Isotropic nuclear magnetic resonance shifts in low-spin iron(III) porphyrin and hemin systems

Calculations, based on the t 2 5 (single vacancy) configuration including spin-orbit coupling, have been carried out to evaluate the magnetic susceptibility anisotropies in low-spin iron(III) heme proteins. The results have been used to assess the importance of second order Zeeman effects and thermal population of excited states when evaluating dipolar nuclear magnetic resonance shifts in such systems. The temperature dependencies predicted by the theory are discussed in light of experimental data in the literature.