Crystal field calculation of g values and zero-field splitting for high spin ferric complexes of rhombic character

For high spin ferric ions in rhombic symmetry, we have used a crystal field model to relate term splittings of the ⁴ T ₁, ² T ₂ and ⁴ T ₂ excited states to zerofield split energies and g values of the ⁶ A ₁ term. In this model five crystal field parameters were used, namely, one cubic parameter, two tetragonal parameters and second and fourth order rhombic parameters. In tetragonal symmetry with only three crystal field parameters, a simpler model including only the ⁴ T ₁ and ² T ₂ excited states is adequate to relate term energies to g values and zero-field split energies. However, we have demonstrated the importance of the ⁴ T ₂ state in rhombic crystalline fields. No higher lying terms other than ⁴ T ₂ can influence the ⁴ T ₁ term directly through the tetragonal or rhombic crystal field, Furthermore, we show that the fourth order rhombic crystal field parameter is a key parameter because the rhombic splitting of the dominant low lying ⁴ T ₁ term of high spin ferric complexes depends to first order on the fourth order crystal field potential. We have performed a computer diagonalization of the spin-orbit, electrostatic and crystal field perturbation matrix, and calculated g values and zero-field splittings in seventeen high spin ferric mixed crystalline species of varying rhombicities and for metmyoglobin and cytochrome P-450. The high spin and spin-mixed regions are developed completely to yield the crystal field term energies, zero-field splittings and basis functions together with g values.