The NMR spectra of porphyrins-121 1 Part 11, R.J. Abraham, G.H. Barnett, G.E. Hawkes and K.M. Smith, Tetrahedron 32, 2949 (1976). 13C and proton NMR spectra of the zinc(ii)coproporphyrin isomers2

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The 13C and proton NMR spectra of the zinc(II) complexes of the tetramethyl esters of the four coproporphyrin type isomers are reported and assigned. Effects of aggregation phenomena on these shifts are discussed and a method involving addition of a slight excess of pyrrolidine is proposed for measurement of the spectra of the "monomeric" species; spectra obtained under these conditions are capable of simple, straight-forward interpretation and assignment in terms of molecular symmetry. Thus, a facile distinction between the type isomers is obtained. The "monomer" chemical shifts so derived allow consistent SCS parameters to be derived. The Cβ-Me SCS are shown to be related to the bond order of the Cβ-Cβ bond in the porphyrin ring, and are thus quite different from the corresponding SCS in pyrroles. Aggregation shifts in the 13C and proton spectra are shown to be consistent with the presence of "stacked" aggregates with the ring current of one molecule affecting the other, together with an additional effect on the chemical shifts of the meso carbons, which is probably steric in origin. © 1977.

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