Reaction Rates and Transition States: A New Perspective on an Old Problem
Ledbetter, Paul Jay
Master of Science
Transition state theory was originally developed in the 1930s as method for calculating chemical reaction rates in simple systems using energetic barrier heights. Unfortunately its usefulness in complex high dimensional chemical processes such as protein dynamics is severely mitigated by the approximate assumption that reacting systems do not recross the transition state. This approximation has been improved for simple systems in modern variational transition state theory (VTST), which gives a close upper bound to the true reaction rate. For complex systems, location of the VTST separatrix is still theoretically challenging. We propose the definition of all alternative transition state separatrix for systems with high friction which has the property that the escape rate through the separatrix gives the exact reaction rate for the system, without the need for recrossing corrections, and, surprisingly, without requiring the escaping trajectories to cross the reaction barrier. However, locating this separatrix presently does require considering trajectories crossing the barrier. In order to lessen the computational challenge associated with barrier crossing, we provide an automatic method to enhance sampling of trajectories crossing the reaction barrier derived from the mathematics of super symmetry. This method provides an effective "ensemble magnifying glass," statistically inducing additional sampling of configurations in regions with large probability flux but without affecting transition rates.
Pure sciences; Chemistry; Theoretical physics