Prediction and interpretation of the 57Fe isomer shift in Mössbauer spectra by density functional theory

The calibration of two popular density functionals (B3LYP and BP86) for the prediction of isomer shifts (IS) in 57Fe Mössbauer (MB) spectra is reported. A linear correlation between non-relativistically calculated theoretical electron densities and experimentally measured ISs is established. Both functionals lead to accurate predictions of MB ISs with standard deviations on the order of approximately 0.1 mm s−1 over a range of approximately 2 mm s−1. The correlation holds for complexes of different total charge, total spin, valence state at the iron, coordination number and coordination geometry. A detailed analysis of the contributions to the IS is presented. It is shown that the important contributions are: (a) the variation of the 3s-contribution which is mainly caused by differing shielding according to the d-population of the metal; and (b) variations in the valence shell contribution. The latter contribution accounts for approximately 70% of the totally observed variation. The interpretation of the valence contribution is complex and involves contributions from changes in covalencies, orbital distortions due to bonding and shielding effects. The changes in metalligand distances are also an important factor, which influences the shape of the 4s-orbital and therefore modulates ISs.