Density functional theory simulations of the structure, stability and dynamics of iron sulphide clusters in water

Abstract

Density Functional Theory-based calculations have been employed to investigate the structure, stability and dynamics of iron sulphide clusters, Fe_xS_y (x, y ≤ 4), in water. Car–Parrinello molecular dynamics simulations of the building unit FeS in explicit water show that the iron is only four-coordinated, which indicates that the effect of sulphur is to significantly reduce the coordination shell of iron compared with the typical octahedral arrangement of hexa-aqua iron complexes in water. The molecular dynamics simulations of Fe_xS_y particles (x, y ≥ 2) in explicit water reveal that these clusters are highly unstable as they dissociate after a few picoseconds. The Gibbs free energies to form the FeS and Fe₂S₂ species have been evaluated in a simulated aqueous environment, using the mPW1B95 density functional theory level for the gas-phase component and the UAHF-CPCM solvation model for the hydration contribution, and the results indicate that while FeS is thermodynamically stable in aqueous solution, the formation of a Fe₂S₂ cluster is endergonic, and dissociation is preferred under natural water conditions.