Ab initio studies on the electronic structure of FeOH

The electronic structure of FeOH has been studied by using the all-electron multireference averaged coupled-pair functional (ACPF) method combined with the scalar-relativistic Douglas-Kroll-Heβ (DKH) Hamiltonian. A linear geometry (ground state 6Δ, RFe-O = 1.7501 Å, RO-H = 0.9444 Å) is found to be the most stable structure for FeOH. Relativistic effects must be taken into account in order to predict the correct ground state. Furthermore, inner-shell correlation effects are also important for the Fe-O bond of FeOH, i.e., the Fe-O distance is lengthened by at least 0.02 Å when the iron 3s and 3p orbitals are frozen at the ACPF level. The calculated ionization energy IE (7.43 eV) and standard gas phase enthalpies of formation ΔHf0 (21.03 kcal/mol) are in good agreement with Gorokhov's measurement (IE: 7.6 ± 0.3 eV, ΔHf0 : 25.4 ± 4.1 kcal/mol). The obtained Fe-OH dissociation energy D0 (3.61 eV) agrees well with the recent experimental value 3.50 ± 0.12 eV obtained by Schröder and Schwarz but exceeds the error bar of Murad's older measurement (3.33 ± 0.17 eV ) by 0.13 eV.