The Development of Spin-Adiabatic Approaches for Studying Spin-Crossing Reactions

WIth support from the Chemical Theory, Models and Computational Methods (CTMC) program in the Division of Chemistry and the Established Program to Stimulate Competitive Research (EPSCoR), Yihan Shao and Zhibo Yang of the University of Oklahoma will develop spin-adiabatic approaches for studying spin-crossing reactions. Spin-crossing reactions are central to a wide range of chemical, biochemical, and photochemical processes such as gas-phase ion-molecule reactions, transition metal complex catalysis, oxygen activation, natural and artificial water splitting, and chemiluminescence/bioluminescence.

However, it remains extremely challenging to model spin-crossing reactions due to several reasons: complex nature of such type of reactions; difficulties of simultaneously exploring multiple spin-diabatic potential energy surfaces; lack of sufficient methodology/software support for efficiently identifying spin-crossing reaction energy pathway (and reaction free energy pathway) and for accurately predicting their reaction rates. To overcome this challenge, the Shao and Yang groups will develop an accurate, efficient, and open-source computational protocol for studying spin-crossing reactions directly on the lowest-energy spin-adiabatic potential energy surface.

Software developed under this program will be released in a free and open-source manner. Under this award, the team also host a qm-mm.org website and hold monthly webinars to provide young researchers an opportunity to learn from the latest applications of computational chemistry tools.

Dr. Shao, Dr. Yang and their research teams at the University of Oklahoma are developing methods to construct and explore spin-adiabatic surfaces, so that one can transition automatically and smoothly from a high-spin state to a lower-spin one (or vice versa) during a transition state search or molecular dynamics simulation.

This research is projected to proceed with the following objectives: (a) implementation of multiple spin-orbit coupling and spin- crossing probability schemes; (b) formulation of analytical energy gradients; (c) incorporation of environment effects through hybrid quantum mechanical molecular mechanical models; (d) combination with pathway optimization methods in free energy calculations; (e) improvement to the energy and free energy results with active-space or coupled-cluster quantum chemistry calculations on selected configurations; and (f) assessment of the contribution from higher-energy spin-diabatic excited states. The team will apply these new computational methods to several spin-crossing ion-molecule reactions studied with mass spectrometry experiments.

They will also investigate oxygenation reactions in chemiluminescence and bioluminescence to gain mechanistic insights.

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.