COROTATION 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-di fferent 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) confi gurations. The expressions for the g tensor, g values, and magnetic ax es were derived on the basis of first-order perturbation theory. The c onditions for co- and counterrotation of magnetic axes with rotation o f planar axial ligands away from the porphyrin nitrogens toward the me so positions and beyond, as well as the order of g values, have been a nalyzed. 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 fa r more common for the (d(xy))(2)(d(xz),d(yz))(3) electron configuratio n. The possibilities of nonlinear co-/counterrotation are also explore d. The predictions of this treatment are then compared to experimental results obtained from single-crystal EPR, glassy sample ESEEM, and so lution NMR spectroscopic studies. It is clear that the majority of exp erimental systems reported thus far follow the major predictions of th is treatment: Most systems exhibit angle-for-angle (linear) counterrot ation of the g or chi tensor with rotation of planar axial ligands awa y 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 lig and 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 solu tion ESEEM experiments.