The Primary Steps in Excited‐State Hydrogen Transfer: The Phototautomerization of o‐Nitrobenzyl Derivatives

The quantum yield for the release of leaving groups from o‐nitrobenzyl “caged” compounds varies greatly with the nature of these leaving groups, for reasons that have never been well understood. We found that the barriers for the primary hydrogen‐atom transfer step and the efficient nonradiative processes on the excited singlet and triplet surfaces determine the quantum yields. The excited‐state barriers decrease when the exothermicity of the photoreaction increases, in accord with Bell–Evans–Polanyi principle, a tool that has never been applied to a nonadiabatic photoreaction. We further introduce a simple ground‐state predictor, the radical‐stabilization energy, which correlates with the computed excited‐state barriers and reaction energies, and that might be used to design new and more efficient photochemical processes.