The prediction of Fe Mössbauer parameters by the density functional theory: a benchmark study

Abstract

We report the performance of eight density functionals (B3LYP, BPW91, OLYP, O3LYP, M06, M06-2X, PBE, and SVWN5) in two Gaussian basis sets (Wachters and Partridge-1 on iron atoms; cc-pVDZ on the rest of atoms) for the prediction of the isomer shift (IS) and the quadrupole splitting (QS) parameters of Mössbauer spectroscopy. Two sources of geometry (density functional theory-optimized and X-ray) are used. Our data set consists of 31 iron-containing compounds (35 signals), the Mössbauer spectra of which were determined at liquid helium temperature and where the X-ray geometries are known. Our results indicate that the larger and uncontracted Partridge-1 basis set produces slightly more accurate linear correlations of electronic density used for the prediction of IS and noticeably more accurate results for the QS parameter. We confirm and discuss the earlier observation of Noodleman and co-workers that different oxidation states of iron produce different IS calibration lines. The B3LYP and O3LYP functionals have the lowest errors for either IS or QS. BPW91, OLYP, PBE, and M06 have a mixed success whereas SVWN5 and M06-2X demonstrate the worst performance. Finally, our calibrations and conclusions regarding the best functional to compute the Mössbauer characteristics are applied to candidate structures for the peroxo and Q intermediates of the enzyme methane monooxygenase hydroxylase (MMOH), and compared to experimental data in the literature.

Figure 1

Some linear correlations observed between electronic density on iron and experimental isomer shift for the X-ray-based geometries. The data in red correspond to the oxidation state +2, and the data in blue correspond to the oxidation states +3 and +4.

Figure 2

Some comparisons between the experimental and computed absolute values on the quadrupole splittings. The red line is y = x: the points lying it represent the perfect agreement between the experiment and the theory. The obvious outliers are indicated by their Cambridge Structural Database code which can also be found in Table I.

Figure 3

The linear correlations observed between electronic density on iron and experimental isomer shift computed with the B3LYP functional and Partridge-1 basis set for the DFT-based geometries. The data in red correspond to the oxidation state +2, and the data in blue correspond to the oxidation states +3 and +4.

Figure 4

The comparison of the quadrupole splittings computed with the B3LYP functional and Partridge-1 basis set using the DFT (a) and X-ray (b) geometries. The red line is y = x: the points lying it represent the perfect agreement between the experiment and the theory. The obvious outliers are indicated by their Cambridge Structural Database code which can also be found in Table I.

Figure 5

The schematic diagrams of the MMOH intermediates active sites. P stands for Fe⁺³, Fe⁺³ peroxo models and Q represents the Fe⁺⁴, Fe⁺⁴ species.