Collaborative Research: Understanding Spin-Forbidden Reactions Mediated by Relativistic Nuclear-Electronic Hyperfine Coupling

Laura Motta of the Woods Hole Oceanographic Institution and Jochen Autschbach of the University at Buffalo are supported by an award from the Chemical Theory, Models, and Computational Methods program in the Division of Chemistry to develop and implement a theoretical method to explore new types of spin-forbidden chemical reactions. The team will implement quantum-theoretical calculations of chemical reactions that would not occur without a magnetic interaction between the spins of the electrons and the spin of the nucleus of a heavy element in these molecules.

This magnetic interaction is called hyperfine coupling (HFC). The resulting theoretical method developments will be used to investigate the magnetic isotope effect (MIE) in reactions of small molecules containing mercury. Phenomena such as the “magnetic compass” in migratory birds may also be facilitated by such processes, and therefore the research has the potential to solve long-standing scientific mysteries.

A greater understanding of spin-forbidden reactions and whether they can be facilitated by HFC will have broad-ranging impacts across various scientific disciplines, from chemistry to environmental sciences. The proposed research will be integrated exclusively into open-source quantum chemistry software. The methodologies will also be integrated into STEM education through participation in the REU program, outreach lectures, and involvement of a graduate student and a postdoctoral scholar in the research.

The team will develop electronic-structure and non-adiabatic molecular dynamics (NAMD) methodology to simulate spin-forbidden reactions driven by HFC. Existing quantum dynamics methods lack a proper spin-independent (scalar effects, SR) and spin-dependent (spin-orbit coupling, SOC) relativistic treatment, preventing the simultaneous evaluation of SOC and HFC interactions, thus limiting the assessment of intersystem crossing in radical pair intermediates.

The limitations of quantum dynamics in explaining spin-forbidden reactions are highlighted by the observable photochemical mercury MIE. The objectives are: (1) Expand the current implementation of Exact 2-component (X2C) relativistic HFC in OpenMolcas at the multireference level to include multiple excited states with different multiplicities; (2) Develop and implement the calculation of the matrix elements of the total HFC; (3) Incorporate HFC in the simulation of nonadiabatic molecular dynamics over long time scales using the SHARC program as interfaced with OpenMolcas.

The outcomes of this work will directly impact our understanding of photochemically derived isotope effects observed in natural samples and thus contribute to a comprehensive understanding of important environmental processes. This project will foster the scientific development of summer students, graduate students, and postdoctoral researchers in the field of computational photochemistry.

These researchers will receive extensive training in the application of advanced computational methods.

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.