Variable-temperature solid-state 13C CPMAS NMR analysis of meso-tetrapropylporphyrin and of octaethylporphyrin

The high-resolution solid-state 13C NMR spectra of meso-tetrapropyl- and octaethylporphyrin were recorded with use of the cross-polarization (CP), high-power decoupling, magic-angle spinning (MAS) technique. Doublings in the pyrrole carbon signals of these compounds were ascribed to kinetic solid-state effects involved in the central hydrogen migration. It was found that whereas in meso-tetrapropylporphyrin the positions of the pyrrole carbon signals in the low temperature solution 13C NMR spectrum are similar to those in the room temperature solid-state NMR spectrum, this is not the case for octaethylporphyrin. In order to characterize the kinetic behavior of these compounds in the solid phase, 13C CPMAS NMR spectra were recorded at temperatures from slow to fast exchange limits. The changes observed in the spectra were analyzed assuming that the hydrogen migration proceeds between two tautomers of different energies. The shifts that are induced by the aromatic clouds of neighboring molecules were also calculated. With use of these ring current shifts and the low temperature solution chemical shift values, it was possible to obtain kinetic parameters of the solid-state reaction at different temperatures. The implications that the obtained results have for the understanding of the N-H tautomerization process in free base porphyrins are discussed, as well as the relevance that the latter may have for obtaining a mean structure of these compounds. 66 references, 10 figures, 3 tables