Theory of the temperature dependence of the NMR shift of intermediate spin (S = 1) four-coordinate ferrous porphyrins

The theory of the NMR shift (pseudocontact and contact) for the intermediate spin state (S = 1) of four-coordinate ferrous porphyrins in developed for the general case of axial symmetry. The theory is then fitted to temperature data for the 1H NMR shifts of Fe(OEP) [iron (II)], Fe(OEC) [iron(II)], and Fe(TPP). It is also fitted to temperature data for the magnetic susceptibility and magnetic anisotropy of solid Fe(TPP) and Fe(PC) [(phthalocyaninato)iron(II)]. The theory assumes the ground state to be a 3A2g state that is spin-orbit coupled with the 3Eg state at energy Δ. In low symmetry the 3Eg state is split by the energy δ. In solution Δ = 600 cm -1 for Fe(OEC) and Fe(OEC) and ∼ 1000 cm -1 for Fe(TPP). Δ is reduced to 400 cm -1 in solid Fe(TPP) and is -900 cm-1 in solid Fe(PC) (3Eg ground state). In solution δ = -700 cm-1 in Fe(OEC). In Fe(OEP) and Fe(OEC), enough experimental data are available to allow the determination of the geometrical factors for the pseudocontact shift of the methyl resonances from the NMR shifts. The theory is successful in explaining the temperature dependence of the pseudocontact shifts and magnetic susceptibilities but does not fully account for the temperature behavior of the contact shifts