Understanding the reactivity bottleneck in the spin-forbidden reaction FeO+ + H2 → Fe+ + H2O

Abstract

New coupled cluster theory calculations, including extrapolation to the complete basis set limit, are reported for key species on the [Fe,O,H2]+ potential energy surfaces. Test calculations including Bruecker orbital methods suggest that the single-reference coupled-cluster approach is reliable for this system. The minimum energy crossing point (MECP) between the sextet and quartet states has been found to lie close in energy and structure to the quartet reactant complex 4FeO+H2. Non-adiabatic transition state theory is used to calculate the rate constant for hydrogen oxidation, and is found to agree reasonably well with experiment, considering the remaining uncertainties in the ab initio energies and the kinetic modelling. The isotope effects are reproduced fairly well also. The transition state theory calculations suggest that the bottleneck to reaction is an adiabatic quartet transition state for insertion of FeO+ into the HH bond. Spin state change at the MECP is calculated to be considerably faster, even allowing for errors in the relative energies.