Multireference Ab Initio Study of Ligand Field d–d Transitions in Octahedral Transition-Metal Oxide Clusters

Abstract

We have used multiconfigurational (MC) and multireference (MR) methods (CASSCF, CASPT2, and MRCI) to study d–d transitions and other optical excitations for octahedral [M(H2O)6]n+ clusters (M = Ti, V, Mn, Cr, Fe, Co, Ni, Cu) as models of hematite and other transition-metal oxides of interest in solar fuels. For [Fe(H2O)6]3+, all calculations substantially overestimate the d–d transition energies (∼3.0 versus ∼1.5 eV) compared to what has been experimentally assigned. This problem occurs even though theory accurately describes (1) the lowest d–d transition energy in the atomic ion Fe3+ (∼4.4 eV), (2) the t2g–eg splitting (∼1.4 eV) in [Fe(H2O)6]3+, and (3) the ligand-to-metal charge transfer (LMCT) energy in [Fe(H2O)6]3+. Indeed, the results for Fe3+ and the t2g–eg splitting suggest that the lowest d–d excitation energy in the hexa-aqua complex should be ∼3 eV (or slightly below because of Jahn–Teller stabilization), as we find. Possible origins for the d–d discrepancy are examined, including Fe2+ and low-s...