Carbon-13 nuclear magnetic resonance in isotopically enriched high-spin iron(III) porphyrins: π-electron spin distribution

Abstract

The ¹³C n.m.r. spectrum of high-spin chloro(5,10,15,20-tetraphenylporphyrinato)iron(III) has been unambiguously assigned using porphyrins isotopically enriched at selected positions. The unexpected linewidth of the resonances of the porphyrin core carbons is explained by the dipolar interaction of the carbon nuclei with the ligand unpaired π-electron spin densities. This interaction is the dominant mechanism of relaxation for the meso carbons and the pyrrole β carbons. The relative contribution of this mechanism increases for nuclei farther from the iron ion. As a result, it is also the dominant mechanism for the relaxation of the porphyrin protons. The π spin distribution on the porphyrin ring has been derived from the dipolar contribution to the linewidth for the carbon nuclei. Furthermore, the experimental β proton resonance linewidth can be interpreted only by considering a normalized iron spin density lower than unity. A value of 3.5/5 is proposed in good agreement with earlier molecular-orbital calculations. This analysis provides a set of self-consistent data for the quantitative description of the various contributions to the observed shifts, including the ligand-centred pseudo-contact contribution, and to the linewidth for both the proton and the carbon nuclei.