EAGER: Quantum Transition State Theory for Electronically Nonadiabatic Processes

Cornell University, Ithaca NY

Investigators

Abstract

With this EAGER award, the Chemical Theory, Models and Computational Methods Program in the Chemistry Division is supporting Professor Nandini Ananth at Cornell University to develop models that describe processes where quantum mechanics is important for understanding the dynamics of complex chemical and physical systems. Characterizing chemical processes by understanding interactions at a sub-atomic level and by quantifying reaction rates is central to our understanding of chemistry, physics and biology. The introduction of classical transition state theory (TST) revolutionized our understanding of chemical reactions, because it allowed the efficient calculation of reaction rates for large systems in which it is not possible to do exact, detailed quantum mechanical calculations. Despite its many successes this theory cannot correctly describe processes where quantum mechanical behavior plays an important role. Recently, a quantum TST (QTST) was derived that incorporates quantum effects into formalism that resemble classical TST. In this work, Ananth and her coworkers are working to extend the QTST to a fundamentally important class of reactions for which it is currently inapplicable - electron and energy transfer processes. Developing such a theory could provide unprecedented insights into the mechanism of complex photo-induced chemical reactions. There are two significant technical challenges that must be met in extending QTST to electron and energy transfer processes. First, the discrete electronic states of the quantum system must be replaced with continuous, classical variables that are necessary for the TST formulation. Second, nonadiabatic effects arising from coupling between electronic state transitions and nuclear motions must be accurately described. Ananth employs a mapping protocol based on Schwinger's angular momentum theory to map discrete states to Cartesian phase-space variables and uses the path-integral representation of quantum mechanics to incorporate nonadiabatic effects into an exact, classically isomorphic expression for a real-time, flux-side correlation function. The desired TST expression will be obtained as the short-time limit of the flux-side correlation function. This nonadiabatic QTST will be general, and can be used to provide a good estimate of reaction rates and insights into reaction pathways for any nonadiabatic, quantum mechanical chemical process. Further, the research team aims to establish a rigorous derivation of an approximate, path-integral based dynamics to provide quantitate reaction rates and detailed mechanistic information from real-time simulations of complex chemical systems.