Such degenerate self-exchange reactions 9,15 can be followed by stationary spectroscopy techniques. The most popular one is NMR line broadening in solution as a function of temperature, which can however have several origins beyond degenerate self exchange. Indeed, initial attempts to determine the self-exchange rate between TH and T in this way have reportedly been unsuccessful. Instead, pseudo self-exchange reactions between different TEMPO derivatives were studied and yielded empirical activation barriers on the order of 15 kJ/mol, but with large deuterium isotope effects. Therefore, the actual electronic transition states assuming classical nuclear motion should be considerably higher in energy. Indeed, early calculations in the gas phase suggest values on the order of 90 kJ/mol. Hydrogen tunnelling through these high but narrow barriers can explain the lower effective activation energy for the overall process in solution, where the binding partners have to meet by diffusion, bond to each other, transfer the hydrogen, and re-dissociate. The present work focuses on the actual HAT step in the pre-formed TH-T complex without significant thermal excitation or solvent influence and is thus particularly sensitive to the central barrier.