Variable-temperature solid-state C-13- and N-15-CPMAS NMR analyses of alkyl-substituted porphycenes

The tautomeric exchange of the inner hydrogens of free base alkylporphycene s was examined with the help of C-13-CPMAS and N-15-CPMAS NMR recorded at v ariable temperatures. The porphycenes used were octaethylporphycene 2, 2,7, 12,17-tetrapropylporphycene 3, and 9,10,19,20-tetrapropylporphycene 4. The CPMAS spectra revealed that the interconversion of NH tautomers in 2-4 is s o fast that even in the solid state it was not possible to determine rate c onstants for the migration of the central hydrogens by simulating the exper imental data. However, using N-15-CPMAS spectra recorded at natural abundan ce and variable temperature, it was possible to calculate the equilibrium c onstant (K) of NH tautomerism for each of the alkylporphycenes 2-4. By meas uring variations in the T-1 relaxation times as a function of temperature, it was possible to detect rotations in the alkyl side chains of 2-4.