Variable-Temperature Solid-State 13C- and 15N-CPMAS NMR Analyses of Alkyl-Substituted Porphycenes

The tautomeric exchange of the inner hydrogens of free base alkylporphycenes was examined with the help of 13 C-CPMAS and 15 N-CPMAS NMR recorded at variable 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 so fast that even in the solid state it was not possible to determine rate constants for the migration of the central hydrogens by simulating the experimental data. However, using 15 N-CPMAS spectra recorded at natural abundance and variable temperature, it was possible to calculate the equilibrium constant ( K ) of NH tautomerism for each of the alkylporphycenes 2 − 4 . By measuring 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 .