Calculation of the Spin-Spin and Spin-Orbit Contribution to the Zero-Field Splitting in Hemin

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Abstract
Earlier theoretical studies of the electronic structure of hemin [iron (III) protoporphyrin (IX) chloride] have been extended to include calculation of both the spin‐spin and spin‐orbit contributions to the observed zero‐field splitting in this molecule. The wavefunctions used for the calculations were obtained by the self‐consistent‐charge extended Hückel method. The iron‐based one‐center spin‐spin result for D is 0.0407 cm‐1. The two‐center spin‐spin contribution to D is 0.0345 cm‐1, and is dominated by chlorine‐3p‐iron‐3d interactions. The total spin‐spin result (0.0752 cm‐1) is much smaller than the observed zero‐field splitting of 6.95 cm‐1, and the major contribution is shown to arise from spin‐orbit effects. The spin‐orbit contribution is large and strongly dominant because of the low‐lying 4 A 2 electronic state in hemin. The accuracy of the spin‐orbit calculation is limited by difficulties in calculating the energy of excitation to the 4 A 2 state, but, with reasonable choices of parameters in terms of which this excitation energy may be expressed, the experimental zero‐field splitting is well accounted for. We conclude that in most hemin‐type molecules, the zero‐field splitting would be strongly dominated by spin‐orbit effects.