Figure 7. Isomer shifts and absolute quadrupole splitting
values for the 20 reference complexes (A) and selected signals from FeNC
catalysts (B). In (A), calculated values are shown as open circles (ᴏ)
and experimental values as filled squares (■); the error bars are taken
as the mean absolute deviation. In (B), the labels differ from the
nomenclature used in the original papers (S1 corresponds to D1, S2 and
S4 to D2-like species, S3 to D3 in porphyrin-type catalysts, S5 to D3
often obtained after an ammonia treatment step or from MOF-based
catalysts; roman numerals indicate doublets that originate from a
specific treatment or appear only within distinct synthesis routes). The
error bars in (B) correspond to standard deviations for the values of
different synthesis routes where applicable; for individual data points,
e.g. roman numerals, the error bar is taken as the 95 % confidence
interval.
The challenge posed by these systems for the computational chemist is to
identify structural and electronic models for the various
FeNx sites that will permit the identification of the
environment of the catalytically active site. Among the characteristic
doublets, S1–S4 fall within a similar
range of isomer shifts between 0.28–0.54 mm s−1. The
variation within a group of Si signals from
different preparations appears approximately as large as the deviation
between groups. In contrast, signal S5 is well separated
at an isomer shift of 0.98–1.01 mm s−1. With the
B3LYP trust region for the isomer shift of 0.13 mm
s−1, it is evident that
S1–S4 will not be distinguishable by
different computational models although it can be expected that one will
be able to clearly identify S5. The quadrupole splitting
values of the S1–S4 and
S5 signals are spread over a wider range of 0.71–2.83
mm s−1. This not only allows a better distinction of
the individual signals, but also provides clear evidence that different
types of local environment are present for the various
FeN4 sites observed experimentally. Taking the B3LYP
trust region for the quadrupole splitting of 0.45 mm
s−1, it appears conceivable that S1,
S2 and S3 will be distinguishable while
the exclusive consideration of the quadrupole splitting will not enable
to differentiate between S2, S4, S5, IVx and
IIIb. If for instance upon poisoning, a chemical
connection between one of the Si doublets and a
species labelled with a roman numeral in Figure 7B can be made, this
could be used as additional information to better characterize the
geometric and electronic structures of typical FeN4sites.