Density functional calculations of g-tensors of low-spin iron(I) and iron(III) porphyrins

The g-values of some selected low-spin Fe(I) and Fe(III) porphyrin complexe s have been calculated, using a method based on density functional theory a nd the zeroth order regular approximation to the Dirac equation. In agreeme nt with experimental observations the calculated g-values of these complexe s differ strongly from the free electron g(e)-value. Optimization of the ge ometries gives iron-ligand distances in good agreement with the experimenta lly derived values. The effects of strong ruffling of the porphyrin core an d of the relative orientation of the planes of axial ligands on the g-value s are evaluated quantitatively. Mechanisms reported for these effects are c onfirmed.