The tautomerization dynamics of porphycene and its isotopomers – Concerted versus stepwise mechanisms

To determine whether the tautomerization of porphycene is a concerted or a stepwise process, we characterize its stationary points on the potential energy surface employing density functional theory as well as Møller–Plesset perturbation theory, and calculate proton-transfer rates and the zero-level tunneling splitting using the approximate instanton method. The results are compared with experimental rate constants and activation energies measured in solid state by NMR techniques, and with the zero-level tunneling splitting measured in gas phase. An apparent discrepancy between the two sets of experimental data emerges from this comparison, which is reassessed in terms of general relations that are independent of the quality of the calculated potentials. We show how kinetic isotope effects can be used to distinguish the two mechanisms experimentally.