Pair Of Iron Atoms Research Articles

Orbital and magnetic ordering in single-layer FePS3 : A DFT+U study

Among the numerous two-dimensional (2D) systems that can be prepared via exfoliation, iron phosphorus trisulfide $({\mathrm{FePS}}_{3})$ attracted a lot of attention recently due to its broad-range photoresponse, its unusual Ising-type magnetic order and possible applications in spintronic nanodevices. Despite various experimental and theoretical-computational reports, there are still uncertainties in identifying its magnetic ground state. In this paper, we investigate the structural and magnetic properties of single-layer ${\mathrm{FePS}}_{3}$ by using density functional theory. Our findings show that orbital ordering leads to a variation in distance between pairs of iron atoms by 0.14 \AA{}. These lattice distortions, albeit small, trigger different (ferromagnetic and antiferromagnetic) exchange couplings so that the ground state consists of ferromagnetically aligned zigzag chains along the long Fe-Fe bonds which couple antiferromagnetically along the shorter Fe-Fe bonds. Within the $\mathrm{DFT}+U$ framework, we parametrize a spin Hamiltonian including Heisenberg, single-ion anisotropy, Dzyaloshinskii-Moriya, and biquadratic interactions. Using $U=2.22$ eV gives a consistent description of both the electronic band gap and the N\'eel temperature in 2D ${\mathrm{FePS}}_{3}$.

Hydride Conformers of the Nitrogenase FeMo-cofactor Two-Electron Reduced State E2(2H), Assigned Using Cryogenic Intra Electron Paramagnetic Resonance Cavity Photolysis.

Early studies in which nitrogenase was freeze-trapped during enzymatic turnover revealed the presence of high-spin (S = 3/2) electron paramagnetic resonance (EPR) signals from the active-site FeMo-cofactor (FeMo-co) in electron-reduced intermediates of the MoFe protein. Historically denoted as 1b and 1c, each of the signals is describable as a fictitious spin system, S′ = 1/2, with anisotropic g′ tensor, 1b with g′ = [4.21, 3.76, ?] and 1c with g′ = [4.69, ∼3.20, ?]. A clear discrepancy between the magnetic properties of 1b and 1c and the kinetic analysis of their appearance during pre-steady-state turnover left their identities in doubt, however. We subsequently associated 1b with the state having accumulated 2[e–/H+], denoted as E2(2H), and suggested that the reducing equivalents are stored on the catalytic FeMo-co cluster as an iron hydride, likely an [Fe–H–Fe] hydride bridge. Intra-EPR cavity photolysis (450 nm; temperature-independent from 4 to 12 K) of the E2(2H)/1b state now corroborates the identification of this state as storing two reducing equivalents as a hydride. Photolysis converts E2(2H)/1b to a state with the same EPR spectrum, and thus the same cofactor structure as pre-steady-state turnover 1c, but with a different active-site environment. Upon annealing of the photogenerated state at temperature T = 145 K, it relaxes back to E2(2H)/1b. This implies that the 1c signal comes from an E2(2H) hydride isomer of E2(2H)/1b that stores its two reducing equivalents either as a hydride bridge between a different pair of iron atoms or an Fe–H terminal hydride.

Ferromagnetism in Fe2CrAl nanowires

The authors have successfully grown Fe2CrAl nanowires on polished Si/SiO2 substrates using the electrospinning method. The diameter of nanowires varies from 50 to 300 nm. These nanowires exhibit a cubic crystal structure with lattice disorder. The nanowires are ferromagnetic with a Curie temperature greater than 400 K, much higher than that the bulk reported value. This higher Curie temperature is attributable to disordering in lattice sites, which causes an enhanced exchange interaction between pairs of iron atoms.

Bioinspired Hydrogenase Models: The Mixed-Valence Triiron Complex [Fe3(CO)7(μ-edt)2] and Phosphine Derivatives [Fe3(CO)7-x (PPh3) x (μ-edt)2] (x = 1, 2) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] as Proton Reduction Catalysts.

The mixed-valence triiron complexes [Fe3(CO)7–x(PPh3)x(μ-edt)2] (x = 0–2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution occurs at the apical positions of the outer iron atoms, while the diphosphine complexes exist only in the dibasal form in both the solid state and solution. The carbonyl on the central iron atom is semibridging, and this leads to a rotated structure between the bridged diiron center. IR studies reveal that all complexes are inert to protonation by HBF4·Et2O, but addition of acid to the pentacarbonyl complexes results in one-electron oxidation to yield the moderately stable cations [Fe3(CO)5(PPh3)2(μ-edt)2]+ and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2]+, species which also result upon oxidation by [Cp2Fe][PF6]. The electrochemistry of the formally Fe(I)–Fe(II)–Fe(I) complexes has been investigated. Each undergoes a quasi-reversible oxidation, the potential of which is sensitive to phosphine substitution, generally occurring between 0.15 and 0.50 V, although [Fe3(CO)5(PPh3)2(μ-edt)2] is oxidized at −0.05 V. Reduction of all complexes is irreversible and is again sensitive to phosphine substitution, varying between −1.47 V for [Fe3(CO)7(μ-edt)2] and around −1.7 V for phosphine-substituted complexes. In their one-electron-reduced states, all complexes are catalysts for the reduction of protons to hydrogen, the catalytic overpotential being increased upon successive phosphine substitution. In comparison to the diiron complex [Fe2(CO)6(μ-edt)], [Fe3(CO)7(μ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials. Electronic structure calculations have been carried out in order to fully elucidate the nature of the oxidation and reduction processes. In all complexes, the HOMO comprises an iron–iron bonding orbital localized between the two iron atoms not ligated by the semibridging carbonyl, while the LUMO is highly delocalized in nature and is antibonding between both pairs of iron atoms but also contains an antibonding dithiolate interaction.

Influence of the Silicon Composition on the Crystallographic Properties of the New Ferromagnetic Ternary Silicides U 2Fe 17- ySi y (3.3 ≤ y ≤ 4.5) and U 2Co 15Si 2

After melting and quenching, the ternary silicides U 2Fe 17- y Si y exist far 3.3 ≤ y ≤ 4.5 but they partially decompose after annealing at 850-900°C. Their crystal structure, determined by X-ray diffractometry on both single crystals and powder, derives from the hexagonal Th 2Ni 17-type but depends strongly on the silicon content. For y = 3.7 some uranium sites are partially replaced by pairs of iron atoms and conversely. This structure shows some similarities to that observed for the binary compound Ho 2Fe 17. On the other hand, for y = 4.2, all the uranium atoms and pairs of iron atoms are statistically distributed. In contrast, the ternary silicide U 2Co 15Si 2, which is obtained as single phase after annealing at 850°C, adopts the Th 2Ni 17-type structure. In all compounds, iron or cobalt atoms of the pair are never substituted by silicon atoms.

MÖSSBAUER AND SUSCEPTIBILITY STUDY OF A SPIN-COUPLED HIGH-SPIN FERROUS SYNTHETIC HEME COMPOUND

High-field, variable-temperature Mossbauer and magnetic susceptibility data have been recorded for the synthetic five-coordinate ferrous heme [Fe(TPP)(OH)]—as its polycrystalline salt [Na(dibenzo-18-crown-6)(THF)2][Fe(TPP)OH)]—(TPP=tetraphenylporphyrinate, OH= hydroxide). This compound is a model for the P-460 center of hydroxylamine oxidoreductase from Nitrosomonas europae, an ammonia-oxidizing bacterium. An analysis of the data suggests that the extra d-electron of the iron is located in an oblate orbital. The data are also consistent with an intermolecular spin-coupling between pairs of iron atoms. Although a crystal structure is not available for this compound, a structure determination for a closely related compound is consistent with a pairwise iron-iron exchange but does not rule out more complex schemes.

Analysis of the high-spin states of the 2[4Fe-4Se]+ ferredoxin from Clostridium pasteurianum by Mössbauer spectroscopy.

A Mössbauer study of the three spin states of the reduced selenium-substituted 2[4Fe-4Se]+ ferredoxin from Clostridium pasteurianum was carried out at T = 1.6 K, in perpendicular and parallel applied magnetic fields of up to 10 T. In low (0.1 T) applied fields, the spectra of the classical S = 1/2 state exhibit magnetic hyperfine patterns arising from two pairs of iron atoms with opposite hyperfine fields. The S = 7/2 state is in the slow relaxation limit at T less than or equal to 4 K in the absence of applied field. The corresponding Mössbauer spectrum can be understood in terms of anti-parallel coupling between one high-spin Fe3+ ion (Hhf = +21.2 T) and three high-spin Fe2+ ions (Hhf,xy = -25.5 T, Hhf,z = -28.5 T). For the S = 3/2 spin state, the use of high (8-10 T) applied fields was necessary to ensure the validity of the high-field approximation and overcome the intercluster spin-spin interaction. The spectral data obtained under such conditions allowed the determination of the hyperfine field (-4.2 T) and a tentative estimation of the zero-field splitting (D approximately less than 3 cm-1).

Interpretation of the Mössbauer spectra of the high-potential iron protein from Chromatium.

The Mössbauer spectra of both reduced and oxidized high‐potential iron protein (Hipip) from Chromatium have been analysed using computer fits to theoretical spectra derived from a spin Hamiltonian. Fits to spectra obtained over a range of temperatures between 4.2 and 195 K and in applied magnetic fields up to 10.0 T lead to a consistent set of hyperfine parameters.These results are interpreted in terms of a model of the four‐iron four‐sulphide active centre which is consistent with its electronic and magnetic properties in both redox states. In the model for the reduced centre all four iron atoms have essentially the same valence, intermediate between ferric and ferrous, with the spins being coupled antiferromagnetically to give the centre zero net spin. The oxidized centre has one less electron which at low temperatures appears to have come predominantly from one pair of iron atoms which thus become ferric with the other pair remaining substantially unchanged. It is clear from the Mössbauer hyperfine parameters obtained from the computer fits to the low‐temperature spectra that a larger magnetic moment is associated with the ferric/ferrous pair of iron atoms than with the ferric pair of iron atoms. This also explains the g values with an average of greater than 2 which are observed in electron paramagnetic resonance (EPR) measurements. At higher temperatures the differences between the electron charge density at the different iron atoms in the oxidized centre appear to become smeared out.

Interpretation of the Mössbauer spectra of the four-iron ferredoxin from Bacillus stearothermophilus.

The Mössbauer spectra of both oxidized and reduced ferredoxin from Bacillus stearothermophilus have been analysed using computer fits to theoretical spectra obtained from a spin Hamiltonian. A consistent set of parameters was obtained from fits to spectra obtained over a wide range of temperature and magnetic field. These results are interpreted in terms of a model for the active centre which is consistent with its electronic and magnetic properties in both redox states. In the model for the oxidized centre all four iron atoms have essentially the same valence, intermediate between ferric and ferrous, with one pair spin-up and the other pair spin-down. On reduction the extra electron goes predominantly to one pair of iron atoms which become ferrous with the other pair remaining substantially unchanged. Using this model it is possible to obtain relationships between the spin Hamiltonian parameters for individual iron atoms and those for the coupled centre. This can give further insight into the relation between the observed electron paramagnetic resonance and Mössbauer spectra.

Structure of the iron complex in methemerythrin.

The coordination of the ligands about the iron atoms in methemerythrin from Themiste dyscritum has been deduced from a 2.8 A resolution electron density map. The complex can be described in terms of two trigonal antiprisms about the pair of iron atoms in each subunit, the antiprisms having one face in common. Ligands at eight of the nine coordination positions are protein side chains, the ninth presumably being water. Comparison of the electron density map for T. dyscritum methemerythrin with the sequence of Phascolopsis gouldii hemerythrin suggests six aromatic side chain ligands (five histidine and one tyrosine) and two nonaromatic side chain ligands. The latter provide atoms at two of the three vertices of the face shared by the two antiprisms, and these along with the presumed water at the third vertex form bridges between the iron atoms of each pair.

Applications of the Mössbauer Effect in Biophysics

The Mössbauer effect may be applied to biology in two ways: (i) to study the electronic state of an atom in a biological molecule and (ii) as a method of chemical identification. The former can be very powerful since many biological molecules have a transition-metal atom at the active center, and hence data on its chemical state and on the electron transfer which takes place during a biochemical reaction may be obtained from magnetic measurements. Mössbauer effect measurements of the hyperfine interaction of 57Fe provide detailed information on the electron distribution in iron atoms, even when the molecular structure is not known. A review is given of some molecules to which this technique has been applied. The most extensive measurements have been made on hemoglobin and its derivatives and on the iron-sulfur proteins. In the latter, pairs of iron atoms are antiferromagnetically coupled. As an example of possible applications in analysis the Mössbauer spectra of iron from a healthy and a hemosiderosis lung are compared and found to be very different.