Nonadiabatic Transition State Theory: Application to Intersystem Crossings in the Active Sites of Metal-Sulfur Proteins

Abstract

Nonadiabatic transition state theory (NA-TST) is a powerful tool to investigate the nonradiative transitions between electronic states with different spin multiplicities. The statistical nature of NA-TST provides an elegant and computationally inexpensive way to calculate the rate constants for intersystem crossings, spin-forbidden reactions, and spin-crossovers in large complex systems. The relations between the microcanonical and canoni- cal versions of NA-TST and the traditional transition state theory are shown, followed by a review of the basic steps in a typical NA-TST rate constant calculation. These steps include evalua- tions of the transition probability and coupling between electronic states with different spin multiplicities, a search for the minimum energy crossing point (MECP), and computing the densities of states and partition functions for the reactant and MECP structures. The shortcomings of the spin-diabatic version of NA-TST related to ill-defined state coupling and state count- ing are highlighted. In three examples, we demonstrate the application of NA-TST to intersystem crossings in the active sites of metal-sulfur proteins focusing on [NiFe]-hydrogenase, rubre- doxin, and Fe2S2-ferredoxin. 2016 Wiley Periodicals, Inc.