Mechanistic Study of Hydrogen Bonding and Proton Coupled Electron Transfer in Two Separate Redox Systems, Each Containing a Phenylenediamine Derivative PDF Download
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Category : Electronic books
Languages : en
Pages : 150
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Book Description
This thesis presents a mechanistic study of two phenylenediamine derivatives. The first is a disubstituted phenylenediamine with a phenyl-urea substituted para to a dimethylamino group. The phenyl-urea moiety offers two N-H sites for hydrogen bonding and proton transfer. This is UHH. The second, is a disubstituted phenylenediamine with an isocytosine-urea moiety substituted para to a dimethylamino group. The combined isocytosine-urea-phenylenediamine forms a redox active 4-hydrogen bond array where the urea moiety offers two N-H sites for proton donation and the isocytosine offers two sites for hydrogen bond acceptance. This is UpyH. Initial cyclic voltammetry (CV) experiments for UHH show reversible CV behavior in CH2Cl2 and irreversible CV behavior in CH3CN. With the inclusion of two UHH analogs, one with both N-H sites "blocked" with methyl substituents, UMeMe, and a second analog with a single urea N-H site, UMeMe, CV analysis continued. From these studies, in addition to a UV-vis/ CV study, it was determined that the dimethylamino on a fully reduced UHH or UMeH could abstract a proton from a second radical cation urea N-H. This was immediately followed by a thermodynamically favorable second electron transfer. Thus the products at the end of the first oxidation wave from a 2 e-, 1H+ transfer are a quinoidal cation and a fully reduced/protonated UHH or UMeH. On the return scan, UHH in CH3CN and UMeH in both solvents undergo a thermodynamically non-favored back proton transfer at a more energetic reduction potential. UHH in CH2Cl2 accesses a lower energy pathway through the formation of a hydrogen bond complex as part of a wedge scheme. Both pathways are supported by results from concentration and scan rate dependent CV studies that show two return waves correlating to two pathways. UV-vis results show a protonated/reduced species, but no radical cation. In the UpyH project, by using the same CV and UV-vis analysis in addition to an 1HNMR study in CH2Cl2/NBu4PF6, we show UpyH favors a dimerized form but as the dimer undergoes oxidation it breaks apart then reforms on the return scan. To our knowledge this the first account for electrochemically breaking apart a Upy derivative.
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ISBN:
Category : Electronic books
Languages : en
Pages : 150
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Book Description
This thesis presents a mechanistic study of two phenylenediamine derivatives. The first is a disubstituted phenylenediamine with a phenyl-urea substituted para to a dimethylamino group. The phenyl-urea moiety offers two N-H sites for hydrogen bonding and proton transfer. This is UHH. The second, is a disubstituted phenylenediamine with an isocytosine-urea moiety substituted para to a dimethylamino group. The combined isocytosine-urea-phenylenediamine forms a redox active 4-hydrogen bond array where the urea moiety offers two N-H sites for proton donation and the isocytosine offers two sites for hydrogen bond acceptance. This is UpyH. Initial cyclic voltammetry (CV) experiments for UHH show reversible CV behavior in CH2Cl2 and irreversible CV behavior in CH3CN. With the inclusion of two UHH analogs, one with both N-H sites "blocked" with methyl substituents, UMeMe, and a second analog with a single urea N-H site, UMeMe, CV analysis continued. From these studies, in addition to a UV-vis/ CV study, it was determined that the dimethylamino on a fully reduced UHH or UMeH could abstract a proton from a second radical cation urea N-H. This was immediately followed by a thermodynamically favorable second electron transfer. Thus the products at the end of the first oxidation wave from a 2 e-, 1H+ transfer are a quinoidal cation and a fully reduced/protonated UHH or UMeH. On the return scan, UHH in CH3CN and UMeH in both solvents undergo a thermodynamically non-favored back proton transfer at a more energetic reduction potential. UHH in CH2Cl2 accesses a lower energy pathway through the formation of a hydrogen bond complex as part of a wedge scheme. Both pathways are supported by results from concentration and scan rate dependent CV studies that show two return waves correlating to two pathways. UV-vis results show a protonated/reduced species, but no radical cation. In the UpyH project, by using the same CV and UV-vis analysis in addition to an 1HNMR study in CH2Cl2/NBu4PF6, we show UpyH favors a dimerized form but as the dimer undergoes oxidation it breaks apart then reforms on the return scan. To our knowledge this the first account for electrochemically breaking apart a Upy derivative.
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: Sebastião J. Formosinho
Publisher: Royal Society of Chemistry
ISBN: 1849731411
Category : Science
Languages : en
Pages : 169
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This book covers the most recent developments in the field of PCET reactions, from the theoretical and experimental points of view.
Author:
Publisher:
ISBN:
Category : Electronic books
Languages : en
Pages : 71
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Proton-coupled electron transfer (PCET) reactions are essential to many of the fundamental chemical processes of life. In the Smith group we have studied a p-phenylenediamine-based urea, U(H)H, by applying PCET. In this study, the phenyl group in U(H)H is replaced by an imidazole group to form a three hydrogen bond array, UImH. Therefore, AAD arrays, such as APy, are needed as guest compounds to form three intermolecular non-covalent contacts. The initial hypothesis was that oxidation of the phenylenediamine of UImH should lead to stronger H-bonding with the guest compound, and this would make the oxidation easier leading to a negative shift in the E1/2 in the presence of the guest compound. The cyclic voltammetry (CV) of UImH has been examined in methylene chloride and acetonitrile with platinum (Pt) and glassy carbon (GC) electrodes. On both electrodes, CV shows two, closely-spaced, reversible waves of similar height, but a single wave is observed at the lowest concentration on the high scan rate (5.0 V/s). Interestingly, the single wave decreases in relative size as the concentration increases and appears to gradually split into the two smaller peaks seen at slow san rate. Based on the results of CV plus DFT calculations, it is likely that ? dimerization is occurring during the electrochemical reaction. The newest mechanism hypothesizes two electrons per UImH with one intramolecular proton transfer at low concentrations and high scan rate and two sequential one electron transfers per two UImH at high concentration, producing a net one electron oxidation per UImH at high concentration. Addition of the guest, APy, results in a slight increase in the current of the CV waves of UImH. However, very little change in the potential of the CV wave is observed upon addition of the guest, indicating that oxidation does not change binding strength. The increase in current that is observed is most likely due to hydrogen bonding interfering with the ? dimerization. Even though UImH did not show the expected behavior, the ? dimerization is arguably far more interesting, and may have applications in supramolecular chemistry, including the design of smart materials.
Author: Christina J. Hanson
Publisher:
ISBN:
Category :
Languages : en
Pages : 111
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Book Description
Proton-Coupled Electron Transfer (PCET) is an important mechanistic motif in chemistry, which allows for efficient charge transport in many biological systems. We seek to understand how the proton and electron motions are coupled in a bidirectional system allowing for individual turning of the kinetics and thermodynamics. The target of interest is a biomimedic heme system allowing for a detailed mechanistic study of the formation of the oxidation states of heme, of particular interest the highly reactive Fe(IV)=O species. The bidirectional model is prepared using a hangman porphyrin with an axially coordinated to the metal center, and the electron transfer event is triggered by excitation of the porphyrin. The synthesis of this motif is discussed as well as initial studies into the binding of a coordinated electron acceptor to the metal center. In the future, the excited state of the acceptor will be used to trigger the electron transfer portion of the PCET event. To understand the signatures of different electron acceptors and binding to the metal center, a redox inactive zinc porphyrin is used as a model to allow for longer excited state lifetimes and well known transient signatures. Three diimide acceptors have been coordinated through a pyridine ring to the metal center of the porphyrin, and electron transfer was triggered both by excitation of the porphyrin and the acceptor. Lifetimes of the charge separated state were determined using picoseconds and nanosecond transient absorption. The acceptors are then coordinated to a symmetrical iron porphyrin in an attempt to understand the behavior of charge separation in the more complicated open d shell system. Spectroscopic data of both systems is shown.
Author: James A. Roberts
Publisher:
ISBN:
Category : Charge exchange
Languages : en
Pages : 384
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Author: David Sarauli
Publisher:
ISBN:
Category :
Languages : en
Pages : 156
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Author: T. Bountis
Publisher: Springer
ISBN: 9781461365242
Category : Science
Languages : en
Pages : 365
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Book Description
Charge transport through the transfer of protons between molecules has long been recognized as a fundamental process, which plays an important role in many chemical reactions. In particular, proton transfer through Hydrogen (H-) bonds has been identified as the main mechanism, via which many bio logical functions are performed and many properties of such basic substances as proteins and ice can be understood. In this volume, several of these important aspects of the H-bond are rep resented. As the division in different sections already indicates, present day research in proton teansfer in biochemistry, biology, and the physics of water and ice remains highly active and very exciting. Nearly a decade ago, a novel approach to the study of collective proton motion in H-bonded systems was proposed, in which this phenomenon was explained by the propagation of certain coherent structures called solitons. In the years that followed, the approach ofsoliton dynamics was further extended and developed by many researchers around the world, into a legitimate and useful method for the analysis of proton transfer in H-bonded systems. Dr. Stephanos Pnevmatikos, the original Director of this ARW, was one of the pioneers in the application ofsoliton ideas to the study ofcharge transport through H-bonds. Having used similar concepts himself in his research on 2D lattices) he was convinced energy transfer through molecular chains (and that solitons can play an important role in enhancing our understanding of protonic conductivity.
Author: American Chemical Society
Publisher:
ISBN:
Category : Chemistry
Languages : en
Pages : 1042
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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.
