Density Functional Calculations for Prediction of 57Fe Mössbauer Isomer Shifts and Quadrupole Splittings in β-Diketiminate Complexes

Abstract

The relative ease of Mössbauer spectroscopy and of density functional theory (DFT) calculations encourages the use of Mössbauer parameters as a validation method for calculations, and the use of calculations as a double check on crystallographic structures. A number of studies have proposed correlations between the computationally determined electron density at the iron nucleus and the observed isomer shift, but deviations from these correlations in low-valent iron β-diketiminate complexes encouraged us to determine a new correlation for these compounds. The use of B3LYP/def2-TZVP in the ORCA platform provides an excellent balance of accuracy and speed. We provide here not only this new correlation and a clear guide to its use but also a systematic analysis of the limitations of this approach. We also highlight the impact of crystallographic inaccuracies, DFT model truncation, and spin states, with intent to assist experimentalists to use Mössbauer spectroscopy and calculations together.

Figure 1

Diagram of a typical zero-field Mössbauer spectrum with isomer shift (δ) and quadrupole splitting parameters (|ΔE_Q|) indicated.

Figure 2

Structure and labeling scheme of β-diketiminate ligands L^Me, L^tBu, and ^MeL^Me,Me.

Figure 3

Plots of calculated electron density at Fe against the experimentally observed isomer shifts for the training set compounds for def2-TZVP (top) and CP(PPP) (bottom). Data are shown in red and the linear fits are shown in black with given fit parameters (α and β) as well as the square of the correlation coefficient (R²).

Figure 4

Complexes examined for effects of ligand truncation on Mössbauer parameter prediction. (a) L^tBuFe(HCCPh), (b) (SiP^iPr₃)FeCOSi(CH₃)₃, (c) L^S,SFe(THF)₂.