We report the results of density functional theory (DFT) calculations of the 57Fe Mössbauer isomer shifts (δFe) for a series of 24 inorganic, organometallic, and metalloprotein/metalloporphyrin model systems in S = 0, 1/2, 1, 3/2, 2, and 5/2 spin states. We find an excellent correlation between calculation and experiment over the entire 2.34 mm s-1 range of isomer shifts: a 0.07-0.08 mm s-1 rms deviation between calculation and experiment (corresponding to 3-4% of the total δFe range, depending on the functionals used) with R2 values of 0.973 and 0.981 (p < 0.0001). The best results are obtained by using the hybrid exchange-correlation functional B3LYP, used previously for 57Fe Mössbauer quadrupole splittings and 57Fe NMR chemical shifts and chemical shielding anisotropies. The relativistically corrected value of α, αrel, converges with the large basis set used in this work, but the exact values vary somewhat with the methods used: -0.253 a03 mm s-1 (Hartree-Fock; HF); -0.316 a03 mm s-1 (hybrid HF-DFT; B3LYP), or -0.367 a03 mm s-1 (pure DFT; BPW91). Both normal and intermediate spin state isomer shifts are well reproduced by the calculations, as is the broad range of δFe values: from [FeVIO4]2- (-0.90 mm s-1 expt; -1.01 mm s-1 calc) to KFeIIF3 (1.44 mm s-1 expt; 1.46 mm s-1 calc). Molecular orbital analyses of all inorganic solids as well as all organometallic and metalloporphyrin systems studied reveal that there are three major core MO contributions to ρtot(0), the total charge density at the iron nucleus (and hence δFe), that do not vary with changes in chemistry, while the valence MO contributions are highly correlated with δFe (R2 = 0.915-0.938, depending on the functionals used), and the correlation between the valence MO contributions and the total MO contribution is even better (R2 = 0.965-0.976, depending on the functionals used). These results are of general interest since they demonstrate that DFT methods now enable the accurate prediction of δFe values in inorganic, organometallic, and metalloporphyrin systems in all spin states and over a very wide range of δFe values with a very small rms error.
ASJC Scopus subject areas
- Colloid and Surface Chemistry