Effect of Aromaticity and Anti-Aromaticity on Transition States and Intrinsic Barriers of Reactions

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With the support of the Organic and Macromolecular Chemistry Program, Professor Claude F. Bernasconi, of the Department of Chemistry and Biochemistry at the University of California, Santa Cruz, will continue experimental and computational studies of the effect of aromaticity and anti-aromaticity on the intrinsic barriers to a number of fundamental chemical reactions. Investigations of intrinsic barriers, that is the barriers in the absence of a thermodynamic driving force, allow critical evaluation of the factors affecting transition state energies and contribute to our basic understanding of the rates of chemical reactions. Both experimental and computational approaches will be used in these investigations. The experimental approach will involve kinetic and equilibrium measurements of proton transfers, nucleophilic additions to highly electrophilic systems, and hydride ion transfers. Reactions will be chosen which allow modulation of the degree of aromaticity or anti-aromaticity in products or reactants and intrinsic barriers will be determined experimentally. The computational approach will focus on gas phase proton, hydride ion, hydrogen atom, halonium ion, and halide ion transfers involving aromatic and anti-aromatic species. Intrinsic barriers will be calculated and compared to the respective barriers of corresponding model reactions that do not involve aromatic or anti-aromatic species. The broader impacts of this research will be two-fold. At the scientific level, the project addresses fundamental questions about chemical reactions and our understanding of how structure affects chemical reactivity, a central theme of physical organic chemistry. At the level of human resource development, the proposed work provides students with a broad educational experience, ranging from synthesis and spectroscopic characterization to kinetics, data analysis, and computational chemistry.