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Author: Azadeh Nazemi
Publisher:
ISBN:
Category :
Languages : en
Pages : 111
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Book Description
Computational techniques, mostly density functional theory (DFT), were applied to study metal-based catalytic processes for energy conversion reactions. In the first and second projects, the main focus was on activation of the light alkanes such as methane, which have thermodynamically strong and kinetically inert C-H bonds plus very low acidity/basicity. Two Mo-oxo complexes with the different redox non-innocent supporting ligands, diamide-diimine and ethylene-dithiolate, were modeled. These Mo-oxo complexes are modeled inspired by active species of a metalloenzyme, ethylbenzene dehydrogenase (EBDH). The results for the activation of the benzylic C-H bond of a series of substituted toluenes by modeled Mo-oxo complexes show there is a substantial protic character in the transition state which was further supported by the preference for [2+2] addition over HAA for most complexes. Hence, it was hypothesized that C-H activation by these EBDH mimics is controlled more by the pKa than by the bond dissociation free energy of the C-H bond being activated. The results suggest, therefore, promising pathways for designing more efficient and selective catalysts for hydrocarbon oxidation based on EBDH active site mimics. Also, it is found that the impact of supporting ligand and Brønsted/Lowry acid/base conjugate is significant on the free energy barrier of C-H bond activation. In the third project the focus was on assessing the nature of hydrogen in the transition state related to the transfer of hydrogen between a carbon and nitrogen in an experimentally studied hydroaminoalkylation process by a five-coordinate Ta complex. It was revealed that, for the studied substituents, pKa is a larger driving force in the rate-determining hydrogen transfer reaction than the BDFE, which suggest a reasonable amount of protic character in the transition state, and possible routes to the design of more active catalysts with greater substrate scope. Finally, for the last project, the focus was on hydrotris(1,2,4-triazol-1-yl)borate complex as an electrocatalyst and study the impact of metal identity down a group or across a period of the d-block on proton-coupled electron transfer (PCET), which is a key process in many electrocatalytic cycles. The studied thermodynamics and kinetics trends for a series of mid to late 3d- and 4d-transition metals show the metal and its electronic structure greatly impact the nature of the PCET processes.
Author: Azadeh Nazemi
Publisher:
ISBN:
Category :
Languages : en
Pages : 111
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Book Description
Computational techniques, mostly density functional theory (DFT), were applied to study metal-based catalytic processes for energy conversion reactions. In the first and second projects, the main focus was on activation of the light alkanes such as methane, which have thermodynamically strong and kinetically inert C-H bonds plus very low acidity/basicity. Two Mo-oxo complexes with the different redox non-innocent supporting ligands, diamide-diimine and ethylene-dithiolate, were modeled. These Mo-oxo complexes are modeled inspired by active species of a metalloenzyme, ethylbenzene dehydrogenase (EBDH). The results for the activation of the benzylic C-H bond of a series of substituted toluenes by modeled Mo-oxo complexes show there is a substantial protic character in the transition state which was further supported by the preference for [2+2] addition over HAA for most complexes. Hence, it was hypothesized that C-H activation by these EBDH mimics is controlled more by the pKa than by the bond dissociation free energy of the C-H bond being activated. The results suggest, therefore, promising pathways for designing more efficient and selective catalysts for hydrocarbon oxidation based on EBDH active site mimics. Also, it is found that the impact of supporting ligand and Brønsted/Lowry acid/base conjugate is significant on the free energy barrier of C-H bond activation. In the third project the focus was on assessing the nature of hydrogen in the transition state related to the transfer of hydrogen between a carbon and nitrogen in an experimentally studied hydroaminoalkylation process by a five-coordinate Ta complex. It was revealed that, for the studied substituents, pKa is a larger driving force in the rate-determining hydrogen transfer reaction than the BDFE, which suggest a reasonable amount of protic character in the transition state, and possible routes to the design of more active catalysts with greater substrate scope. Finally, for the last project, the focus was on hydrotris(1,2,4-triazol-1-yl)borate complex as an electrocatalyst and study the impact of metal identity down a group or across a period of the d-block on proton-coupled electron transfer (PCET), which is a key process in many electrocatalytic cycles. The studied thermodynamics and kinetics trends for a series of mid to late 3d- and 4d-transition metals show the metal and its electronic structure greatly impact the nature of the PCET processes.
Author: Timothy M. Alligrant
Publisher:
ISBN:
Category : Electrochemistry
Languages : en
Pages :
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Book Description
This research has investigated proton-coupled electron transfer (PCET) of quinone/hydroquinone and other simple organic PCET species for the purpose of furthering the knowledge of the thermodynamic and kinetic effects due to reduction and oxidation of such systems. Each of these systems were studied involving the addition of various acid/base chemistries to influence the thermodynamics and kinetics upon electron transfer. It is the expectation that the advancement of the knowledge of acid/base catalysis in electrochemistry gleaned from these studies might be applied in fuel cell research, chemical synthesis, the study of enzymes within biological systems or to simply advance the knowledge of acid/base catalysis in electrochemistry. Furthermore, it was the intention of this work to evaluate a system that involved concerted-proton electron transfer (CPET), because this is the process by which enzymes are believed to catalyze PCET reactions. However, none of the investigated systems were found to transfer an electron and proton by concerted means. Another goal of this work was to investigate a system where hydrogen bond formation could be controlled or studied via electrochemical methods, in order to understand the kinetic and thermodynamic effects complexation has on PCET systems. This goal was met, which allowed for the establishment of in situ studies of hydrogen bonding via 1H-NMR methods, a prospect that is virtually unknown in the study of PCET systems in electrochemistry, yet widely used in fields such as supramolecular chemistry. Initial studies involved the addition of Brønsted bases (amines and carboxylates) to hydroquinones (QH2's). The addition of the conjugate acids to quinone solutions were used to assist in the determination of the oxidation processes involved between the Brønsted bases and QH2's. Later work involved the study of systems that were initially believed to be less intricate in their oxidation/reduction than the quinone/hydroquinone system. The addition of amines (pyridine, triethylamine and diisopropylethylamine) to QH2's in acetonitrile involved a thermodynamic shift of the voltammetric peaks of QH2 to more negative oxidation potentials. This effect equates to the oxidation of QH2 being thermodynamically more facile in the presence of amines. Conjugate acids were also added to quinone, which resulted in a shift of the reduction peaks to more positive potentials. To assist in the determination of the oxidation process, the six pKa's of the quinone nine-membered square scheme were determined. 1H-NMR spectra and diffusion measurements also assisted in determining that none of the added species hydrogen bond with the hydroquinones or quinone. The observed oxidation process of the amines with the QH2's was determined to be a CEEC process. While the observed reduction process, due to the addition of the conjugate acids to quinone were found to proceed via an ECEC process without the influence of a hydrogen bond interaction between the conjugate acid and quinone. Addition of carboxylates (trifluoroacetate, benzoate and acetate) to QH2's in acetonitrile resulted in a similar thermodynamic shift to that found with addition of the amines. However, depending on the concentration of the added acetate and the QH2 being oxidized, either two or one oxidation peak(s) was found. Two acetate concentrations were studied, 10.0 mM and 30.0 mM acetate. From 1H-NMR spectra and diffusion measurements, addition of acetates to QH2 solutions causes the phenolic proton peak to shift from 6.35 ppm to as great as ~11 ppm, while the measured diffusion coefficient decreases by as much as 40 %, relative to the QH2 alone in deuterated acetonitrile (ACN-d3). From the phenolic proton peak shift caused by the titration of each of the acetates, either a 1:1 or 1:2 binding equation could be applied and the association constants could be determined. The oxidation process involved in the voltammetry of the QH2's with the acetates at both 10.0 and 30.0 mM was determined via voltammetric simulations. The oxidation process at 10.0 mM acetate concentrations involves a mixed process involving both oxidation of QH2 complexes and proton transfer from an intermediate radical species. However, at 30.0 mM acetate concentrations, the oxidation of QH2-acetate complexes was observed to involve an ECEC process. While on the reverse scan, or reduction, the process was determined to be an CECE process. Furthermore, the observed voltammetry was compared to that of the QH2's with amines. From this comparison it was determined that the presence of hydrogen bonds imparts a thermodynamic influence on the oxidation of QH2, where oxidation via a hydrogen bond mechanism is slightly easier. In order to understand the proton transfer process observed at 10.0 mM concentrations of acetate with 1,4-QH2 and also the transition from a hydrogen bond dominated oxidation to a proton transfer dominated oxidation, conjugate acids were added directly to QH2 and acetate solutions. Two different acetate/conjugate acid ratios were focused on for this study, one at 10.0 mM/25.0 mM and another at 30.0 mM/50.0 mM. The results of voltammetric and 1H-NMR studies were that addition of the conjugate acids effects a transition from a hydrogen bond oxidation to a proton transfer oxidation. The predominant oxidation species and proton acceptor under these conditions is the uncomplexed QH2 and the homoconjugate of the particular acetate being studied, respectively. Furthermore, voltammetry of QH2 in these solutions resembles that measured with the QH2's and added amines, as determined by scan rate analysis. In an attempt to understand a less intricate redox-active system under aqueous conditions, two viologen-like molecules were studied. These molecules, which involve a six-membered fence scheme reduction, were studied under buffered and unbuffered conditions. One of these molecules, N-methyl-4,4'-bipyridyl chloride (NMBC+), was observed to be reduced reversibly, while the other, 1-(4-pyridyl)pyridinium chloride (PPC+), involved irreversible reduction. The study of these molecules was accompanied by the study of a hypothetical four-membered square scheme redox system studied via digital simulations. In unbuffered solutions each species, both experimental and hypothetical, were observed to be reduced at either less negative (low pH) or more negative (high pH), depending on the formal potentials, pKa's of the particular species and solution pH. The presence of buffer components causes the voltammetric peaks to thermodynamically shift from a less negative potential (low pH buffer) to a more negative potential (high pH buffer). Both of these observations have been previously noted in the literature, however, there has been no mention, to our knowledge, of kinetic effects. In unbuffered solutions the reduction peaks were found to separate near the pKa,1. While in buffered solutions, there was a noted peak separation throughout the pH region defined by pKa's 1 and 2 (pKa,1 and pKa,2) of the species under study. The cause for this kinetic influence was the transition from a CE reduction at low pH to an EC reduction process at high pH in both buffered and unbuffered systems. This effect was further amplified via the study of the hypothetical species by decreasing the rate of proton transfer. In an effort to further this work, some preliminary work involving the attachment of acid/base species at the electrode surface and electromediated oxidation of phenol-acetate complexes has also been studied. The attachment of acid/base species at the surface is believed to assist in the observation of heterogeneous acid/base catalysis, similar to that observed in homogeneous acid/base additions to quinone/hydroquinone systems. Furthermore, our efforts to visualize a concerted mechanism are advanced in our future experiments involving electromediated oxidation of phenol-acetate complexes by inorganic species. It may be possible to interrogate the various intermediates more efficiently via homogeneous electron-proton transfer rather than heterogeneous electron transfer/homogeneous proton transfer.
Author: Gui-Juan Cheng
Publisher: Springer
ISBN: 9811045216
Category : Science
Languages : en
Pages : 140
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Book Description
This thesis presents detailed mechanistic studies on a series of important C-H activation reactions using combined computational methods and mass spectrometry experiments. It also provides guidance on the design and improvement of catalysts and ligands. The reactions investigated include: (i) a nitrile-containing template-assisted meta-selective C-H activation, (ii) Pd/mono-N-protected amino acid (MPAA) catalyzed meta-selective C-H activation, (iii) Pd/MPAA catalyzed asymmetric C-H activation reactions, and (iv) Cu-catalyzed sp3 C-H cross-dehydrogenative-coupling reaction. The book reports on a novel dimeric Pd-M (M = Pd or Ag) model for reaction (i), which successfully explains the meta-selectivity observed experimentally. For reaction (ii), with a combined DFT/MS method, the author successfully reveals the roles of MPAA ligands and a new C-H activation mechanism, which accounts for the improved reactivity and high meta-selectivity and opens new avenues for ligand design. She subsequently applies ion-mobility mass spectrometry to capture and separate the [Pd(MPAA)(substrate)] complex at different stages for the first time, providing support for the internal-base model for reaction (iii). Employing DFT studies, she then establishes a chirality relay model that can be widely applied to MPAA-assisted asymmetric C-H activation reactions. Lastly, for reaction (iv) the author conducts detailed computational studies on several plausible pathways for Cu/O2 and Cu/TBHP systems and finds a reliable method for calculating the single electron transfer (SET) process on the basis of benchmark studies.
Author: Gui-Juan Cheng
Publisher:
ISBN: 9789811045226
Category : Activation (Chemistry)
Languages : en
Pages : 126
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Book Description
This thesis presents detailed mechanistic studies on a series of important C-H activation reactions using combined computational methods and mass spectrometry experiments. It also provides guidance on the design and improvement of catalysts and ligands. The reactions investigated include: (i) a nitrile-containing template-assisted meta-selective C-H activation, (ii) Pd/mono-N-protected amino acid (MPAA) catalyzed meta-selective C-H activation, (iii) Pd/MPAA catalyzed asymmetric C-H activation reactions, and (iv) Cu-catalyzed sp3 C-H cross-dehydrogenative-coupling reaction. The book reports on a novel dimeric Pd-M (M = Pd or Ag) model for reaction (i), which successfully explains the meta-selectivity observed experimentally. For reaction (ii), with a combined DFT/MS method, the author successfully reveals the roles of MPAA ligands and a new C-H activation mechanism, which accounts for the improved reactivity and high meta-selectivity and opens new avenues for ligand design. She subsequently applies ion-mobility mass spectrometry to capture and separate the [Pd(MPAA)(substrate)] complex at different stages for the first time, providing support for the internal-base model for reaction (iii). Employing DFT studies, she then establishes a chirality relay model that can be widely applied to MPAA-assisted asymmetric C-H activation reactions. Lastly, for reaction (iv) the author conducts detailed computational studies on several plausible pathways for Cu/O2 and Cu/TBHP systems and finds a reliable method for calculating the single electron transfer (SET) process on the basis of benchmark studies.
Author: Sebastião J. Formosinho
Publisher: Royal Society of Chemistry
ISBN: 1849731411
Category : Science
Languages : en
Pages : 169
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Book Description
This book covers the most recent developments in the field of PCET reactions, from the theoretical and experimental points of view.
Author: Felix Schneck
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
The influence of metal coordination on carbon-centered proton-coupled electron transfer processes is an attractive field of research. Reported literature is limited to thermodynamic investigations and substrate oxidation using C-H bond formation at metal complexes is not reported. Comparison of organic HAT processes with related processes in coordination compounds suggests similar linear free energy relationships for N-H and O-H bond dissociation/formation. This thesis reports the synthesis of nickel pincer complexes containing a C-basic site and their redox and protonation reactivity. Oxid...
Author: Steven Kennedy
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Proton-coupled electron transfer (PCET) is an important phenomenon for controlling charge mobility in chemistry and biology because it allows the simultaneous movement of a proton and electron with a lower energy barrier than otherwise possible. Much work has been done on PCET systems, particularly for excited state processes in which charge mobility can be easily followed using pump-probe methods. However, while excited state PCET is utilized for the initial step of many solar energy-driven processes, including photosynthesis, ground state PCET is critical for all subsequent processes, including regeneration of solar cells. Homogeneous ground state PCET systems are of particular interest for this regeneration, but no convenient method exists for measuring parameters governing such reactions. Our work is directed toward understanding homogeneous ground state PCET reactions as probed using solution-phase steady-state methods.In order to establish a probe for these homogeneous ground state PCET reactions, we design self-exchange model systems for PCET in analogy to classical electron transfer. With our first model system, [Ni(2,3-pyrazinedithiol)2], we demonstrate that protonation of a mixed valence species, generating a mixed valence mixed protonated (MVMP) state, results in severe reduction of the electronic coupling intimately connected with electron transfer kinetics. This ligand-based mixed valence molecule can be asymmetrically protonated, rendering the MVMP state. We characterize the structural, electronic, vibrational, and magnetic properties of this complex in five different states, including the mixed valence and MVMP states, and then analyze the intervalence charge transfer (IVCT) band to demonstrate a five-fold reduction in electronic coupling upon protonation. We conclude that the reduction in electronic coupling is a result of the asymmetry of the electronic orbitals of the redox sites that results from the asymmetric protonation. As a result, the IVCT band is established as a probe for interrogating the electronic coupling in the MVMP state, which reflects the change in the PCET potential energy landscape as a result of protonation. This conclusion suggests that many systems designed to link electron and proton transfer will also exhibit a decrease in electronic coupling upon protonation as the strength of the interaction between redox and protonation sites is increased.After having established the MVMP state as a useful model system to study homogeneous ground state PCET, we explored structural modifications to control the communication between electron transfer and protonation sites. These studies allow for a more fine-tuned response to protonation in a series of metal dithiolene complexes when moving from the mixed valence to the MVMP state. We investigate the effect of changing the bridge between ligands simply by changing the metal center. In this study, we find nearly five-fold decreases in electronic coupling for both Ni and Pt, while, for the Pd complex, the electronic coupling is reduced to the point that the IVCT band is no longer observable. We ascribe the reduction in electronic coupling to charge pinning induced by asymmetric protonation. The more severe reduction in coupling for the Pd complex is a result of greater energetic mismatch between ligand and metal orbitals, reflected in the smaller electronic coupling for the pure mixed valence state. This work demonstrates that the bridging metal center can be used to tune the electronic coupling in both the mixed valence and MVMP states, as well as the magnitude of change of the electronic coupling that accompanies changes in protonation state.In addition, we explore 2,3-quinoxalinedithiol and 2,3-pyridinedithiol ligands, which are structurally altered versions of the above dithiolene ligands in which the aromatic rings are extended and the number of ring nitrogen atoms is reduced, respectively. With these complexes, we find that these modifications cause changes in the electronic coupling both in the mixed valence and MVMP states, and the degree of response to protonation, generating the MVMP state, is controlled as well. For [Ni(2,3-quinoxalinedithiol)2], the only complex with the 2,3-quinoxalinedithiol ligand that reversibly generated its MVMP state, the IVCT band, and hence the electronic coupling, disappeared upon protonation. This disappearance of electronic coupling resulted from additional electron density being placed on the ligands and not being channeled into ligand-ligand electronic coupling through the metal center. The complex [Ni(2,3-pyridinedithiol)2] retained its IVCT band in the MVMP state, but with less electronic coupling than in the 2,3-pyrazinedithiol analogue. This lower value of electronic coupling is a result of higher energy ligand orbitals that overlap with the metal orbitals to a lesser extent.Lastly, we explore the [Au(2,3-pyrazinedithiol)2] complex, which is appealing for the non-innocent character of its ligands. We report its synthesis and characterization, along with electrochemistry and spectrophotometric response to acid titration. This molecule did not exhibit generation of its singly oxidized mixed valence species, so it does not permit direct comparison to the mixed valence species of the other metal dithiolene compounds in this study.Ultimately, our investigations of these metal dithiolene MVMP model systems allow for more informed control over PCET self-exchange transformations, as interrogated through their IVCT bands. The IVCT band is established as a probe for monitoring the effect of asymmetric protonation upon electronic coupling, seeking to extend classical electron transfer model systems into the domain of PCET. The interdependence of asymmetric protonation and electron transfer will allow for better control over PCET charge mobility through structural modifications, which will allow for more rational design of systems that undergo ground state PCET in device applications.
Author: Hunter Hiatt Ripberger
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Proton-coupled electron transfer, or PCET, is a process described by the transfer of both a proton and an electron. This type of reactivity is established in biology as a fundamental method of accomplishing proton transfer, electron transfer, and radical transfer in proteins. A subclass of PCET reactions, known as concerted, multi-site PCETs, advantageously alters the thermodynamics and kinetics of bond homolysis, enabling the generation of open-shell intermediates under mild conditions. This dissertation describes two distinct applications of concerted, multi-site PCET in organic synthesis to yield highly reactive radical intermediates and to improve the lifetime of these intermediates in photoredox catalysis. In Chapter 2, a method for the homolysis of strong, C(sp3)-H bonds is described in the context of an intermolecular C-H alkylation, catalyzed by a cationic iridium(III) photocatalyst and anionic Bronsted base. In-depth mechanistic studies of the reaction demonstrate that ground state ion-pairing between the photo-oxidant and base enables the concerted activation of the C-H bond via multi-site PCET. In this case, ion-pairing is essential for lowering the kinetic barrier of the concerted activation step. In Chapter 3, redox relays built into the ligand framework of an iridium(III) photocatalyst are described and utilized to improve the quantum yield of a preparative scale photoredox reaction. These ligands are shown to undergo reversible PCET upon excitation of the catalyst, which enables long-lived charge separation and subsequently alters the thermodynamics and kinetics of charge recombination, leading to the observed improvement in reaction efficiency.
Author: Achim Müller
Publisher: Elsevier Publishing Company
ISBN:
Category : Science
Languages : en
Pages : 420
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Book Description
Various aspects of electron and proton transfer in chemistry and biology are described in this volume. The joint presentation was chosen for two reasons. Rapid electron and proton transfer govern cellular energetics in both the most primitive and higher organisms with photosynthetic and heterotrophic lifestyles. Further, biology has become the area where the various disciplines of science, which were previously diversified, are once again converging. The book begins with a survey of physicochemical principles of electron transfer in the gas and solid phase, with thermodynamic and photochemical driving force. Inner and outer sphere mechanisms and the coupling of electron transfer to nuclear rearrangements are reviewed. These principles are applied to construct artificial photosynthesis, leading to biological electron transfer involving proteins with transition metal and/or organic redox centres. The tuning of the free energy profile on the reaction trajectory through the protein by single amino acids or by the larger ensemble that determines the electrostatic properties of the reaction path is one major issue.Another one is the transformation of one-electron to paired-electron steps with protection against hazardous radical intermediates. The diversity of electron transport systems is represented in various chapters with emphasis on photosynthesis, respiration and nitrogenases. The book will be of interest to scientists in chemistry, physics and the life sciences.
Author: Jacob N. Israelachvili
Publisher: Academic Press
ISBN: 0123919339
Category : Science
Languages : en
Pages : 708
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Book Description
Intermolecular and Surface Forces describes the role of various intermolecular and interparticle forces in determining the properties of simple systems such as gases, liquids and solids, with a special focus on more complex colloidal, polymeric and biological systems. The book provides a thorough foundation in theories and concepts of intermolecular forces, allowing researchers and students to recognize which forces are important in any particular system, as well as how to control these forces. This third edition is expanded into three sections and contains five new chapters over the previous edition. - Starts from the basics and builds up to more complex systems - Covers all aspects of intermolecular and interparticle forces both at the fundamental and applied levels - Multidisciplinary approach: bringing together and unifying phenomena from different fields - This new edition has an expanded Part III and new chapters on non-equilibrium (dynamic) interactions, and tribology (friction forces)
