Theoretical Study on Rotation of Pyrrole Rings in Porphyrin and N-Confused Porphyrin

Rotation of pyrrole rings in regular porphyrins and N-confused porphyrins is theoretically investigated by DFT calculations. While the inversion of the pyrrole rings in the regular porphyrins requires high activation energies (36.5-49.1 kcal/mol), the inversion of the confused pyrrole rings in the N-confused porphyrins requires much lower activation energies (18.1-24.5 kcal/mol). This marked difference can be explained by the intramolecular hydrogen bondings and aromatic stabilization due to the [18]annulenic substructures, where confusion and NH tautomerism play an important role. In both of the macrocycles, 360 degrees rotation of their pyrrole rings would be difficult possibly due to the small cavity. Alternatively, a reaction pathway for the production of N-fused porphyrin from N-confused porphyrin is obtained, which is consistent with the experimental observation.