyl−protected phenols with tBuOOtBu as an oxidant can take place at CuI b−diketiminates under a radical−relay mechanism. Mechanistic studies revealed that while in some cases the copper(II) alkylamide [CuII]−NHR′ is capable of HAA, the principle role of the copper intermediates [CuII]−NHR′ or [CuII]−OAr is to capture an alkyl radical R• (generated in the HAT of R−H substrate via tBuO• radical) to form R−NHR’ or R’OAr, respectively.We sought to extend this protocol to Ni−catalysis to expand the scope of functional groups that can be installed via radical relay approaches to C−H functionalization (chapter 3). Besides potentially offering access to a wider range of functional groups, a radical−relay approach that employs [NiII]−FG species more stable than their [CuII]−FG could potentially offer different site−selectivities comparing to the established Cu system. Hence, mild activation of tBuOOtBu at [NiI] center was shown and the resulted {[NiII]}2(μ−OtBu)2 complex was shown to be a suitable precursor to prepare several [NiII]−FG complexes were prepared where FG = nitromethanoato, enolato, amido or phenolato ligands. These complexes were carefully characterized and their radical capture behavior was studied to assess the capability of this system for catalytic reactions.The Glaser coupling reaction, an oxidative cross−coupling of alkynes to form 1,3−diynes is an important reaction in synthetic chemistry and material sciences. This transformation was discovered by Carl Glaser in 1869 with the use of cuprous chloride as catalyst and aqueous ammonia as the base under an oxygen atmosphere. Since discovery of this oxidative coupling, chemists have been synthesizing molecules by this means featuring conjugated C≡C bonds, yet its mechanism is unclear up to date. Although several mechanisms have been proposed, the most currently accepted one has been reported by Bohlmann and coworkers in 1964 and relies on dimeric copper acetylides as intermediates in the oxidative acetylenic coupling. Herein, transalkynylation of a sterically encumbered copper(II) tert−butoxide with and sterically bulky, electron−deficient terminal arylakyne allows for isolation of the first three-coordinate, terminal copper(II) alkynyl species [CuII]-C≡CArCl2 (Cl2Ar = 2,6−Cl2C6H3) (chapter 4).Facile reduction of [CuII]−C≡CArCl2 (E1/2 = −645 mV vs Cp2Fe+/Cp2Fe) to the copper(I) alkynide complex {[CuII]−C≡CArCl2}− was achieved using cobaltocene. This reduction triggers redox disproportionation upon coordination of the Lewis base, such as MeCN, 2,4−lutidine, or an alkynylate anion to provide [CuII]−C≡CArCl2(LB), which reductively eliminates 1,3−diyne Cl2ArC≡C−C≡CArCl2. This new mechanistic findings provide insight into the mechanism of C−C coupling in the Glaser coupling and support a redox disproportionation pathway involving CuI, CuII, and CuIII organometallic intermediates
Mechanistic Insights into Catalytic Bond Formation and Electrocatalytic Ammonia Oxidation
No full textPh.D.Computational chemistry is a useful technique to gain insight into reaction mechanisms when utilized in combination with synthesis and experimental characterization. Organometallic chemistry particularly benefits from a greater understanding of the mechanism for reactions especially when intermediates are too transient to be capable of isolation. When intermediates can be isolated, however, they can aid in the determination of reaction mechanisms illustrating how the pairing of synthesis and computational chemistry can work together. The isolation of rare copper(II) and nickel(II) organometallic complexes provide the opportunity to computationally investigate a variety of copper-catalyzed reactions. Utilizing these copper and nickel(II) complexes as starting points for reaction mechanisms, this study investigates a variety of reactions including cross-coupling, C-H functionalization, and radical capture. The unique electronic structure of copper(II) organometallic complexes inspire the development of a new series of reactions through copper-catalyzed ammonia oxidation.
Computational studies on a rare, three-coordinate copper(II) aryl species supported by a β-diketiminate ligand serve as the foundation for an investigation into the mechanism similar to that of Ullman and Chan-Lam-Evans cross-coupling reactions with the resulting PhO-C6F5 and PhO-C6F5 products, respectively. The addition of the phenolate anion PhO– to [CuII]-C6F5 results in the formation of a {[CuII](C6F5)(OPh)}− intermediate which undergoes redox disproportionation with a [CuII]-C6F5 to give {[CuI]-C6F5}− and [CuIII](C6F5)(OPh). This [CuIII](C6F5)(OPh) is unstable toward reductive elimination to [CuI](solvent) and PhO-C6F5. In addition, the copper(II) aryl species serves as a starting point for insight into radical capture reactions in which a [CuIII](C6F5)(R) (where R = radical) intermediate is formed and reductively eliminates to form the corresponding Csp2-Csp3 products.
Computational studies of isolated copper(II) alkynyl species [CuII]-C≡CAr supported by a β-diketiminate ligand provides insight into the copper-catalyzed synthesis of diynes ArC≡C–C≡CAr from alkynes HC≡CAr similar to that of the Glaser coupling reaction. The addition of nucleophiles to this copper(II) alkynyl results in the formation of Csp-Csp, and Csp-Csp2 coupled products ArC≡C–C≡CAr and Ph–C≡CAr, respectively. Mechanistic study aided by computation indicates that [CuII]-C≡CAr species are susceptible to redox-disproportionation enabling the formation of [CuIII](C≡CAr)(R) species in polar aprotic media that reductively eliminates to form R–C≡CAr products (where R = alkynyl or phenyl).
Computational studies of a series of nickel(II) complexes [NiII]-FG (FG = functional group) supported by a β-diketiminate ligand illustrate C-C, C-N, and C-O bond formation at nickel via radical capture. Two competing pathways, alternatively metal- or functional group-centered, enable the capture of the benylic PhCH(•)CH3 at [NiII]-FG species to enable the formation of the functionalized product PhCH(FG)CH3. The favored pathway is dependent upon the nature of the nickel-bound functional group that delivers products featuring new C–C, C–N, or C–O bonds.
Inspired by prior work where a copper(II) amido complex acts as a key intermediate for the formation of diazines via bimolecular N-N coupling, computational studies describe a copper electrocatalyst capable of ammonia oxidation. This computational investigation reveals that limiting ammonia coordination to a single ammine ligand in sterically hindered -diketiminato copper(II) cations {[CuII]-NH3}+ enables deprotonation by ammonia to give highly reactive [CuII]-NH2 intermediate capable of N-N coupling to form hydrazine bound to copper(I) in [CuI]-NH2NH2-[CuI]. A related study also investigates the electrocatalytic oxidation of ammonia with ferrocene via both an inner-sphere and outer-sphere mechanism, considering specific intermediates in ammonia oxidation that do not involve metal centers.
This body of computational studies provides new insights into new methods for C-C, C-N, and C-O bond formation, via reactions similar to that of Chan-Lam-Evans couplings, Glaser couplings, and Kharasch-Sosnovky reactions, that can enable the development reactions of along with electrocatalysts to employ ammonia as a fuel
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