Structure and relative spin-state energetics of [Fe(H2O)6]3+: A comparison of UHF, Møller-Plesset, nonlocal DFT, and semiempircal 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 starting structure using unrestricted Hartree-Fock (UHF), Møller-Plesset (MP2), and nonlocal density functional theoretical (DFT) methods. All three methods indicate a high?spin ground state for the iron?water complex, consistent with experimental results. The optimized ground-state 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 cluster 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 of 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(H2) bond distances from the UHF, MP2, and DFT methods, with additional distortions present in the DFT excited-state structures. The most significant difference in these results was in the calculated vertical transition energies between the sextet ground state and the low lying quartet state: specifically UHF (30 253 cm-1), MP2 (20 040 cm-1), DFT (9895 cm-1 (BPW91); 9302 cm-1 (BLYP)), and semiempirical INDO/S (10 875 cm-1) with the DFT and INDO/S result in good agreement with the experimental determined vertical transition energy of 12?300 cm-1. Comparison of the lowest computed DFT sextet-quartet transition energies for the iron?water cluster (9895 cm-1) and the Fe3+ ion (30?890 cm-1) indicates the coordination of Fe3+ by water ligands to be a significant perturbation on the excited-state energies relative to the ground state.