Theoretical and Experimental Investigation of Photoheterolysis Reactions

Iowa State University, Ames IA

Investigators

Abstract

With the support of the Chemical Structure Dynamics and Mechanisms-B Program in the Division of Chemistry, Professor Arthur Winter at Iowa State University is working to uncover structure-reactivity principles for photoheterolysis reactions of carbon-leaving group (C-LG) bonds. The long term goal of this project is to generate a predictive theoretical framework that relates chemical structure to reactivity in the excited state for heterolytic bond scission. Heterolytic bond cleavage represents one of the two possible ways in which a chemical bond can break, with both of the two electrons in the bond being given to one of the atoms in the bond. This work is focused on studying how this bond breaking can occur when a molecule is irradiated. Photoheterolysis reactions are important in biological, materials, and environmental chemistry applications. Understanding the mechanistic principles for a simple archetypal bond-breaking photoreaction will contribute to the broader problem of understanding the guiding mechanistic principles that direct photochemical reactions. Professor Winter will also be involved with a Freshman Research Initiative that involves first-year college students in an early multidisciplinary research experience. He will continue to recruit undergraduates and students from underrepresented groups into his lab. Under this award, Professor Winter and his team seek to advance from static computations of excited state barriers and conical intersections and move toward dynamic excited state trajectories. Non-adiabatic trajectories begin from the initial light absorption event, evolve on the excited state surface, and hop to the ground state ion pair photoproduct. These simulations and experiments will answer key questions, including: (i) Are photoheterolysis reactions governed by transition state control or by the presence of productive conical intersections (CI’s) near the ion pair; (ii) Does the Hammond postulate apply for S1 and T1 as it does for thermal (S0) SN1 reactions? (iii) How does the topography of the CI and the CI seam influence the branching ratios of photoheterolysis reactions at funnels? (iv) How important is the minimum energy crossing point, and the surrounding topology, of the CI for understanding the mechanisms of non-adiabatic surface crossings? (iv) How does spin state and solvent alter the mechanism? High level, excited state trajectory simulations in combination with ultrafast laser flash photolysis experiments will jointly provide a powerful approach for answering these fascinating questions of broad photochemical relevance. 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.