The Electrochemical Reduction of Superoxide in Acetonitrile: A Concerted Proton-Coupled Electron Transfer (PCET) Reaction.

Persistent Link:
http://hdl.handle.net/10150/194763
Title:
The Electrochemical Reduction of Superoxide in Acetonitrile: A Concerted Proton-Coupled Electron Transfer (PCET) Reaction.
Author:
Singh, Pradyumna Shaakuntal
Issue Date:
2005
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
Superoxide, the product of the one-electron reduction of dioxygen, is a molecule of enormous importance. It participates in a variety of critical physiological processes and is also an important component of fuel cells where it is an intermediate in the cathodic reaction. However, the electrochemical behavior of superoxide, mainly its reduction, is not well understood. Here, the electrochemical behavior of superoxide has been investigated in acetonitrile on glassy carbon electrodes, through cyclic voltammetry experiments. By stabilizing the electrogenerated superoxide, aprotic solvents afford an opportunity to study its electrochemical reactions further. Superoxide was generated electrochemically from dioxygen at the first voltammetric peak. In the presence of hydrogen-bond donors (water, methanol, 2-propanol), the superoxide forms a complex with the donor resulting in a positive shift in the formal potential which can be analyzed to obtain formation constants for these complexes. Stronger acids (2,2,2- trifluoroethanol, 4-tert-butylphenol) result in protonation of superoxide followed by reduction to produce HO₂-. On scanning to more negative potentials a second peak is observed which is irreversible and extremely drawn out along the potential axis indicating a small value of the transfer coefficient α. Addition of hydrogenbond donors, HA, brings about a positive shift in this peak, without a noticeable change in shape. The reaction occurring at the second peak is a concerted proton-coupled electron transfer (PCET) in which the electron is transferred to superoxide and a proton is transferred from HA to superoxide forming HO₂- and A- in a concerted process. We estimate the standard potential for this reaction for the case of water as the donor. This value suggests that the reaction at the second peak occurs at very high driving forces. Kinetic simulations using both Butler-Volmer and Marcusian schemes were performed to estimate the kinetic parameters. The unusually low rate constants obtained suggest high nonadiabaticity for this PCET reaction. The reaction was also found to proceed with an unusually large reorganization energy. Consistent with a PCET, a kinetic isotope effect, HA vs. DA, was detected for the three hydrogen-bond donors.
Type:
text; Electronic Dissertation
Keywords:
electrochemical reduction of superoxide; superoxide; proton-coupled electron transfer; PCET; concerted proton and electron transfer (CPET); non-adiabatic electron transfer
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Chemistry; Graduate College
Degree Grantor:
University of Arizona
Advisor:
Evans, Dennis H.
Committee Chair:
Evans, Dennis H.

Full metadata record

DC FieldValue Language
dc.language.isoenen_US
dc.titleThe Electrochemical Reduction of Superoxide in Acetonitrile: A Concerted Proton-Coupled Electron Transfer (PCET) Reaction.en_US
dc.creatorSingh, Pradyumna Shaakuntalen_US
dc.contributor.authorSingh, Pradyumna Shaakuntalen_US
dc.date.issued2005en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractSuperoxide, the product of the one-electron reduction of dioxygen, is a molecule of enormous importance. It participates in a variety of critical physiological processes and is also an important component of fuel cells where it is an intermediate in the cathodic reaction. However, the electrochemical behavior of superoxide, mainly its reduction, is not well understood. Here, the electrochemical behavior of superoxide has been investigated in acetonitrile on glassy carbon electrodes, through cyclic voltammetry experiments. By stabilizing the electrogenerated superoxide, aprotic solvents afford an opportunity to study its electrochemical reactions further. Superoxide was generated electrochemically from dioxygen at the first voltammetric peak. In the presence of hydrogen-bond donors (water, methanol, 2-propanol), the superoxide forms a complex with the donor resulting in a positive shift in the formal potential which can be analyzed to obtain formation constants for these complexes. Stronger acids (2,2,2- trifluoroethanol, 4-tert-butylphenol) result in protonation of superoxide followed by reduction to produce HO₂-. On scanning to more negative potentials a second peak is observed which is irreversible and extremely drawn out along the potential axis indicating a small value of the transfer coefficient α. Addition of hydrogenbond donors, HA, brings about a positive shift in this peak, without a noticeable change in shape. The reaction occurring at the second peak is a concerted proton-coupled electron transfer (PCET) in which the electron is transferred to superoxide and a proton is transferred from HA to superoxide forming HO₂- and A- in a concerted process. We estimate the standard potential for this reaction for the case of water as the donor. This value suggests that the reaction at the second peak occurs at very high driving forces. Kinetic simulations using both Butler-Volmer and Marcusian schemes were performed to estimate the kinetic parameters. The unusually low rate constants obtained suggest high nonadiabaticity for this PCET reaction. The reaction was also found to proceed with an unusually large reorganization energy. Consistent with a PCET, a kinetic isotope effect, HA vs. DA, was detected for the three hydrogen-bond donors.en_US
dc.typetexten_US
dc.typeElectronic Dissertationen_US
dc.subjectelectrochemical reduction of superoxideen_US
dc.subjectsuperoxideen_US
dc.subjectproton-coupled electron transferen_US
dc.subjectPCETen_US
dc.subjectconcerted proton and electron transfer (CPET)en_US
dc.subjectnon-adiabatic electron transferen_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineChemistryen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorEvans, Dennis H.en_US
dc.contributor.chairEvans, Dennis H.en_US
dc.contributor.committeememberArmstrong, Neal R.en_US
dc.contributor.committeememberEnemark, John H.en_US
dc.contributor.committeememberGlass, Richard S.en_US
dc.identifier.proquest1377en_US
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