DFT calculations of ⁵⁷Fe Mössbauer isomer shifts and quadrupole splittings for iron complexes in polar dielectric media: Applications to methane monooxygenase and ribonucleotide reductase

Abstract

Abstract:To predict the isomer shifts of Fe complexes in different oxidation and spin states more accurately, we have performed linear regression between the measured isomer shifts (␦ exp ) and DFT (PW91 potential with all-electron triple-plus polarization basis sets) calculated electron densities at Fe nuclei [ (0)] for the Fe 2ϩ,2.5ϩ and Fe 2.5ϩ,3ϩ,3.5ϩ,4ϩ complexes separately. The geometries and electronic structures of all complexes in the training sets are optimized within the conductor like screening (COSMO) solvation model. Based on the linear correlation equation.0] ϩ C, the best fitting for 17 Fe 2ϩ,2.5ϩ complexes (totally 31 Fe sites) yields ␣ ϭ Ϫ0.405 Ϯ 0.042 and C ϭ 0.735 Ϯ 0.047 mm s Ϫ1 . The correlation coefficient is r ϭ Ϫ0.876 with a standard deviation of SD ϭ 0.075 mm s Ϫ1 . In contrast, the linear fitting for 19 Fe 2.5ϩ,3ϩ,3.5ϩ,4ϩ complexes (totally 30 Fe sites) yields ␣ ϭ Ϫ0.393 Ϯ 0.030 and C ϭ 0.435 Ϯ 0.014 mm s Ϫ1 , with the correlation coefficient r ϭ Ϫ0.929 and a standard deviation SD ϭ 0.077 mm s Ϫ1 . We provide a physical rationale for separating the Fe 2ϩ,2.5ϩ fit from the Fe 2.5ϩ,3ϩ,3.5ϩ,4ϩ fit, which also is clearly justified on a statistical empirical basis. Quadrupole splittings have also been calculated for these systems. The correlation between the calculated (⌬E Q(cal) ) and experimental (⌬E Q(exp) ) quadrupole splittings based on ͉⌬E Q(exp) ͉ ϭ A ͉⌬E Q(cal) ͉ ϩ B yields slope A, which is almost the ideal value 1.0 (A ϭ 1.002 Ϯ 0.030) and intercept B almost zero (B ϭ 0.033 Ϯ 0.068 mm s Ϫ1 ). Further calculations on the reduced diferrous and oxidized diferric active sites of class-I ribonucleotide reductase (RNR) and the hydroxylase component of methane monooxygenase (MMOH), and on a mixed-valent [(tpb)Fe 3ϩ ( -O)( -CH 3 CO 2 )Fe 4ϩ (Me 3 [9]aneN 3 )] 2ϩ (S ϭ 3/2) complex and its corresponding diferric state have been performed. Calculated results are in very good agreement with the experimental data.

A Mössbauer study on iron(II) complex of 2-acetyl-1,3-indandione ⎼ spin-crossover or structural changes

“…For the optimized structures all-electron single-point calculations, with TZP basis set for all elements, were performed in order to obtain the electronic density at the iron nucleus, which was done with the hypers2003 program [19], kindly provided by Han and Noodleman. For calculations of the Mössbauer isomer shifts the following correlation formulas, as suggested by Han and Noodleman [16], were used:…”

“…The calculated ΔEQ is calibrated using the following correlation formulae [16] ΔEQ(exp) = 1,055×ΔEQ(calc) +0,227 [mm/s] for PW91 (6)…”

“…Han and Noodleman have proposed several correlation formula by doing linear regression fit of the calculated ρ(0) and δexp isomer shift, IS, as well as for the quadrupole splitting, QS, on large sets of experimentally availble data and using different theoretical models [14][15][16]. By using some of the proposed correlations, we attempted to find explanation of the experimentally observed Mössbauer spectral features and propose the most probable structure of the Fe(II) complex of 2AID.…”

The Density Functional Theory

“…The estimation of DE Q requires the calculation of the electric gradient field at the iron nucleus, which can be done with basis sets of sufficient flexibility in the core region (Neese 2002). Many studies at the B3LYP level have demonstrated that the sign and the magnitude of DE Q is predicted accurately, although absolute errors ranging from 0.3 to 1.00 mm s -1 are not uncommon (Berry et al 2008;Godbout et al 1999;Han et al 2006;Salzmann et al 1999;Sinnecker et al 2005). Moreover, it has been shown that the computed DE Q values react fairly sensitively to details of the surrounding, such as counter ions.…”

Fe4 Cluster and a Buckled Macrocycle Complex from the Reduction of [(dmgBF2)2Fe(L)2] (L = MeCN, tBuiNC)

“…Geometry optimization and orbital calculations on the crystal structure coordinates of 4 was performed in the Firefly software package 4 (which is partially based on the GAMESS (US) source code 5 ) using the 6-31G* basis set and the pure functional PW91 6,7 which has been shown to be accurate for Fe systems. [8][9][10][11][12][13][14] Orbitals were visualized using MacMolPlt, 15 and spin density plots were generated using gOpenMol. …”