Co- and Counterrotation of Magnetic Axes and Axial Ligands in Low-Spin Ferriheme Systems

The orientation of the principal axes of the g tensor with respect to the relationship of axial ligand planes to the porphyrin nitrogens has been studied in the framework of the one-electron crystal field model for tetragonal and rhombic low-spin d 5 complexes such as ferriheme centers. All five d atomic orbitals were taken into account for two different ground-state electronic configurations, the “normal” (d xy ) 2 (d xz,d yz ) 3 and the “novel” (d xz,d yz ) 4 (d xy ) 1 configurations. The expressions for the g tensor, g values, and magnetic axes were derived on the basis of first-order perturbation theory. The conditions for co- and counterrotation of magnetic axes with rotation of planar axial ligands away from the porphyrin nitrogens toward the meso positions and beyond, as well as the order of g values, have been analyzed. It is found that counterrotation is the only possibility for the (d xz,d yz ) 4 (d xy ) 1 configuration and that it is also by far more common for the (d xy ) 2 (d xz,d yz ) 3 electron configuration. The possibilities of nonlinear co-/counterrotation are also explored. The predictions of this treatment are then compared to experimental results obtained from single-crystal EPR, glassy sample ESEEM, and solution NMR spectroscopic studies. It is clear that the majority of experimental systems reported thus far follow the major predictions of this treatment: Most systems exhibit angle-for-angle (linear) counterrotation of the g or χ tensor with rotation of planar axial ligands away for the N−Fe−N axes. Hence, knowing the structure of a model heme or heme protein, and in particular, the orientation of (fixed) axial ligand planes, one should be able to predict the approximate orientation of the in-plane magnetic axes. This knowledge provides a check on the values obtained in new solution NMR, single-crystal EPR or frozen solution ESEEM experiments.