Proton NMR relaxation study on low-spin pyridine complexes of ferriporphyrins

Abstract Proton NMR shifts, spin-lattice, and spin-spin relaxation times have been measured on low-spin pyridine complexes of Fe(III)-protoporphyrin(IX)dimethylester and Fe(III)-tetraphenylporphyrine in chloroform between 203 K ⩽ T ⩽ 253 K. Deviations between measured and calculated chemical shifts have been observed for the protons (e.g., methyl group) in Fe(III)-protoporphyrin but not for the pyrrole protons in Fe(III)-tetraphenylporphyrin. These deviations have been analyzed theoretically and it was found that higher-order terms to the pseudocontact shift are too small to explain the deviations observed. Instead, a considerable temperature dependence of the hyperfine coupling constant A for the protons of Fe(III)-protoporphyrin had to be assumed in order to explain the deviations observed, whereas A was found to be independent against temperature variations for Fe-(III)-tetraphenyl-porphyrin. This result was confirmed by independent relaxation time measurements and the values for the hyperfine coupling constants extracted from the two methods were found to be in excellent agreement.