Figure 2. Typical values for the isomer shift (mm s−1) and the absolute value of the quadrupole splitting (mm s−1) for iron in oxidation states I–VI (A) and I–IV (B). The reference compounds for the isomer shift are both molecular complexes and solid state materials, whereas for the quadrupole splitting only complexes with an FeN4environment were chosen. The values shown in (A) are adapted from Ref.70; the values in (B) are adapted from Ref.8.
In this contribution, computational Mössbauer spectroscopy is calibrated using a set of 20 complexes with FeN4-6 environments, specifically chosen such that they are representative of plausible active site structures in FeNC catalysts. We find that the TPSSh, B3LYP and PBE0 density functionals are able to predict isomer shift and quadrupole splitting values with approximately equal accuracy. The mean absolute and maximum deviations for the isomer shift are ca. 0.05–0.06 mm s−1 and 0.12–0.13 mm s−1, and for the quadrupole splitting mean absolute and maximum deviations of ca. 0.23–0.26 mm s−1 and 0.57–0.93 mm s−1 are found. Important raw data including the computed densities is provided in the SI for future use in gauging deviations between computational calibration studies. In addition, we introduce an interactive notebook that based on the reference data set presented here derives predicted Mössbauer parameters with individual associated error margins from computed contact densities and quadrupole splitting values. Defining the computational trust region as twice the mean absolute deviation obtained in the calibration study, we discuss for which of the Mössbauer signatures typically observed in FeNC catalysts one can expect that accurate computational models will be able to differentiate them.