Paramagnetic relaxation in high-spin ferrous compounds

Measurements of three high-spin Fe^IIS₄ complexes at various temperatures and applied magnetic fields are reported and the results are compared with those of reduced rubredoxin. Simulations of the measured spectra by means of the fully developed stochastic theory of lineshape prove that intermediate relaxation behavior is present and is determined essentially by the phonon energy distribution and by one single parameter W₀, the scaling factor, which contains the strength of spin-lattice coupling.     

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

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 BaFeO₄ 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 BaFeO₄) which satisfy the relation Δδ = αΔρ(0). The isomer shift calibration constant is in the range -0.195 mms^?1a₀³ to -0.25 mms^?1a₀³. Calculated quadrupole splittings are comparable with experimental data.     

Archimedean Synthesis and Magic Numbers: "Sizing" Giant Molybdenum-Oxide-Based Molecular Spheres of the Keplerate Type

Pythagorean harmony can be found in the spherical polyoxometalate clusters described here (see illustration for an example of a structure), since there are interesting relationships between the so-called magic numbers (12, 32, 42, 72, 132) relevant for spherical viruses and the number of the building blocks in the cluster. The size of these Keplerate clusters can be tailored by varying the type of connections between the pentagons by means of different spacers.     

Mössbauer, magnetic, X‐ray fluorescence and transmission electron microscopy study of natural magnetic materials from speleothems: haematite and the Morin transition

Detrital magnetic materials within cave stalagmitic formations, e.g., haematite or magnetite, carry remanence whose vector is of value in dating. Magnetometry measurements on a particular haematite‐bearing sample reveal that remanence was substantially restored and/or conserved on rewarming after cooling below the Morin transition temperature. Mössbauer measurements indicate the presence of two types of haematite, distinguished primarily by particle size. The majority is small in size, partially exhibiting superparamagnetism, and does not undergo a Morin transition above liquid nitrogen temperature. Superparamagnetic goethite is the second major component. Mine haematite samples of surface location with different color and mineralogical composition have also been studied. Possible relations between the mineralogical composition of the mine samples and detrital stalagmitic magnetic material, the modifications and the origin of this mineralization are discussed. Special attention is paid to the “irreversible” Morin transition in large enough (>20 nm) haematite particles and the possible loss of natural remanent magnetization and hence of palaeomagnetic records.     

Synthesis and characterization of metal-centered, six-membered, mixed-valent, heterometallic wheels of iron, manganese, and indium

Heptanuclear metal-centered, six-membered, mixed-valent, heterometallic wheels 1-3 of iron, manganese, and indium were prepared in a one-pot reaction from N-benzyldiethanolamine (H2L(1)), cesium carbonate, [PPh4]2[MnCl4], and FeCl3 or InCl3. All three complexes were characterized by the combination of elemental analysis, FAB mass spectroscopy, X-ray diffraction and cyclic voltammetry and in the case of 1 additionally by M?ssbauer spectroscopy. In 1, four Mn(II) ions in the periphery are arranged in pairs alternating with one Fe(III) ion each, with an Fe(III) ion located in the center. In 2, three Mn(II) ions alternate with three In(III) ions, whereas in 3, four In(III) ions are arranged in pairs and alternate with one Mn(II) ion each. In 2 and 3 an Mn(II) ion is encapsulated in the center.     

Molecular structure,quadrupole splitting,and magnetic susceptibility of iron in deoxygenated myoglobin and hemoglobin

For three stereo-structural models of deoxymyoglobin (Mb) and deoxyhemoglobin (Hb) we derive electronic configurations and their mutual spin-orbit coupling. From the temperature dependent molecular electric field gradient (EFG) tensor we calculate temperature dependent quadrupole splittings, E _q(T), asymmetry parameters, (T), and orientations of the EFG component V _zz(T) with respect to the heme group. Comparing theoretical and experimental data we find a molecular electronic structure, which then is used to compute temperature dependent magnetic susceptibilities, (T). Theoretical and experimental (T) data are in reasonable agreement. From the consistency of our model calculations with experimental results we conclude that iron in Mb and Hb probably is pentacoordinated and considerably out of the heme plane by 0.4–0.8 Å.Supported in part by Stiftung Volkswagenwerk, by Deutsche Forschungsgemeinschaft, by the European Molecular Biology Organization, by an award from the Biomedical Sciences Support Grant at the University of Utah, and the National Science Foundation.