High-Resolution Solid-State 13C NMR Spectra of Porphine and 5,10,15,20-Tetraalkylporphyrins: Implications for the N-H Tautomerization Process

Article abstract of DOI:10.1021/ja00210a002

The solid-state high-resolution 13C nuclear magnetic resonance spectra of porphine and of 5,10,15,20-tetraalkylporphyrins were recorded with use of the CP/MAS technique.The solution kinetics of the hydogen migration between the two tautomers of porphine, meso-tetrapropyl- and meso-tetrahexylporphyrins, were also studied, and the activation parameters were found to be similar to those reported in the literature for tetraarylporphyrins.Porphine showed the same dynamical behavior in the solid state as in solution, while in the solid state the tetraalkylporphyrins showed doubling of the pyrrole carbon resonances at room temperature.The results obtained with the tetraalkylporphyrins can be explained assuming either a proton-transfer reaction slow on the NMR time scale or a fast exchange between two unequally populated tautomers.Three hypotheses are discussed with regard to the kinetic solid-state effects involved: (a) a quenching of the tunneling contribution to the proton-migration process; (b) a fixed geometry adopted by the four nitrogen atoms in the crystal that controls the migration; (c) a coupling between the proton-exchange process and the deformation of the porphyrin skeleton, the latter being hindered by neighboring molecules.