Theoretical estimates of equilibrium Fe-isotope fractionations from vibrational spectroscopy

The magnitude and direction of equilibrium iron-isotope (Fe-54-Fe-56) fractionations among simple iron-bearing complexes and alpha-Fe metal are calculated using a combination of force-field modeling and existing infrared, Raman, and inelastic neutron scattering measurements of vibrational frequencies. Fractionations of up to several per mil are predicted between complexes in which iron is bonded to different ligands. Similar fractionations are predicted between the different oxidation states of iron. The heavy iron isotopes will be concentrated in complexes with high-frequency metal-ligand stretching vibrations, which means that Fe-56/Fe-54 will be higher in complexes with strongly bonding ligands such as CN and H2O relative to complexes with weakly bonding ligands like Cl(-) and Br(-). (Author)