The predicted values are listed in Table 3; Figure 4 shows that all density functionals perform almost equally well in the prediction of isomer shifts (left) and quadrupole splitting values (right). Note that the calibration lines shown in the main text are obtained after exclusion of complexes 9 and 10 (shown as solid symbols in Figure 4) as is explained in detail below. The fits including all 20 complexes are shown in the Supporting Information (Figure S2).

Beginning with a closer inspection of the isomer shift predictions, the experimental values range from –0.29 mm s⁻¹ to +1.11 mm s⁻¹. The linear fits obtained with the three density functionals show almost identical R-values of 0.989 (TPSSh), 0.985 (B3LYP) and 0.988 (PBE0, see Table 4). With the B3LYP density functional, the largest discrepancy with experiment is seen for complex9 (0.159 mm s⁻¹ absolute deviation). The maximum absolute discrepancy from the regression line for the B3LYP functional is 0.131 mm s⁻¹ (see Table 4).

For the quadrupole splittings, the experimental values lie between 0.23 mm s⁻¹ and 4.25 mm s⁻¹ in absolute numbers, with the only negative sign reported for complex 17(–1.76 mm s⁻¹). Since in previous studies the sign was calculated correctly in all cases where it was measured explicitly,²¹ the correlation line assumes the sign of the experimental values to match that predicted computationally. As expected, the scatter about the regression line is greater than for the isomer shifts. Overall, the quality of the fit is quite satisfactory with R-values of 0.989 (TPSSh), 0.991 (B3LYP) and 0.987 (PBE0). It can be seen readily that the most significant outliers are complexes9 and 10 , with absolute deviations of 1.601 mm s⁻¹ (9 , B3LYP) and 1.848 mm s⁻¹ (10 , B3LYP) from the experimental value. These much larger than usual deviations led us to exclude them from the fitting procedure, and the underlying electronic structure reasons will be discussed below. Among the set of 18 complexes the fit is based on, the largest deviation is observed for complexes 6 and7 with the B3LYP density functional (0.850 mm s⁻¹ and 0.727 mm s⁻¹ absolute deviation from experiment). The maximum absolute deviation from the B3LYP regression line is smaller at 0.570 mm s⁻¹.

To summarize, all three density functionals yield roughly the same quality of fit. Because B3LYP provides better predictions for the quadrupole splitting values in terms of the mean absolute deviation and maximum deviations while also performing very well for the isomer shift, any specific discussion in the following makes use of the B3LYP data. We define a trust region for the later evaluation of FeNC catalyst models as twice the mean absolute deviation, i.e. 0.131 mm s⁻¹ for the isomer shift and 0.451 mm s⁻¹ for the quadrupole splitting using the B3LYP density functional.