Quadrupole Doublet Research Articles

Synthesis and properties of NaFeGe2O6 polycrystals

NaFeGe2O6 polycrystals were synthesized and their x-ray diffraction, magnetic, electrical, and Mossbauer characteristics were measured. It is established that this monoclinic compound is a dielectric with a temperature of antiferromagnetic ordering of 15 K. The Mossbauer spectrum at 300 K is a quadrupole doublet. The isomer shift is 0.40 mm/s, which is characteristic of the high-spin Fe3+ ion in the octahedral coordination. The quadrupole splitting is 0.34 mm/s, which indicates that the oxygen octahedron around the iron cation is distorted. The exchange interactions are estimated, and the crystal magnetic structure is discussed.

ESEEM Measurements of Local Water Concentration in D2O-Containing Spin-Labeled Systems

To calibrate electron spin echo envelope modulation (ESEEM) amplitudes with respect to the deuterium water content in spin-labeled biological systems, ESEEM of nitroxide TEMPO has been studied in frozen glassy D2O-dimethylsulfoxide mixtures of different composition. The interaction between the unpaired electron of nitroxide and the deuterium nuclei manifests itself in a cosine Fourier transform spectrum as the sum of a narrow line with the doublet quadrupole splitting and of a broad one. The narrow line arises from interaction with distant deuterium nuclei, the broad one arises from interaction with nearby nuclei belonging to nitroxide-water molecule complexes. The dependence on water concentration was found to be nonlinear for the intensity of the narrow line and close to linear for the intensity of the quadrupole doublet. Therefore, the intensity of the quadrupole doublet is suggested as a measure of concentration of free water around a spin label in biological objects. Fourier transform line shape was theoretically simulated for different model distributions of water molecules around the spin label. Simulations confirm the linear dependence of the quadrupole doublet intensity on water concentration seen in the experiment. The suggested approach was applied to analyze data for spin-labeled dipalmitoylphosphatidylcholine (DPPC) and DPPC-cholesterol D2O-hydrated model membranes. The concentration of free water near the spin-labeled fourth carbon atom along the lipid chain was estimated as 5.2 and 7.2 M for DPPC and DPPC-cholesterol membranes, respectively.

Biogenesis of Iron-Sulfur Clusters in Photosystem I: HOLO-NfuA FROM THE CYANOBACTERIUM SYNECHOCOCCUS SP. PCC 7002 RAPIDLY AND EFFICIENTLY TRANSFERS [4Fe-4S] CLUSTERS TO APO-PsaC IN VITRO

The NfuA protein has been postulated to act as a scaffolding protein in the biogenesis of photosystem (PS) I and other iron-sulfur (Fe/S) proteins in cyanobacteria and chloroplasts. To determine the properties of NfuA, recombinant NfuA from Synechococcus sp. PCC 7002 was overproduced and purified. In vitro reconstituted NfuA contained oxygen- and EDTA-labile Fe/S cluster(s), which had EPR properties consistent with [4Fe-4S] clusters. After reconstitution with 57Fe2+, Mössbauer studies of NfuA showed a broad quadrupole doublet that confirmed the presence of [4Fe-4S]2+ clusters. Native gel electrophoresis under anoxic conditions and chemical cross-linking showed that holo-NfuA forms dimers and tetramers harboring Fe/S cluster(s). Combined with iron and sulfide analyses, the results indicated that one [4Fe-4S] cluster was bound per NfuA dimer. Fe/S cluster transfer from holo-NfuA to apo-PsaC of PS I was studied by reconstitution of PS I complexes using P700-F(X) core complexes, PsaD, apo-PsaC, and holo-NfuA. Electron transfer measurements by time-resolved optical spectroscopy showed that holo-NfuA rapidly and efficiently transferred [4Fe-4S] clusters to PsaC in a reaction that required contact between the two proteins. The NfuA-reconstituted PS I complexes had typical charge recombination kinetics from [F(A)/F(B)](-) to P700+ and light-induced low-temperature EPR spectra. These results establish that cyanobacterial NfuA can act as a scaffolding protein for the insertion of [4Fe-4S] clusters into PsaC of PS I in vitro.

Magnetic properties of the magnetoelectricAl2−xFexO3 (x = 0.8,0.9 and 1)

By using x-ray diffraction, magnetization and Mössbauer spectroscopy techniques we have studied themagnetoelectric Al2−xFexO3 (x = 0.8,0.9 and1.0) compound. Ac-susceptibility and magnetization measurements revealed magnetic transitions atTN = 180,210 and260 K for x = 0.8, 0.9 and 1.0 respectively, that can be attributed to the Néel temperatures of ferrimagneticto paramagnetic phase transition for all samples. Mössbauer spectra for the three sampleswere recorded between 4.2 and 295 K. Above the Néel temperature the paramagnetic spectracan be analyzed by three quadrupole doublets associated with the octahedral Fe1, Fe2 andFe4 sites. The values of the hyperfine parameters show that iron ions are in the high spinFe3+ state. The spectrum area of the doublet with larger quadrupole splitting increases withx, andin combination with x-ray diffraction results it can be attributed to the iron which occupies the Fe4 site.Below TN(x) the Mössbauer spectra are magnetically split and atT = 4.2 K consist of six broad lines, indicating either a hyperfine magnetic field distribution(P(Hhyp)) or that the three octahedral sites give three unresolved sextets. The most probable value ofHhyp (the maximumvalue of P(Hhyp)) follows a power law indicative of a second order transition, in agreementwith ac-susceptibility and magnetization measurements. The width ofP(Hhyp) increases drastically toward low hyperfine magnetic fields as temperature increases. In addition,an appreciable percentage of the iron nuclei sense a hyperfine field with values in the interval[0,Hmax]. This behavior can be explained by assuming that several magnetic sites with differentsuperexchange parameters exist.

Fe3+ intercalation/deposition on vermiculites and interaction with pyrrole: a Mössbauer spectroscopic investigation

AbstractThe Fe3+ uptake from aqueous and methanol solutions of FeCl3·6H2O in two vermiculites from Santa Olalla and Ojén (Spain) and in a montmorillonite from Wyoming has been investigated by Mössbauer spectroscopy at room temperature and 4.2 K. The samples were further characterized by X-ray diffraction. The low temperature Mössbauer spectra reveal the deposition of akaganeite and perhaps some ferrihydrite on the external surface of the clay minerals. The room temperature spectra exhibit two Fe3+ quadrupole doublets in addition to the Fe2+ components of the structural iron in the sheet silicates. On treatment in ⩾0.04 M Fe3+ solutions the Fe3+ quadrupole splittings of 0.90 mm/s and 0.55 mm/s can be assigned to the mineral akaganeite; at smaller Fe3+ concentrations one of these doublets has a quadrupole splitting similar to that of the Fe3+ in the original vermiculites, the other one a quadrupole splitting of 0.6–0.7 mm/s, which can be considered as a hint for an uptake of Fe dimers in the interlayer space irrespective of the solvent used for iron deposition. Preliminary results show that the adsorption of pyrrole in all of our Fe3+-containing samples, regardless of the preparation in aqueous or methanol solution, results in the formation of polypyrrole and a concomitant reduction of some of the Fe3+. An uptake of pyrrole into the interlayer space has been observed only in the Fe3+-containing Wyoming montmorillonite and in the Ojén vermiculite treated with the methanol solution of iron chloride.

Mössbauer study of synthetic jarosites

57Fe Mossbauer spectroscopy and PXRD were used to study artificially prepared jarosites with the compositions of KFe3(SO4)2(OH)6 − x F x x = 0 − 1.6 PXRD measurements revealed single phase jarosite samples. All Mossbauer spectra taken at room temperature exhibit a quadrupole doublet corresponding to mineral jarosite. However, at low temperature where the mineral jarosite has a well resolved sextet, the synthetic jarosite even with x = 0 F − content shows a relaxation transition. The spectra indicate that with increasing F − concentration, the paramagnetic–antiferromagnetic transition temperature is decreasing. The results can be used in the analysis of artificial jarosites or those formed during biomineralization processes.

Iron environment in calcium-soda-phosphate glasses and vitroceramics

The structure of calcium-soda-phosphate glasses and vitroceramics with relatively high iron content was investigated by X-ray diffraction, electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. The X-ray diffraction analysis proves the vitreous state of the as prepared samples and the development of crystalline phases in the annealed samples. The ferric ions disposed in sites that give rise to the absorption line with g ef ≈ 4.3 in the EPR spectra of vitreous samples are not more evidenced in the spectra of the annealed samples, from which only of a symmetric and narrowed line with g ef ≈ 2 is recorded. The room temperature 57Fe Mössbauer spectra both of glass and vitroceramic samples consist of two quadrupole doublets characteristic for octahedral sites of Fe 2+ and Fe 3+ ions. The isomer shift for glass samples decreases and for vitroceramic samples increases with the iron oxide content.

One-step aqueous synthesis of stoichiometric Fe–Cu nanoalloy

Fe 50 − x Cu 50 − x B 2 x alloy was obtained by an aqueous chemical reduction at pH = 7.0, using fast and slow NaBH 4 solution addition methods. According to X-ray diffraction patterns, TEM and HRTEM observations, smaller than 10 nm nanocrystals of fcc-FeCuB were formed when the fast method was employed. The room temperature Mössbauer spectrum of this FeCuB nanoalloy shows a quadrupole doublet with an isomer shift of 0.355(1) mm/s and a quadrupole splitting of 0.772(2) mm/s, and a broadened magnetic sextet with a quadrupole splitting of 0.020(3) mm/s and an isomer shift of 0.436(5) mm/s, exhibiting a distinctive high average magnetic field of 40.57(5) T. Grains with a size larger than 10 nm were obtained for a slow method nanoalloy, and its Mössbauer spectrum consisted only of a broadened sextuplet characteristic of an fcc-FeCuB alloy.

Mössbauer Evidence for an Exchange-Coupled {[Fe4S4]1+ Nip1+} A-Cluster in Isolated α Subunits of Acetyl-Coenzyme A Synthase/Carbon Monoxide Dehydrogenase

The active site A-cluster in the alpha subunit of the title enzyme consists of an Fe4S4 cluster coordinated to a [Nip Nid] subcomponent. The cluster must be activated for catalysis using low-potential reductants such as Ti(III) citrate. Relative to the inactive {[Fe4S4]2+ Nip2+ Nid2+} state, the activated state appears to be 2-electrons more reduced, but the location of these electrons within the A-cluster is uncertain, with {[Fe4S4]2+ Nip0 Nid2+} and {[Fe4S4]1+ Nip1+ Nid2+} configurations proposed. Recombinant apo-alpha subunits oligomerize after activation with NiCl2. The dimer fraction, upon reduction with excess Ti(III)citrate, exhibited Mössbauer spectra consisting of two quadrupole doublets representing 51% and 21% of the Fe, with parameters indicating [Fe4S4]1+ states. Spectra recorded in strong magnetic fields were typical of diamagnetic systems, indicating an exchange-coupled S = 0 {[Fe4S4]1+ Nip1+} state. Additional treatment with CO altered the doublet Mössbauer parameters, suggesting an interaction with CO, but maintaining the cluster in the {[Fe4S4]1+ Nip1+} state. Reduction with substoichiometric equivalents of Ti(III) citrate afforded an EPR signal typical of Ni1+ ions, with g parallel = 2.10 and g perpendicular = 2.02. Addition of more Ti caused the signal intensity to decline, suggesting that it arises from the semireduced {[Fe4S4]2+ Nip1+} state.

A study of ordinary chondrites by Mössbauer spectroscopy with high‐velocity resolution

Abstract— An improvement in the velocity resolution and quality of Mössbauer spectra has been applied to a group of ordinary chondrites. This improvement permitted us to carry out a more detailed study of the iron bearing phases in these samples than has previously been possible. Mössbauer spectra of 11 ordinary chondrites of L and H chemical groups were measured using 4096 channels and presented for further analysis in 1024 channels. Subspectra of the metal grains of several chondrites demonstrated the presence of at least two magnetic sextets related to the main Fe(Ni, Co) phases. Moreover, Mössbauer study of extracted metal grains from Tsarev L5 revealed three sextets and one singlet spectral components related to various α‐Fe(Ni, Co), α‘‐Fe(Ni, Co), α2‐Fe(Ni, Co), and γ‐Fe(Ni, Co) phases. Each subspectrum of olivine and pyroxene in Mössbauer spectra of ordinary chondrites was fitted by superposition of two quadrupole doublets related to M1 and M2 sites in minerals for the first time. An analysis of relative areas and Mössbauer hyperfine parameters was performed and some differences for L and H chondrites as well as for M1 and M2 sites were observed. Mössbauer parameters of troilite and oxidized iron were analyzed. In contrast to a previous study with 512‐channel spectra, the presence of oxidized iron was found in all chondrites.

A Two-Dimensional Iron(II) Carboxylate Linear Chain Polymer that Exhibits a Metamagnetic Spin-Canted Antiferromagnetic to Single-Chain Magnetic Transition

A two-dimensional iron(II) carboxylate coordination polymer, [Fe(pyoa)2]infinity, where pyoa is 2-(pyridin-3-yloxy)acetate, has been prepared by hydrothermal synthesis. Its crystal structure reveals a single iron(II) site with an elongated octahedral coordination environment containing four equatorial carboxylate oxygens and two axial pyridyl nitrogens; the iron(II) sites are linked by syn-anti micro-carboxylates to form chains along the b axis that have an Fe...Fe separation of 4.910 A. The shortest interchain and interlayer Fe...Fe distances are 6.453 and 11.125 A, respectively. The 4.2-295 K Mössbauer spectra of [Fe(pyoa) 2] infinity consist of a single paramagnetic high-spin iron(II) quadrupole doublet. The axial Fe-N bond direction defines the Jahn-Teller axis at an iron(II) site and, consequently, the orientation of the single-ion magnetic anisotropy. Thus, along the b axis in a given chain, the spins are collinear and parallel to the Jahn-Teller axis. The Jahn-Teller axes of adjacent intralayer chains have different orientations with an angle of 79.2 degrees between the axes in adjacent chains in a bc layer. [Fe(pyoa)2]infinity exhibits field-induced metamagnetic behavior such that, in an applied field smaller than the critical field, the iron(II) spin-canted moments experience intrachain ferromagnetic interactions and weak interchain antiferromagnetic interactions; the spin canting yields weak ferromagnetism. In an applied field larger than the critical field, the weak antiferromagnetic interchain interactions are overwhelmed to yield superparamagnetic-like slow-magnetic relaxation with an energy barrier of 23(3) K. Single-crystal magnetic studies reveal a quasi-uniaxial magnetic anisotropy with the a axis as the easy-magnetic axis and the b axis as the hard-magnetic axis; the susceptibility measured along the easy a axis may be fit with the Glauber model to yield an effective intrachain exchange coupling constant of 2.06(8) K. A dynamic analysis of the susceptibility yields a 6.3(1) K energy barrier for intrachain domain wall creation. The observed field-assisted superparamagnet-like behavior is consistent with the dynamics of a single-chain magnet. Thus, [Fe(pyoa)2]infinity is best considered as a "metamagnetic-like" single-chain magnet.

(TAML)FeIV═O Complex in Aqueous Solution: Synthesis and Spectroscopic and Computational Characterization

Recently, we reported the characterization of the S = (1)/ 2 complex [Fe (V)(O)B*] (-), where B* belongs to a family of tetraamido macrocyclic ligands (TAMLs) whose iron complexes activate peroxides for environmentally useful applications. The corresponding one-electron reduced species, [Fe (IV)(O)B*] (2-) ( 2), has now been prepared in >95% yield in aqueous solution at pH > 12 by oxidation of [Fe (III)(H 2O)B*] (-) ( 1), with tert-butyl hydroperoxide. At room temperature, the monomeric species 2 is in a reversible, pH-dependent equilibrium with dimeric species [B*Fe (IV)-O-Fe (IV)B*] (2-) ( 3), with a p K a near 10. In zero field, the Mössbauer spectrum of 2 exhibits a quadrupole doublet with Delta E Q = 3.95(3) mm/s and delta = -0.19(2) mm/s, parameters consistent with a S = 1 Fe (IV) state. Studies in applied magnetic fields yielded the zero-field splitting parameter D = 24(3) cm (-1) together with the magnetic hyperfine tensor A/ g nbeta n = (-27, -27, +2) T. Fe K-edge EXAFS analysis of 2 shows a scatterer at 1.69 (2) A, a distance consistent with a Fe (IV)O bond. DFT calculations for [Fe (IV)(O)B*] (2-) reproduce the experimental data quite well. Further significant improvement was achieved by introducing hydrogen bonding of the axial oxygen with two solvent-water molecules. It is shown, using DFT, that the (57)Fe hyperfine parameters of complex 2 give evidence for strong electron donation from B* to iron.

Mössbauer spectroscopy study of aluminium substituted nickel-ferrites

Powders of Al substituted Ni-ferrites (NiAl x Fe2 − x O4 x = 0, 0.5, 1.0 and 1.5) were prepared using the calcination method. Mossbauer spectroscopy results of all samples show two well defined magnetic sextets corresponding to Fe3 + ions in the A (tetrahedral) and B (octahedral) sites of the spinel structure. In addition, a quadrupole doublet with quadrupole splitting of (~0.53 mm/s) starts evolving upon Al substitution. This doublet is associated with a new Fe–Al-oxide phase. The X-ray diffraction patterns show, in addition to the original peaks, new peaks evolve and increase in intensity as the Al concentration increases, confirming the evolution of this new phase.

X-ray diffraction and Mössbauer spectroscopy of high energy ball-milled α-Fe2O3/TiO2 composite powders

α–Fe2O3/TiO2 Composite powders have been prepared by high energy ball-milling for different times. The composites were studied using Mossbauer Spectroscopy (MS) and X-ray diffraction (XRD). The patterns of XRD show broadening in the diffraction peaks, indicating a decrease in the particle size of the composites with milling time. Also, the XRD patterns show an evolving new structural phase correlated with an evolving Titanium ferrite species with milling time. Mossbauer Spectroscopy shows the evolving titanium ferrite species characterized by a quadrupole doublet at the expense of the α–Fe2O3 represented by the magnetic sextet. The doublet corresponding to the Ti-ferrite phase dominates the Mossbauer spectra at long milling time (greater than 100 h of milling).

Combined Analysis of C-18 Unsaturated Fatty Acids Using Natural Abundance Deuterium 2D NMR Spectroscopy in Chiral Oriented Solvents

The quantitative determination of isotopic (2H/1H)i ratios at natural abundance using the SNIF-NMR protocol is a well-known method for understanding the enzymatic biosynthesis of metabolites. However, this approach is not always successful for analyzing large solutes and, specifically, is inadequate for prochiral molecules such as complete essential unsaturated fatty acids. To overcome these analytical limitations, we use the natural abundance deuterium 2D NMR (NAD 2D NMR) spectroscopy on solutes embedded in polypeptide chiral liquid crystals. This approach, recently explored for measuring (2H/1H)i ratios of small analytes (Lesot, P.; Aroulanda, C.; Billault, I. Anal. Chem. 2004, 76, 2827-2835), is a powerful way to separate the 2H signals of all nonequivalent enantioisotopomers on the basis both of the 2H quadrupolar interactions and of the 2H chemical shift. Two significant advances over our previous work are presented here and allow the complete isotopic analysis of four mono- and polyunsaturated fatty acid methyl esters: methyl oleate (1), methyl linoleate (2), methyl linolenate (3), and methyl vernoleate (4). The first consists of using NMR spectrometers operating at higher magnetic field strength (14.1 T) and equipped with a selective cryoprobe optimized for deuterium nuclei. The second is the development of Q-COSY Fz 2D NMR experiments able to produce phased 2H 2D maps after a double Fourier transformation. This combination of modern hardware and efficient NMR sequences provides a unique tool to analyze the (2H/1H)i ratios of large prochiral molecules (C-18) dissolved in organic solutions of poly(gamma-benzyl-L-glutamate) and requires smaller amounts of solute than previous study on fatty acids. For each compound (1-4), all 2H quadrupolar doublets visible in the 2D spectra have been assigned on the basis of 2H chemical shifts, isotopic data obtained from isotropic quantitative NAD NMR, and by an interspectral comparison of the anisotropic NAD spectra of four fatty acids. The NMR results are discussed in terms of (2H/1H)i isotopic distribution and molecular orientation in the mesophase. For the first time, we show that the investigation of natural isotopic fractionation of complete fatty acids is possible without the need of chemical modifications, hence providing an alternative method to probe the mechanisms of enzymes implied in the biosynthetic pathway of unsaturated fatty acids.

Magnetic quadrupole doublet focusing system for high energy ions

A high energy focused ion beam microprobe using a doublet arrangement of short magnetic quadrupole lenses was used to focus 1-3 MeV protons to spot sizes of 1x1 microm2 and 1-4.5 MeV carbon and silicon ion beams to spot sizes of 1.5x1.5 microm2. The results presented clearly demonstrate that this simple doublet configuration can provide high energy microbeams for microanalysis and microfabrication applications.

Mössbauer and IR analysis of iron sites in four ferruginous smectites

Abstract57Fe Mössbauer spectra of the Na+- and Ca2+-saturated forms of four ferruginous smectites,displaying a range of compositions, have been fitted with Voigtian profiles to represent (principally) two correlated distributions of isomer shifts and quadrupole splittings for the Fe3+ sites. Correlating the areas and hyperfine parameters of these distributions with simulations of the octahedral array of cations based on chemical analyses and infrared spectroscopy has enabled identification of the Fe3+ ions that contribute to the outer quadrupole doublet. The octahedral Fe3+ sites are characterized as having at least one Mg2+ nearest-cation neighbour and probably a cisconfiguration with respect to the OH– ions.

2H 2O quadrupolar splitting used to measure water exchange in erythrocytes

The 2H NMR resonance from HDO (D = 2H) in human red blood cells (RBCs) suspended in gelatin that was held stretched in a special apparatus was distinct from the two signals that were symmetrically arranged on either side of it, which were assigned to extracellular HDO. The large extracellular splitting is due to the interaction of the electric quadrupole moment of the 2H nuclei with the electric field gradient tensor of the stretched, partially aligned gelatin. Lack of resolved splitting of the intracellular resonance indicated greatly diminished or absent ordering of the HDO inside RBCs. The separate resonances enabled the application of a saturation transfer method to estimate the rate constants of transmembrane exchange of water in RBCs. However both the theory and the practical applications needed modifications because even in the absence of RBCs the HDO resonances were maximally suppressed when the saturating radio-frequency radiation was applied exactly at the central frequency between the two resonances of the quadrupolar HDO doublet. More statistically robust estimates of the exchange rate constants were obtained by applying two-dimensional exchange spectroscopy (2D EXSY), with back-transformation analysis. A monotonic dependence of the estimates of the efflux rate constants on the mixing time, t mix, used in the 2D EXSY experiment were seen. Extrapolation to t mix = 0, gave an estimate of the efflux rate constant at 15 °C of 31.5 ± 2.2 s −1 while at 25 °C it was ∼50 s −1. These values are close to, but less than, those estimated by an NMR relaxation-enhancement method that uses Mn 2+ doping of the extracellular medium. The basis for this difference is thought to include the high viscosity of the extracellular gel. At the abstract level of quantum mechanics we have used the quadrupolar Hamiltonian to provide chemical shift separation between signals from spin populations across cell membranes; this is the first time, to our knowledge, that this has been achieved.

A synthetic precedent for the [Fe IV 2 (μ-O) 2 ] diamond core proposed for methane monooxygenase intermediate Q

Intermediate Q, the methane-oxidizing species of soluble methane monooxygenase, is proposed to have an [Fe(IV)(2)(mu-O)(2)] diamond core. In an effort to obtain a synthetic precedent for such a core, bulk electrolysis at 900 mV (versus Fc(+/0)) has been performed in MeCN at -40 degrees C on a valence-delocalized [Fe(III)Fe(IV)(mu-O)(2)(L(b))(2)](3+) complex (1b) (E(1/2) = 760 mV versus Fc(+/0)). Oxidation of 1b results in the near-quantitative formation of a deep red complex, designated 2b, that exhibits a visible spectrum with lambda(max) at 485 nm (9,800 M(-1).cm(-1)) and 875 nm (2,200 M(-1).cm(-1)). The 4.2 K Mössbauer spectrum of 2b exhibits a quadrupole doublet with delta = -0.04(1) mm.s(-1) and DeltaE(Q) = 2.09(2) mm.s(-1), parameters typical of an iron(IV) center. The Mössbauer patterns observed in strong applied fields show that 2b is an antiferromagnetically coupled diiron(IV) center. Resonance Raman studies reveal the diagnostic vibration mode of the [Fe(2)(mu-O)(2)] core at 674 cm(-1), downshifting 30 cm(-1) upon (18)O labeling. Extended x-ray absorption fine structure (EXAFS) analysis shows two O/N scatterers at 1.78 A and an Fe scatterer at 2.73 A. Based on the accumulated spectroscopic evidence, 2b thus can be formulated as [Fe(IV)(2)(mu-O)(2)(L(b))(2)](4+), the first synthetic complex with an [Fe(IV)(2)(mu-O)(2)] core. A comparison of 2b and its mononuclear analog [Fe(IV)(O)(L(b))(NCMe)](2+) (4b) reveals that 4b is 100-fold more reactive than 2b in oxidizing weak C H bonds. This surprising observation may shed further light on how intermediate Q carries out the hydroxylation of methane.

Syntheses, Structure, and a Mössbauer and Magnetic Study of Ba4Fe2I5S4

The compound Ba4Fe2I5S4 has been prepared at 1223-1123 K by the "U-assisted" reaction of FeS, BaS, S, and U with BaI2 as a flux. A more rational synthesis was also found; however, the presence of U appears to be essential for the formation of single crystals suitable for X-ray diffraction studies. Ba4Fe2I5S4 crystallizes in a new structure type with two formula units in space group I4/m of the tetragonal system. The structure consists of a Ba-I network penetrated by (1)infinity[Fe2S4] chains. Each Fe atom, which is located on a site with 4 symmetry, is tetrahedrally coordinated to four S atoms. The FeS4 tetrahedra edge-share to form linear (1)infinity[Fe2S4] chains in the [001] direction. The Fe-Fe interatomic distance in these chains is 2.5630(4) A, only about 3% longer than the shortest Fe-Fe distance in -Fe metal. Charge balance dictates that the average formal oxidation state of Fe in these chains is +2.5. The Mössbauer spectra obtained at 85 and 270 K comprise a single quadrupole doublet that has hyperfine parameters consistent with an average Fe oxidation state of +2.5. The Mössbauer spectrum obtained at 4.2 K consists of a single magnetic sextet with a small hyperfine field of -15.5 T. This spectrum is also consistent with rapid electron delocalization and an average Fe oxidation state of +2.5. The molar magnetic susceptibility of Ba4Fe2I5S4, obtained between 3.4 and 300 K, qualitatively indicates the presence of weak pseudo-one-dimensional ferromagnetic exchange within a linear chain above 100 K and weak three-dimensional ordering between the chains at lower temperatures.