The proton magnetic resonance spectra of porphyrins. Part IV. Coproporphyrin tetramethyl esters

Abstract

Comparison of the n.m.r. spectra of the coproporphyrin I, II, III, and IV tetramethyl esters in trifluoroacetic acid and chloroform solution allows an unambiguous distinction between these isomers. The spectra of the chloroform solutions show a pronounced concentration dependence, all the peaks moving to low field on dilution; the spectra of the type III and IV isomers show additional fine structure in concentrated solution. These results can be interpreted quantitatively in terms of an equilibrium between mono- and di-meric forms. In the dimer the porphyrin rings are parallel to each other and spaced ca. 8 Å apart, owing to steric repulsions of the substituent propionate ester groups. With the centrosymmetrical I and II isomers the rings lie directly above one another, and no fine structure is observed, whereas with the less symmetrical III and IV isomers the steric repulsions due to the side chains cause small lateral displacements of the rings relative to one another, and hence give rise to the fine structure observed in their spectra. The size and direction of this lateral displacement may be calculated from the spectra, and is in good agreement with the model of the complex.