Figure 1: Mössbauer parameters, namely isomer shift and quadrupole splitting, of several tetrapyrrolic FeNC model complexes calculated using density functional theory (black) together with experimental Mössbauer parameters taken from various literature sources (colored ellipsoids). The model investigated is shown in the lower left corner of the corresponding plot. Triangles indicate models without further ligands, whereby the “large” model has an expanded graphene plane. Circles indicate models with intermediates of the oxygen reduction reaction as axial ligands, as denoted by the labels. The color of the ellipsoids represents the allocation to typical experimental FeNC signals, whereas their size illustrates the error in the computed Mössbauer parameters as determined by an in-house calibration study.