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

The tautomerization of porphycene, which can be a concerted or a stepwise process, is studied by density-functional theory and instanton techniques. The zero-level tunneling splitting and tautomerization rate constants are calculated, and compared with gas-phase and NMR measurements, respectively. An apparent discrepancy between the two sets of experimental data are revealed and discussed.