Molecular Interactions of Some Free Base Porphyrins with - and -Acceptor Molecules

The aromatic character of porphyrins can also be seen by NMR spectroscopy. Studies performed in the last decades demonstrated that 1H NMR spectra are very informative and adequately reflect the structural features of porphyrins.[1] The presence of the extended delocalized -electron system of the porphyrin macrocycle gives rise to a strong ring current in the molecules placed in the magnetic field. The ring current causes anisotropic shielding of the protons located in the field of its action and (together with the diamagnetic component of paired -electrons) leads to a substantial shift of their signals in the 1H NMR spectra. It can be stated that the ring current and the aromaticity of porphyrins change in a similar way in response to the analogous changes in the molecular structure of the porphyrin and the medium, which is most clearly seen on comparison of the spectra of porphyrins and their precursors. Due to the anisotropic effect from the porphyrin ring