The NMR spectra of porphyrins: 22—Ring current effects in chlorins versus porphyrins

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Abstract
The proton NMR spectra of tetraphenylporphyrin, octaethylporphyrin and the analogous chlorins (7,8‐dihydroporphyrins) are presented, and the chemical shift changes on chlorin formation are interpreted using a ring current model. In these compounds a general 10% reduction in the ring current occurs upon chlorin formation. Similar comparison of the chemical shifts of the corresponding dications and also of the protonated form of 2‐vinylphylloerythrin methyl ester with the corresponding chlorin, methyl pyropheophorbide‐a, shows that chlorin formation now has a much larger effect on the ring current, this reflecting the increased steric effects within the macrocycle which occur upon protonation. Variable temperature studies on the porphyrins and chlorins examined show clearly the effect of NH exchange processes and, in particular, novel intermolecular exchange processes with trifluoroacetic acid in the protonated species are recorded.