Two-State Reactivity: Personal Recounting of its Conception and Future Prospects

This essay tells the story of the conception of the Two-State Reactivity (TSR) notion. Since scientific career is part of life's flow, the story blends sub-stories of scientific colleagues and events. This is also a story of a beguiling paradigm, which has started from a puzzling reactivity of the diatomic oxidant FeO⁺, has continued to larger oxidants, like the active species of Cytochrome P450 and of nonheme enzymes, and its extension to reductive processes. Finally, the essay discusses prospects of experimental probing of the reactive spin-state, and of the transition state constitution for these reactions by means of tunneling-augmented kinetic isotope effects.