Influence of Hartree−Fock Exchange on the Calculated Mössbauer Isomer Shifts and Quadrupole Splittings in Ferrocene Derivatives Using Density Functional Theory

Abstract

Influence of molecular geometry, type of exchange-correlation functional, and contraction scheme of basis set applied at the iron nuclei have been tested in the calculation of 57Fe Mössbauer isomer shifts and quadrupole splittings for a wide range of ligand types, as well as oxidation and spin states, in inorganic and organometallic systems. It has been found that uncontraction of the s-part of Wachter's full-electron basis set at the iron nuclei does not appreciably improve the calculated isomer shifts. The observed correlations for all tested sets of geometries are close to each other and predominantly depend on the employed exchange-correlation functional with B3LYP functional being slightly better as compared to BPW91. Both hybrid (B3LYP) and pure (BPW91) exchange-correlation functionals are suitable for the calculation of isomer shifts in organometallic compounds. Surprisingly, it has been found that the hybrid B3LYP exchange-correlation functional completely fails in accurate prediction of quadrupole splittings in ferrocenes, while performance of the pure BPW91 functional for the same systems was excellent. This observation has been explained on the basis of relationship between the amount of Hartree-Fock exchange involved in the applied exchange-correlation functional and the calculated HOMO-LUMO energy gap in ferrocenes. On the basis of this explanation, use of only pure exchange-correlation functionals has been suggested for accurate prediction of Mössbauer spectra parameters in ferrocenes.

DFT Approach to the Calculation of Mössbauer Isomer Shifts

“…[12][13][14][15][16] The so-obtained parameters of nuclear transitions may differ by a factor of 2 from the experimentally obtained values. 3 Recently, a new approach 20 to the calculation of MIS was suggested.…”

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

“…7 and the appropriate fitting constants ( Tables Table 1 and Table 2) A note concerning Fe(CO) 5 should be made. As pointed out in a previous study on isomer shifts, 63 this compound has been measured quite often where a wide spread of experimental isomer shifts has been obtained ranging from 0.00 to −0.18. From the linear correlation plot displayed in Figure 2 it is obvious that a value closer to −0.18 will lead to a linear relationship between the effective density and the isomer shift.…”

First principles calculation of Mössbauer isomer shift

“…In Fig. 2, the results of relativistically corrected (with the scalar-relativistic effects included) Hartree-Fock and MP2 calculations [61,69] on a set of iron-containing clusters and molecules standardly used [77,[80][81][82] for the parametrization of 57 [80]. It has also been observed in the HF, MP2, and CCSD(T) calculations on a set of coinage metal atoms [61] that the electron correlation has a strong effect (up to 50%) on the density differences between different states of atoms.…”

“…There exists a large variety of approximate density functionals which enable one to obtain results of acceptable accuracy for geometry and thermochemistry of large molecules. Although the methods related to density functional theory, such as the X␣ method [83], were employed for the interpretation of Mössbauer spectra for a long time [84][85][86][87][88][89], the use of more accurate density functional approaches in these calculations was initiated relatively recently [56,58,77,81,82,[90][91][92].…”

“…Density functional calculations of molecular models of solids [58,77,81,82,[90][91][92] as well as band structure calculations [55,56] have been employed to obtain the contact densities in iron compounds. Large sets of iron-containing species have been used in the calculations to reduce the dependence of the calibration constant on the choice of the training set of compounds.…”