The kinetic isotope effect as a probe of spin crossover in the C-H activation of methane by the FeO(+) cation.

Two-state reactivity (TSR) is often used to explain the reaction of transition-metal-oxo reagents in the bare form or in the complex form. The evidence of the TSR model typically comes from quantum-mechanical calculations for energy profiles with a spin crossover in the rate-limiting step. To prove the TSR concept, kinetic profiles for CH activation by the FeO(+) cation were explored. A direct dynamics approach was used to generate potential energy surfaces of the sextet and quartet H-transfers and rate constants and kinetic isotope effects (KIEs) were calculated using variational transition-state theory including multidimensional tunneling. The minimum energy crossing point with very large spin-orbit coupling matrix element was very close to the intrinsic reaction paths of both sextet and quartet H-transfers. Excellent agreement with experiments were obtained when the sextet reactant and quartet transition state were used with a spin crossover, which strongly support the TSR model.