Author: Timothy M. Alligrant
Publisher:
ISBN:
Category : Electrochemistry
Languages : en
Pages :
Get Book Here
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:
Publisher:
ISBN:
Category : Electronic books
Languages : en
Pages : 150
Get Book Here
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: Sebastião J. Formosinho
Publisher: Royal Society of Chemistry
ISBN: 1849731411
Category : Science
Languages : en
Pages : 169
Get Book Here
Book Description
This book covers the most recent developments in the field of PCET reactions, from the theoretical and experimental points of view.
Author: Kejie Meng
Publisher:
ISBN:
Category : Antioxidants
Languages : en
Pages :
Get Book Here
Book Description
The objective of the research was to investigate proton-coupled electron transfer (PCET) reactions involving antioxidants to gain insight into the detailed mechanisms of glutathione (GSH), Trolox, and [alpha]-tocopherol ([alpha]-TOH). PCET reactions are complex redox reactions that transfer electrons and protons sequentially or in concert. These reactions are ubiquitous in natural or artificial processes that produce electrochemical energy that is extractable as electricity or as chemical fuels of high energy content. Examples of processes based on PCET are photosynthesis, respiration, nitrogen fixation, carbon dioxide reduction, redox fuel cells, and artificial photosynthesis. Antioxidants were selected as a PCET model to understand the coupling between proton transfer (PT) and electron transfer (ET) in order to elucidate structure-reactivity relationships under different experimental conditions. PCET reactions were studied with a set of electrochemical techniques to propose a preliminary mechanism that could be validated with digital simulations matching the electrochemical response. In some cases, other analytical techniques were used to aid in the system characterization. This thesis presents the results and discussion of the effects of oxidant-base pairs on the mediated oxidation of GSH, the -2e−-H process of Trolox in aqueous and nonaqueous solvents with various pH values, and the particle collision electrolysis of [alpha]-tocopherol in oil-in-water emulsion droplets on an ultramicroelectrode. Ultimately our goal was to determine the kinetic and thermodynamic factors that control PCET reactions so that they can be applied in designing artificial systems for the production of energy using more abundant reagents with lower cost and better yields.
Author: Wenbin Zhang
Publisher:
ISBN: 9780494923184
Category :
Languages : en
Pages :
Get Book Here
Book Description
Author: Jean-Michel Savéant
Publisher: John Wiley & Sons
ISBN: 1119292336
Category : Science
Languages : en
Pages : 640
Get Book Here
Book Description
Dieses Fachbuch, geschrieben von zwei weltweit führenden Koryphäen auf dem Gebiet der Elektrochemie, beschreibt detailliert die zentralen elektrochemischen Reaktionen, die als Grundlage für die heutige Erforschung alternativer Energielösungen dienen. - Bietet eine zugängliche und gut lesbare Zusammenfassung zu elektrochemischen Verfahren und der Anwendung elektrochemischer Konzepte bei funktionalen Systemen auf Molekularebene. - Enthält ein neues Kapitel zu dem protonengekoppelten Elektronentransfer, ein vollständig überarbeitetes Kapitel zur molekularen Katalyse bei elektrochemischen Reaktionen sowie durchgängig neue Abschnitte. - Stellt die Verbindung zwischen der Elektrochemie, der Molekular- und Biomolekularchemie her und stärkt deren Zusammenspiel, indem eine Vielzahl von Funktionen präsentiert werden, die sich mit Multi-Komponenten-Systemen und Paradigmen aus beiden Bereichen der Chemie erreichen lassen.
Author: James A. Roberts
Publisher:
ISBN:
Category : Charge exchange
Languages : en
Pages : 384
Get Book Here
Book Description
Author: David Sarauli
Publisher:
ISBN:
Category :
Languages : en
Pages : 156
Get Book Here
Book Description
Author: Marko M. Melander
Publisher: John Wiley & Sons
ISBN: 1119605636
Category : Science
Languages : en
Pages : 372
Get Book Here
Book Description
Atomic-Scale Modelling of Electrochemical Systems A comprehensive overview of atomistic computational electrochemistry, discussing methods, implementation, and state-of-the-art applications in the field The first book to review state-of-the-art computational and theoretical methods for modelling, understanding, and predicting the properties of electrochemical interfaces. This book presents a detailed description of the current methods, their background, limitations, and use for addressing the electrochemical interface and reactions. It also highlights several applications in electrocatalysis and electrochemistry. Atomic-Scale Modelling of Electrochemical Systems discusses different ways of including the electrode potential in the computational setup and fixed potential calculations within the framework of grand canonical density functional theory. It examines classical and quantum mechanical models for the solid-liquid interface and formation of an electrochemical double-layer using molecular dynamics and/or continuum descriptions. A thermodynamic description of the interface and reactions taking place at the interface as a function of the electrode potential is provided, as are novel ways to describe rates of heterogeneous electron transfer, proton-coupled electron transfer, and other electrocatalytic reactions. The book also covers multiscale modelling, where atomic level information is used for predicting experimental observables to enable direct comparison with experiments, to rationalize experimental results, and to predict the following electrochemical performance. Uniquely explains how to understand, predict, and optimize the properties and reactivity of electrochemical interfaces starting from the atomic scale Uses an engaging “tutorial style” presentation, highlighting a solid physicochemical background, computational implementation, and applications for different methods, including merits and limitations Bridges the gap between experimental electrochemistry and computational atomistic modelling Written by a team of experts within the field of computational electrochemistry and the wider computational condensed matter community, this book serves as an introduction to the subject for readers entering the field of atom-level electrochemical modeling, while also serving as an invaluable reference for advanced practitioners already working in the field.
Author: Jonnathan Medina-Ramos
Publisher:
ISBN:
Category : Charge exchange
Languages : en
Pages :
Get Book Here
Book Description
This thesis presents the results and discussion of the investigation of the effects of Brönsted bases on the kinetics and thermodynamics of two proton-coupled electron transfer processes: the mediated oxidation of glutathione and the electrochemical oxidation of hydroquinone. Proton-coupled electron transfer (PCET) is the name given to reactions that involve the transfer of electron(s) accompanied by the exchange of proton(s). PCETs are found in many chemical and biological processes, some of current technological relevance such as the oxygen reduction reaction in fuel cells, which involves the transfer of four electrons and four protons (4e-, 4H+); or the splitting of water into protons (4H+), electrons (4e- ) and oxygen (O2) efficiently achieved in photosynthesis. The study of PCET mechanisms is imperative to understanding biological processes as well as to developing more efficient technological applications. However, there are still many unanswered questions regarding the kinetic and thermodynamic performance of PCETs, and especially about the effect of different proton acceptors on the rate and mechanism of PCET reactions. This study aimed to investigate the effect of Brönsted bases as proton acceptors on the kinetics and thermodynamics of two model PCET processes, the oxidation of glutathione and hydroquinone. The analysis presented in this thesis provides insight into the influence of different proton acceptors on the mechanism of PCET and it does so by studying these reactions from a different angle, that one of the acid-base catalysis theory which has been successfully applied to the investigation of numerous chemical reactions coupled to proton transfer. We hope future research of PCETs can benefit from the knowledge of acid-base catalysis to better understand these reactions at a molecular level.
