¹⁵N‐CPMAS‐NMR Study of the Problem of NH Tautomerism in Crystalline Porphine and Porphycene

Solid state, variable temperature 15 N‐NMR studies make it possible to predict the behavior of the four N atoms and the two internal H atoms in the isomeric porphine and porphycene. The spectra of porphine indicate a statistical disorder of the internal H atoms. In the case of porphycene it can be concluded that there are two non‐equivalent unsymmetric proton transfer systems. Since the NH…N distances are very short, the energy barrier for the rearrangement is very small. Therefore, other than in the case of porphine, in porphycene the interconversion of the tautomers is so rapid that the rate constants for the proton transfer could not be determined by the 15 N‐CPMAS‐NMR method.