STRUCTURE AND RELATIVE SPIN-STATE ENERGETICS OF [FE(H2O)(6)](3- A COMPARISON OF UHF, MOLLER-PLESSET, NONLOCAL DFT, AND SEMIEMPIRICAL INDO())S CALCULATIONS/

The optimized structure and spin-state energetics of the iron(ferric)- hexaaquo complex, [Fe(H2O)(6)](3+), in high (S = 5/2), intermediate (S = 3/2), and low (S = 1/2) spin states were determined from a common s tarting structure using unrestricted Hartree-Fock (UHF), Moller-Plesse t (MP2), and nonlocal density functional theoretical (DFT) methods. Al l three methods indicate a high-spin ground state for the iron-water c omplex, consistent with experimental results. The optimized ground-sta te geometries were similar to each other and in quantitative agreement with known model-system crystal structures and experimental solution phase scattering data. The energy ordering of the spin states of the c luster was also found to be the same at all levels of calculation: E(5 /2) < E(3/2) < E(1/2), in agreement with experiment. This same order o f spin-state energies was found using the semiempirical INDO/S method. Optimized geometries for both the quartet and doublet excited states led to similar Fe-O(H-2) bond distances from the UHF, MP2, and DFT met hods, with additional distortions present in the DFT excited-state str uctures. The most significant difference in these results was in the c alculated vertical transition energies between the sextet ground state and the low lying quartet state: specifically UHF (30 253 cm(-1)), MP 2 (20 040 cm(-1)), DFT (9895 cm(-1) (BPW91); 9302 cm(-1) (BLYP)), and semiempirical INDO/S (10 875 cm(-1)) with the DPT and INDO/S result in good agreement with the experimental determined vertical transition e nergy of 12 300 cm(-1). Comparison of the lowest computed DFT sextet-q uartet transition energies for the iron-water cluster (9895 cm(-1)) an d the Fe3+ ion (30 890 cm(-1)) indicates the coordination of Fe3+ by w ater ligands to be a significant perturbation on the excited-state ene rgies relative to the ground state.