Nonadiabatic transition state theory: Application to intersystem crossings in the active sites of metal‐sulfur proteins

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 canonical 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 evaluations 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 counting 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, rubredoxin, and Fe 2 S 2 ‐ferredoxin. © 2016 Wiley Periodicals, Inc.