Vernon D. Parker
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 directed the emphasis of our research work toward proton, hydride and hydrogen atom transfer reactions involving the C-H bond. We have observed that most of these reactions involve the formation of kinetically significant acceptor/donor complexes. We have found that the kinetics of the reactions can be resolved into microscopic rate constants by making measurements in the pre-steady state period. One of the major consequences of our work is that kinetic isotope effects previously reported for these reaction are apparent values which are often very much smaller than the real kinetic isotopes for the steps in which the C-H bond is broken. 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.