A. Trautwein

Mössbauer measurements of the bipyramidal lattice site in BaFe12O19

Abstract Mossbauer measurements have been made on polycrystalline barium ferrite samples. The subspectrum corresponding to the iron ion in the bipyramidal lattice site in the temperature range from T = 4.2K up to T = 870K was obtained. Based on the temperature dependence of the unusually large quadrupole splitting due to ferric iron in the bipyramidal position, a model of this lattice site is proposed in which the iron ion is located at one of two equivalent positions of Wyckhoff notation 4e, separated by a potential barrier. At sufficiently high temperature the ion jumps between the two 4e sites with a jump frequency which is greater than the inverse of the lifetime of the 14.4 keV state of 57Fe. This is in agreement with Mossbauer measurements, molecular-orbital studies and previous X-ray data.

Molecular orbital structure, Mssbauer isomer shift, and quadrupole splitting in iron complexes

Molecular orbital calculations are made on six iron complexes, using iterative Hückel methods and, where required for proper description of spin states, a spin-projected semi-empirical configuration interaction (CI). Many integrals are avoided in the CI studies by making direct calculation of energy differences between states. From the calculations are obtained charge and spin bond order matrices, dipole moments, and atomic orbital charges. These quantities are used to calculate charge densities at the Fe nucleus, nuclear quadrupole splittings, and spin populations. From calculations of all six complexes we estimate an Fe57 Mössbauer isomer-shift calibration α ≡ Δδ/Δϱ(0)=−0.31 to −0.38 a03mm/sec.ZusammenfassungMit Hilfe iterativer Hückel-Methoden werden MO-Rechnungen für sechs Eisenkomplexe durchgeführt. Wo es für die Beschreibung der Spinzustände notwendig ist, wird nach einem CI-Verfahren mit Spinprojektion gearbeitet. Durch direkte Berechnung von Energiedifferenzen zwischen den einzelnen Zuständen werden viele Integrale in den CI-Berechnungen vermieden. Aus den Rechnungen erhält man die Matrizen der Ladungs- und Spin-Bindungsordnungen, Dipolmomente und AO-Ladungsverteilungen. Diese Größen werden zur Berechnung der Ladungsdichte am Fe-Kern, der Kernquadrupolaufspaltung und der Spinpopulation verwendet. Aufgrund der Berechnungen aller sechs Komplexe wird die Fe57-Mößbauer-Isomerieverschiebung auf αε Δδ/Δϱ(0)=−0,31 bis −0,38 a03mm/sec geschätzt.

Limitation of semi-empirical mo-calculations in deriving charge densities ρ(0) in iron-oxygen compounds

Abstract For fifteen iron-oxygen compounds we carry out semi-empirical MO cluster calculations. The derived electronic structure is used to calculate electron charge densities ρ(0) and electric field gradients V pq at the iron nucleus. The ρ(0)-values correlate with experimental isomer shifts δ, however, extreme cases like the Fe(VI)-compound BaFeO 4 are beyond the scope of accuracy of our method. We discuss variations of our calculational method, with one of them leading to ρ(0)-values for all fifteen compounds (including BaFeO 4 ) which satisfy the relation Δδ = αΔρ (0). The isomer shift calibration constant is in the range -0.195 mms −1 a 0 3 to -0.25 mms −1 a 0 3 . Calculated quadrupole splittings are comparable with experimental data.

Metal- and Ligand-Directed One-Pot Syntheses, Crystal Structures, and Properties of Novel Oxo-Centered Tetra- and Hexametallic Clusters†

Starting from closely related metal-ligand combinations, completely different oligomeric metal clusters are synthesized. Whereas, picoline-tetrazolylamide HL(1) (1) and zinc or nickel acetate afforded [2x2] grids [M(4)(L(1))(8)] (2), slightly different N-(2-methylthiazole-5-yl)-thiazole-2-carboxamide HL(2) (5 a) and nickel acetate yielded the monometallic complex [Ni(L(2))(2)(OH(2))(2)] (6). In contrast, reaction of 5 a with zinc acetate produced the tetrametallic zinc cluster [Zn(4)O(L(2))(4)(OAc)(2)] (7). Even more surprising, when 3-methyl-substituted HL(3) (5 b) instead of 2-methyl-substituted HL(2) (5 a) was allowed to react under identical conditions with zinc acetate, the cluster [Zn(4)O(L(3))(4)Cl(2)] (8) crystallized from dichloromethane. Clusters 7 and 8 are isostructural. As for 7, in 8 two of the edges of the tetrahedron of zinc ions are doubly bridged, two are singly bridged, and the other two are nonbridged. On the other hand, when iron(II) acetate under aerobic conditions was allowed to react with 5 a, the unprecedented complex [[Fe(3)O(L(2))(2)(OAc)(4)](2)O] (9) was isolated. Cluster 9 is composed of two trimetallic, triangular mu(3)-O(2-)-centered [Fe(3)O(L(2))(2)(OAc)(4)](+) modules, linked by an almost linear mu(2)-O(2-) bridge. The Mössbauer spectrum together with cyclic voltammetric and square-wave voltammetric measurements of 9 are reported, and 6-9 were characterized unequivocally by single-crystal X-ray structure analyses.

Mssbauer and molecular orbital study of the myoglobin-CO complex

Experimental Mössbauer spectra of the Fe57-enriched CO complex of sperm whale myoglobin (MbCO) at T= 4.2 K with and without applied magnetic field (H⊥γ) were measured to derive the sign of the electric field gradient (EFG), the quadrupole splitting ΔEQ, and the isomer shift δ of the heme iron. We find a positive EFG, δEQ = 0.363 mm/sec, and δ + 0.266 mm/sec. Molecular orbital calculations were carried out to obtain theoretical estimates of EFG and ΔEQ for several steric arrangements of the CO ligand relative to the heme group. Our results are most consistent with the conclusion that the iron is situated in the heme plane, and that a bent geometry with a Fe-C-O angle of about 135 ° is more favorable than a more symmetric structure with a linear Fe-C-O geometry.

Molecular orbital and mössbauer study of iron-oxygen compounds☆

Abstract Semi-empirical, spin-projected, open-shell, molecular orbital calculations are used to estimate the electron density and electric field gradient at the iron nucleus of the iron-oxygen complexes FeO 6 −9 and FeO 6 −10 from 57 Fe-doped MgO, FeO 5 −7 from BaFe 12 O 19 , and FeO 4 −6 from BaFeSi 4 O 10 . The calculations indicate that the inclusion of covalency effects can provide a consistent interpretation of the Mossbauer data.

Mössbauer effect in bacterial catalase

Abstract The Mossbauer spectra in native catalase (EC 1.11.1.6) prepared from 57Fe-enriched bacteria have been measured from 2.1 to 195 °K. At low temperature the spectra consist of a quadrupole doublet of 0.9 mm/s and a broad magnetic hyperfine pattern. By applying a small magnetic field, the magnetic pattern exhibits six well-defined lines with an effective field of 293 kG and a quadrupole perturbation of (−)0.45 mm/s, while the quadrupole doublet is not influenced. At high temperatures the magnetic subspectrum collapses. The area of the two subspectra are equal within experimental error over a wide temperature range. These results suggest that the four hematins in bacterial catalase show pair-wise different interactions, probably due to different Fe-Fe interatomic distances. Both subspectra are very similar to those of crystalline hemin and dilute hemin, and they are characterized by a ferric high-spin state. This is in agreement with the results of EPR, magnetic susceptibility and optical absorption measurements.

Numerical integration of overlap and ligand contributions to the electric field gradient

The total electric field gradient (EFG) tensor Vpqis calculated by numerical integration of threedimensional integrals. Each of them is solved a) by integrating over one dimension analytically and b) by integrating over the remaining two dimensions on the basis of a Gauss-type integration rule. The use of 100 abscissas in the twodimensional numerical integration scheme yields satisfactory accuracy which was checked by evaluating overlap integrals; an increase to 400 abscissas does not increase the result drastically. Calculating quadrupole splittings ΔEQfrom numerically integrated electric field gradient tensors Vpqwe observe that depending a) on the amount of covalency and b) on the amount of deviation from octahedral or tetrahedral symmetry, involved in a molecular system, overlap and ligand contributions to Vpqplay an important role. Especially for the sandwich compound ferrocene, Fe(C5H5)2, we find a significant difference between ΔEQnum. int.which follows from the numerical integration method, and ΔEQconventionalwhich is derived from effective charges.

Interpretation of experimental Mössbauer quadrupole splittings of iron pentacyanide complexes using molecular orbital theory

Semiempirical self-consistent-field molecular-orbital calculations are carried out for six iron-pentacyanide complexes and are used to interpret their experimental Mössbauer quadrupole splittings. Probable orientations are identified for the C6H5−and NO2−groups in Fe(CN)5NOC6H5−3and Fe(CN)5NO2−4. Calculations on Fe(CN)5NO−2 and Fe(CN)5NO−3 can simultaneously be brought into agreement with experiment by reparametrization to make the NO group more positively charged. All the calculations indicate the importance of including all the Fe 3d and 4p orbitals in the calculations and of considering neighboring-atom effects.

Electronic structure, electron density, electric field gradient-, magnetic susceptability- and G-tensor of K3Fe(CN)6

Abstract Molecular orbital calculations are presented for the electronic structure of K 3 Fe(CN) 6 based on clusters of formula K 8 Fe(CN) 5+ 6 , using a semi-empirical molecular orbital method including mixing of low-lying electronic configurations and spin-orbit coupling. The energy splittings obtained are in qualitative agreement with ligand field studies from several workers, excluding those of Merrithew and Modestino. While other authors interpret either Mossbauer data, or ESR results, or susceptibility values only, we obtain from the electron structure calculations charge densities at the iron nucleus, and electric field gradient, magnetic susceptibility and gyromagnetic tensors, which consistently interpret experimental Mossbauer-, EST- and magnetic anisotropy results. From the electronic structure calculations as well as from the reanalysis of experimental quadrupole line intensities (obtained by Oosterhuis and Lang) we derive that orthorhombic polytypism of K 3 Fe(CN) 6 has to be considered for a consistent interpretation of the experimental data. The successful correlation between calculation and experiment in an energy range of about 300 K above the electronic groundstate is a measure for the adequacy of our electronic structure calculations in this low-energy range.