Theoretical ⁵⁷Fe Mössbauer spectroscopy: isomer shifts of [Fe]-hydrogenase intermediates

Abstract

Mössbauer spectroscopy is an indispensable technique and analytical tool in iron coordination chemistry. The linear correlation between the electron density at the nucleus ("contact density") and experimental isomer shifts has been used to link calculated contact densities to experimental isomer shifts. Here we have investigated for the first time relativistic methods of systematically increasing sophistication, including the eXact 2-Component (X2C) Hamiltonian and a finite-nucleus model, for the calculation of isomer shifts for iron compounds.While being of similar accuracy as the full four-component treatment, X2C calculations are far more efficient. We find that effects from spin orbit coupling can safely be neglected, leading to further speed up. Linear correlation plots using effective densities rather than contact densities versus experimental isomer shift leads to a correlation constant a = −0.32 a −3 0 mm s −1 (PBE functional) which is in close agreement to experimental findings. Isomer shifts of similar quality can thus be obtained both with and without fitting, which is not the case if one pursues a priori a non-relativistic model approach. As an application for a biologically relevant system, we have studied three recently proposed [Fe]-hydrogenase intermediates. The structures for these intermediates were extracted from QM/MM calculations using large QM regions surrounded by the full enzyme and a solvation shell of water molecules. We show that a comparison between calculated and experimentally observed isomer shifts can be used to discriminate between the different intermediates, whereas calculated atomic charges do not necessarily correlate with Mössbauer isomer shifts. Detailed analysis shows that the difference in isomer shifts between two intermediates is due to an overlap effect.

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

“…As a result, calculation of isomer shift values generally depends on an empirically determined correlation between the calculated electron density and the experimental isomer shift. 6,9 These fits are typically linear, but the fit parameters only apply when using a specific set of computational parameters (functional, basis set, relativistic corrections). The calculated EFG yields a predicted quadrupole splitting through the relationship given in eq 1, where e is the charge of an electron, Q is the quadrupole moment of the 57 Fe nucleus (0.16(1) barn, 3 and V zz is the major component of the EFG.…”

Hydrogen-activation mechanism of [Fe] hydrogenase revealed by multi-scale modeling

“…However, experimental and theoretical Mössbauer spectra indicate that no stable hydride or H 2 -bound species exists under turnover conditions. 12,19,20 (ii) Model compounds that accurately mimic the rst coordination sphere of the iron do not even bind H 2 , [21][22][23] hence, the protein environment is likely to be crucial. (iii) In model compounds, protonating the thiolate ligand leads to dissociation of the ligand.…”

Successes, challenges, and opportunities for quantum chemistry in understanding metalloenzymes for solar fuels research

“…The putative structures of these intermediates were obtained from QM/MM calculations and the comparison between calculated and experimental isomer shifts could be used to discriminate between different intermediates following a proper calibration based on iron-based model systems. 274 The same approach was also used for studying the coordination environment of an active site model complex. Based on the calculated isomer shifts and quadrupole splitting, they could determine the protonation states, the presence of a bound water and the redox state of the iron center providing meaningful insight regarding the proton transfer mechanism.…”