Nonvanishing Electric Field Gradients, Zero-Field Splitting, and Magnetic Field Energies ofS6-State Ferric Ion in Tetragonal Symmetry

A calculation has been made of the spin-orbit coupling of the high-spin ground state of ferric ion, the ${}_{}{}^6{}_{}{}^{}S$ state, with excited doublet and quartet electronic states in strong crystal electric fields of tetragonal symmetry. The aim of the calculation was to simulate as closely as possible, without explicitly including bonding, the condition of ferric ion in the heme group of ferriheme compounds. Eigenfunctions and eigenvalues resulting from such a calculation were used as a basis for calculating other properties of interest of the ferric ion in such systems. Explicitly, the zero-magnetic-field splitting of the sextet ground state, magnetic-field energy components, effective magnetic moments, and the lattice and valence contributions to an electricfield gradient at the iron nucleus were calculated. These properties were investigated as a function of the cubic and tetragonal field strengths, the spin-orbit coupling strength, and the number of excited states included in the interaction. The calculated results were compared with existing experimental data from single-crystal electron spin resonance, magnetic-suceptibility measurements, and the observed quadrupole splitting of the Mössbauer resonance of high-spin ferriheme compounds. For a consistent and relatively narrow range of values for input parameters, all of the calculated properties agreed quite well with the experimental results currently available.