Research Interests
Our research is primarily concerned
with electron transfer reactions and the reactions of reactive intermediates
formed during charge transfer. Our experiments provide thermodynamic as
well as kinetic and mechanistic information on these reactions. We depend
heavily on physical measurements, including electrochemistry, calorimetry
and stop-flow kinetics, to obtain our data. Brief descriptions of representative
projects are given in the following paragraphs.
The addition of an electron to or the removal of an electron from a neutral molecule results in the formation of a radical ion. Radical ions have been implicated as reactive intermediates in a number of chemical and biochemical reactions. These intermediates can undergo reactions typical of both ions and of radicals. A common property of radical ions is that both homolytic and heterolytic bond dissociation energies are dramatically lowered upon going from a neutral molecule to the radical ion. For example, the C-H bond dissociation energy of the methyl group of toluene changes from about 88 kcal/mol in the neutral to about 40 kcal/mol in the radical cation. Likewise the acidity of these C-H groups changes dramatically from about 42 in toluene to about -15 in the corresponding radical cation. These large bond energy changes as well as greatly increased electrophilic and nucleophilic reactivity gives rise to a wealth of new chemistry which is readily studied using electrochemical methods.
Doubly charged organic ions available from electron transfer reactions of radical ions are essentially unstudied intermediates. The reactivity patterns mentioned for the radical ions are expected to be greatly accentuated upon addition or removal of the second electron. We have shown that the anthracene dianion is about 30 pK units more basic than the corresponding radical anion. Dianions are very much stronger nucleophiles than are anion radicals. Establishing reactivity patterns of doubly charged ions is one of the current goals of our work.
My group has recently shifted the emphasis of our research work to include electron transfer reactions of proteins. A host of organic molecules and ions are commonly used as electron transfer mediators of redox protein electron transfer. We have developed a titration microcalorimetry method to study the thermodynamics of protein-protein and protein-mediator electron transfer. Together with entropies of charge transfer, obtained by measurements of the temperature dependence of reversible electrode potentials, these studies will define the thermochemistry of a large number of protein redox reactions. These studies will be supported by stop-flow kinetics of the electron transfer reactions.
Our experimental work is on the borderline between biochemistry,
organic, physical and analytical chemistry. Graduate students working in
the group may elect to take their degree in any of these disciplines.
Selected Publications:
The Reaction Pathways of the Cation Radicals of Aromatic Compounds Related to the Anthracenes. Parker, V. D. Acc. Chem. Res. 1984, 17, 243.
The Generation and Reactions of Carbene Ion Radicals in Solution. Bethell, D.; Parker, V. D. Acc. Chem. Res. 1988, 21, 400.
A General Approach to the Thermochemistry of Reactions in Solution. Wayner, D. D. M.; Parker, V. D., Acc. Chem. Res. 1993, 26, 287.
Reactivity of Aromatic Radical Cations. Rate Constants for Reactions of 9-Phenyl and 9,10-Diphenylanthracene Radical Cations with Nitrogen Centered Nucleophiles. Workentin, M.; Johnston, L. J.; Wayner, D. D. M.; Parker, V. D. J. Amer. Chem. Soc.1994, 116, 8279.
Aryl Proton Transfer Reactions of 9-Aryl and 9-Substituted Anthracene Radical Cations with 2,6-Di-tert-butylpyridine. Xue, J.-Y.; Parker, V. D. J. Org. Chem. 1994, 59, 6564.
The Pre-Peak Method to Determine Rate Constants of Rapid
Reactions following Charge Transfer at Electrodes. Parker, V. D.; Sheng,
G.; Wang, H. Acta Chem. Scand. 1995, 49, 351.
Tel. (435) 797-1620; FAX (435) 797-3390
E-mail: vparker@cc.usu.edu