Conclusions
We have presented a calibration study of three density functionals,
TPSSh, B3LYP and PBE0, for their application in computational Mössbauer
spectroscopy. All density functionals were found to perform well for
both the isomer shift and the quadrupole splitting prediction; R-values
exceeded 0.985 in all cases. We defined trust regions as the double mean
absolute deviation of the correlation line and note as exemplary values
0.13 mm s−1 for the isomer shift and 0.45 mm
s−1 for the quadrupole splitting obtained with the
B3LYP density functional.
Besides the notoriously difficult electronic structures of iron(II)
intermediate spin (2S +1 = 3) ions in square-planar coordination
geometry, we discuss other cases where an adequate electronic structure
description was not found when treating DFT purely as a black-box
method. In order to facilitate comparisons of computational data for
iron complexes with complicated electronic structures such as those
expected in the coordination environment of FeNC catalysts, it is thus
strongly recommended to at least report the spin populations on all
relevant atoms.
The focus of this study was placed on iron environments similar to those
thought to be the active sites in FeNC catalysts, thereby guiding the
choice of complexes in the reference set. Despite this intentional
constrain, the results are very similar to those of Pápai et
al .,21 suggesting that the correlation lines obtained
here can be used for other systems as well. The data presented here are
made available in an online notebook that allows researchers to obtain
predicted Mössbauer parameters and the individual uncertainties from the
computed values; the notebook
(tinyurl.com/mbs-notebook) is
open to submission of additional data points for the computational
setups presented here and will accept submissions of entire data sets
produced with different methodological choices. In this way, a
future-proof and ever-growing calibration of computational Mössbauer
spectroscopy is provided that ensures a rigorous and directly comparable
statistical analysis of different computational approaches.
In native FeNC catalysts, up to five distinct Mössbauer signatures can
be found and several other signals can be produced by different
treatments. We discuss the trust regions for isomer shift and quadrupole
splitting deduced from the calibration study in the context of the
relative positions of these experimental values. From this analysis, it
appears probable that appropriate computational models will be able to
differentiate between these characteristic spectral features if isomer
shift and quadrupole splitting are both considered in the analysis. In
other words, through a combination of experimental and computational
Mössbauer spectroscopy, one will likely be able to identify the
structural and electronic basis for the oxygen reduction reaction in
FeNC catalysts.