Shaping Chemical Reactivity via Strong Coupling of Vibrations and Electron Spins to Low-Frequency Optical Cavities: A Quantum Polariton Chemistry Approach

• Applicant: Dr. Eric W. Fischer
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 536826332

This project is concerned with the theoretical investigation of strong light-matter coupling in low-frequency optical cavities aiming at insight into two complementary scenarios: First, a mechanistic understanding of selected experimentally reported but microscopically not yet understood thermal vibro-polaritonic reactions. Second, an explorative study of explicit interactions between electron spins of open-shell molecules and magnetic components of quantized cavity fields potentially relevant for strong coupling altered transition metal or radical chemistry. Conceptually, this project is based on the cavity Born-Oppenheimer (CBO) framework, which treats both nuclei and low-frequency cavity modes as "slow" degrees of freedom in contrast to "fast" electrons. Microscopically, electrons are then described by an extended electronic structure problem, which accounts for interactions between electrons with both nuclei and cavity modes. In this project, computational approaches for the numerical solution of an extended electronic Schrödinger equation will be developed by combining efficient quantum chemical wave function methods with an implementation of the extended electronic CBO Hamiltonian. This ab initio "quantum polariton chemistry" approach will then be exploited to address the mechanism of two recent experimentally reported thermal vibro-polaritonic reactions: A vibrational strong coupling (VSC) induced modification of Woodward-Hoffmann stereoselectivity and VSC-decelerated urethane formation. For both reactions, the influence of strong light-matter coupling on thermochemistry, chemical kinetics and frontier orbital character will be investigated with a focus on experimentally formulated mechanistic hypotheses. In a subsequent step, the collective character of vibro-polaritonic chemistry will be addressed from the perspective of density matrix embedding theory (DMET). DMET is a quantum embedding theory, which allows for exploring the experimentally reported concepts of collective and cooperative VSC, here with a focus on cavity-altered urethane formation. Finally, molecular open-shell systems interacting with low-frequency cavity modes will be investigated. There the extended electronic structure problem is augmented by an explicitly spin-dependent fully quantized Zeeman-type contribution. Spin-adapted multi-reference methods will be employed in a two-step scheme to study well characterized molecular open-shell systems under strong coupling with application in radical pair and transition metal chemistry. Specifically, strong coupling will be explored as a tool for shaping molecular magnetic properties and electronic properties of molecular spin states. The proposed project aims for both augmenting experimental results with a microscopic interpretation and exploration of new strong coupling scenarios in low-frequency optical cavities, here with a focus on electronic spin degrees of freedom.

• DFG Programme: WBP Position