Molecular rotors for studying transition state stabilization by non-covalent interactions

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Professor Ken D. Shimizu of the University of South Carolina is supported by the Macromolecular, Supramolecular, and Nanochemistry Program in the Division of Chemistry to investigate the role of non-bonding interactions in the stabilization of the transition state between reactants and products. Catalysts are frequently used to speed up chemical reactions and make them more selective for the desired product. A key challenge in catalysis development is enhancing the ability of catalysts to bring together molecules into crowded reactive structures know as transitions states. Professor Shimizu and his students are constructing molecular machines (rotors) that assist in the fundamental understanding and optimization of catalyst reactivity. The molecular rotors are designed to assess factors that simplify the formation of these crowded reactive structures via changes in the speed of their rotation. Thus, the molecular rotors provide a simple and systemic way to study and improve catalyst design. The development of new and improved catalysts enables the efficient production of new chemicals, polymers, fuels, and pharmaceuticals. Graduate and undergraduate students are trained in research methods, in particular women chemists through a collaboration with faculty and students at a local women’s college. A graduate course is developed to provide students with fundamental skills, such as how to present seminars, time and research management strategies, and how to write research publications and proposals. The ability of non-covalent interactions to stabilize transition states is measured using a series of molecular rotors. The rotors form non-covalent interactions in their planar transition states. The stabilizing effects of the non-covalent interactions can be measured from the increase in speed of the molecular rotors. The versatility and modularity of the rotor framework enables the study of a wide range of non-covalent interactions including hydrogen bonds, cation-pi, anion-pi, n-pi*, chalcogen-chalcogen, CH-pi, arene-arene, OH-pi, and metal-pi interactions. The transition state effects are easily and accurately assessed by measuring the rotational barriers using dynamic NMR spectroscopy. Finally, the rigid framework limits the degrees of freedom in the ground and transition states enabling accurate modeling and simulation using standard DFT methods, which provide computational-corroboration of the experimentally measured barriers, transition state structures, and insights into the origins of the stabilizing effects. The project broadly impacts the development of new strategies to improve synthetic catalysts and provides insights into the large rate accelerations in enzymatic systems. In addition, the development of a new strategy for studying and interrogating transition state energies using molecular rotors provides chemists with new tools for fundamental kinetic studies.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.