Predicting Mössbauer Parameters of Nonheme Diiron Complexes with Density Functional Theory

Abstract

Mossbauer spectroscopy provides significant insights into the electronic structure and environment of the metal centers. Herein, we investigate the electronic structures of a set of nonheme diiron complexes by evaluating two key parameters pertaining to Mossbauer spectroscopy, namely, the isomer shift (δ) and quadrupole splitting (|ΔE Q |), using different levels of density functional theory (DFT). The diiron systems investigated here span diverse oxidation states, bridging motifs, and spin coupling patterns, which present a challenging case for theoretical predictions. We demonstrate that the combination of B97-D3/ def2-TZVP is an efficient approach in modeling both the δ and |ΔE Q | values with high accuracy for the representative nonheme diiron complexes. We also show that δ is accurately predicted irrespective of the choice of approximate density functional while the |ΔE Q | is sensitive to the level of theory employed. Further investigation shows that the present methodology assessed using synthetic nonheme diiron complexes could be extended to nonheme diiron enzyme active sites, featuring both ferromagnetic and antiferromagnetic coupling between the iron centers.

Calculation of electric field gradients with the exact two-component (X2C) quasi-relativistic method and its local approximations

“…The Mössbauer NQS reveals the asymmetry of the electron distribution around the 57 Fe nucleus, thus offering insight into the occupation of Fe 3d orbitals. Nearly all theoretical EFG studies of 57 Fe were performed using the DFT calculations, 12–33 usually non-relativistic, whereas relativistic effects were considered only in a few studies. 13,19,23,25,27,32,33 It is well known that iron compounds often involve strong correlations, and the DFT results can be questionable.…”

“…Nearly all theoretical EFG studies of 57 Fe were performed using the DFT calculations, 12–33 usually non-relativistic, whereas relativistic effects were considered only in a few studies. 13,19,23,25,27,32,33 It is well known that iron compounds often involve strong correlations, and the DFT results can be questionable. Consequently, in a previous paper, 62 we used the selected configuration interaction method ICE-SCF with quite large active spaces to study the Mössbauer isomer shifts of 57 Fe, showing the superb ability of this method to handle strong correlations in iron compounds.…”

“…11 To date, the EFG and NQCC values of a large number of nuclides have been theoretically studied in the literature, mainly focusing on Fe. 12–33 For systems containing heavy atoms, such as Bi, 6,34–39 U, 40,41 and Np, 42 relativistic effects become crucial and have been taken into account through various approximate or (nearly) exact methodologies. In the early 1970s, Desclaux and Bessis presented the first four-component Dirac calculation of EFGs.…”