Valence states of 57Fe decayed from 57Mn implanted into KMnO4

In-beam Mössbauer spectra of ⁵⁷Fe, decayed from short-lived ⁵⁷Mn (T_1/2 = 1.45 min) implanted into potassium permanganate, KMnO₄, were measured at temperatures between 11 K and 130 K. This is the first application of a secondary RI beam to the study of valence states after nuclear transformation. The in-beam Mössbauer spectra obtained below 90 K could be analyzed with two components, a doublet and a singlet. From the calculations of the molecular orbital wave functions, the singlet is suggested to be substitutional ⁵⁷Fe atoms for Mn-sites in tetrahedral [MnO₄]^– with an unusually high valence state of Fe⁸⁺.     

57Fe Mössbauer study of volcanic hornblende

A Mössbauer study has been made of Fe sites in volcanic hornblende of chemical composition (Si_5.7Al_2.3) (Al_1.5Ti_0.4Fe_0.5³⁺Fe_0.6²⁺Mn_0.01Mg_3.2)(Ca_2.0Na_0.5K_0.4P_0.01)(O, OH, F)₃₂ obtained from inside a volcanic bomb in the ash ejecta of Eifel region in Germany. The spectrum has been computer fitted to two quadrupole doublets and assigned to Fe²⁺ and Fe³⁺ oxidation states at the octahedral M₁, M₂ and M₃ sites; the M₄ site being considered to be occupied primarily by Ca atoms.     

Zur simultanen Bestimmung von 55Fe und 59Fe in biologischen Proben

Ohne Zusammenfassung     

Direct observation of 57Fe chemical shifts in diamagnetic organoiron complexes

⁵⁷Fe NMR spectra of 35 organoiron complexes predominantly of the type [Fe(CO)₃(diene)] have been measured by direct detection at 2.9 MHz and at natural isotope abundance (2.2%). The ⁵⁷Fe chemical shift range so far observed is 3000 ppm, corresponding to about 9 KHz. The majority of the resonances are at higher frequency than that of Fe(CO)₅, which is proposed as a secondary standard. The ⁵⁷Fe resonance frequencies are also reported as ratios relative to the ⁷³Ge frequency of GeCl₄.The ⁵⁷Fe shielding is discussed qualitatively in terms of charge distribution in the complexes, very large deshielding effects being observed for cationic olefinic ligands. For η⁴-diene complexes, the shielding also depends upon ligand geometry and decreases with increasing ring size or CCC bond angle. Two types of ferrocenylcarbenium ions may be distinguished, indicated by high- or low-frequency shifts relative to ferrocene. For a number of [Fe(CO)₃(olefin)] complexes ¹J(⁵⁷Fe, ¹³C) coupling constants involving the carbonyl carbon are also reported.     

Mössbauer 57Fe spectra exhibiting “ferrous character”

The term “ferrous character” reported extensively in the literature for the Mössbauer spectroscopy of octahedrally coordinated iron atoms in chalcogenides and antimonides is critically evaluated. The significance of a formal valence state for the iron is examined. The magnitude of the isomer shift is shown to provide a guide not only to the existence of high-spin versus low-spin states, but also to localized versus itinerant electrons or to extended electrons within FeXM interactions (M = transition metal and X = anion). The existence of a quadrupole splitting for iron atoms in intrinsically cubic fields is only possible where the β-spin electron outside a closed α-spin half-shell is localized and Jahn-Teller coupled to lattice vibrations to form vibronic states. This situation is distinguished from quadrupolar fields associated with local-site symmetries deformed from cubic symmetry as well as from the case where the formation of itinerant β-spin electrons inhibits the formation of vibronic states.     

57Fe NMR Chemical shifts and 57fe, 13C coupling constants in α-ferrocenyl carbocations. Direct metal participation in the stabilization of metallocenyl carbocations

The ¹³C{⁵⁷Fe} double resonance method has been used to investigate ⁵⁷Fe-enriched samples of ferrocene derivatives, α-ferrocenyl carbocations and carbonyl complexes with various σ- and π-hydrocarbon ligands. In the α-ferrocenyl carbocations the ⁵⁷Fe resonances span a 1200 ppm range, being a sensitive tool of direct iron participation in the stabilization. The ⁵⁷Fe resonances in the carbocations [FcCH₂][HSO₄] (I), [FcCHMe][HSO₄] (II), [FcCHPh][HSO₄] (III), [FcCHC₅H₄ Mn(CO)₃][CF₃CO₂] (IV), [(Fc)₂CH][BF₄] (V), [FcCHC₅H₄RuC₅Hs][BF₄] (VI) and [FcCMe₂][HSO₄] (VII), ?523.6, ?219.3, 221.0, 368.7, 699.0, 405.0 and 288.5 ppm, respectively, relative to ferrocene, are interpreted in terms of rehybridization of the iron non-bonding d orbitals (shielding effect and the electron withdrawing effect of the substituent in the cyclic ligand (deshielding effect). The role of rehybridization of non-bonding iron orbitals in the low-frequency shift of the ⁵⁷Fe resonances, in addition to that in the previously investigated complex [(C₅H₅)₂FeH][BF₃OH] (?1109.3 ppm), has been demonstrated for bridge-substituted [3]ferrocenophanes, whose ring tilting induces a low-frequency shift of up to 340 ppm relative to their unbridged analogues.The ¹³C NMR spectra of carbocations V and VI reveal a temperature dependence due to the rotation around the C(1)C_α exocyclic bonds. In carbocation VI the ruthenium atom effectively competes with the iron atom to bond with C_α whereas in carbocation V two equivalent metal atoms possess the same ability for such bonding; as a result, complex V has a more pronounced “carbenium ion” nature than IV and VI, as indicated by the relative positions of the ¹³C_α resonances in carbocations IV, V and VI: δ 122.4, 147.2 and 116.9 ppm, respectively.The values of ⁵⁷Fe, ¹³C coupling constants for α-ferrocenyl carbocations exclude Fe-C_α σ-bonding and support a structure in which the iron atom is π-bonded with six carbon atoms of a fulvenoid ligand. According to the data on ⁵⁷Fe resonances and ⁵⁷Fe, ¹³C coupling constants in α-ferrocenyl carbocations the strength of FeC_α bonding is markedly influenced by the electronic effect of the substituent at C_α, being even lower in carbocation I than that of Fe-cyclopentadienyl carbon atoms.     

Overlap contribution to internal conversion coefficients of the 14.4 keV M1 transition in 57Fe

It is suggested that while evaluating contact charge densities from internal conversion coefficients it is important to consider overlap effects. Ratios of the internal conversion coefficients

and

are evaluated as a function of interatomic separation for Fe³⁺ in an octahedral arrangement of Br^? ions.     

Hyperfine splitting of 57Fe3+ Mössbauer spectra in diamagnetic host lattices

The Mössbauer spectra of ⁵⁷Fe³⁺ substituted in 0.2 to 0.4 atom % of the octahedral sites in the host lattices Na_0.9Mg_0.45Ti_1.55O₄, TiO₂, Sc₂O₃, LiScO₂ and MgAl₂O₄ have been examined. Hyperfine splittings of the spectra were observed for all cases except Sc₂O₃. In all cases the electron spin resonance spectrum was found, or has been shown to be highly anisotropic as a result of the presence of strong axial or rhombic fields and consequent splitting of the 3d⁵ ground state energy levels. In several cases resolution of the $\text{±}\text{5}\text{2}$ and $\text{±}\text{3}\text{2}$ Mössbauer spectra was observed. Mössbauer spectra of the solid solutions Na_0.9Mg_0.45Ti_1.55O₄Na_0.9Fe_0.9Ti_1.1O₄, LiScO₂LiFeO₂ and TiO₂FeNbO₄ have also been examined.     

Mössbauer 57Fe isomer shift as a measure of valence in mixed-valence iron sulfides

An empirical formula relating the room-temperature isomer shift δ for high-spin iron in sulfur coordination to the effective iron valence 〈m +〉, of the form δ = A ? B 〈m +〉, has been refined for tetrahedral coordination and modified by an inclusion of orbital degeneracy into the definition of 〈m+〉; it has also been extended to include octahedral coordination by adding 0.17 mm/sec to A. For a given 〈m+〉, the I.S. is shown to be sensitive to nearest-neighbor anion coordination, but much less sensitive than the hyperfine field to the competitive bonding with these anions. However, the sign of the deviation from the empirical line indicates the direction of any net superexchange charge transfer to or from the iron atom. The utility of the relationship is illustrated by application to a wide range of problems.     

Die chromatographische Trennung und Bestimmung von 51Cr, 54Mn, 60Co und 59Fe

Ohne Zusammenfassung     

Anomalous spin state of iron(II) in 57Fe Mössbauer emission spectra of 57Co-Labeled tris(2-methylphenanthroline) cobalt(II) perchlorate

⁵⁷Fe Mössbauer emission spectra of the ⁵⁷Co labeled complex compound [⁵⁷Co(2-CH₃-phen)₃] (ClO₄)₂·2H₂O have been measured as a function of temperature between 293 and 4.6 K. The spectra exclusively show high-spin iron(II) resonances beside a small fraction of an high-spin iron(II) species, whereas the corresponding iron(II) compound is known to exhibit thermally induced high-spin ⁵T_2g(O_h) ? low spin ¹A_1g(O_h) transition. The electronic nature of the anomalous spin state has been found to be ⁵A₁(D₃) by a theoretical treatment of the temperature dependence of the quadrupole splitting. The results are in good agreement with those obtained from Mössbauer absorption measurements on [⁵⁷Fe_0.01Co_0.99(2-CH₃-(phen)₃] (ClO₄)₂·2H₂O.     

Eu0.5RE0.5Fe0.5Mn0.5O3的57Fe Msosbauer谱研究

利用固相反应合成了Eu0.5RE0.5Fe0.5Mn0.5O3(RE=La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Y)等化合物。测量了其XRD谱及57FeMssbauer谱。实验发现,随着RE原子序数的增加,样品的晶胞体积减小,Fe在化合物中处于Fe3+的高自旋状态,57Fe的四极裂矩与样品的畸变参数D成线性关系。     

Origin of the quadrupole splitting in the Mössbauer 57Fe spectrum of cubic (disordered) LiFeO2

The room-temperature Mössbauer ⁵⁷Fe spectrum of polycrystalline disordered cubic α-LiFeO₂ contains a quadrupole splitting |Δ_obs| of 0.65(2) mm/sec. This value is relatively large for an Fe atom in an essentially Fe³⁺_HS state. To account for its magnitude, the distribution of the electric-field gradient (EFG) values associated with the Fe atoms was investigated by means of exact geometric analysis involving the 12 nearest cation neighbors (model A) as well as large-scale computer simulation involving more distant cations (models B to E). It is found that (1) the major contribution to |Δ_obs| comes from the distribution of +1 and +3 charges among the 12 nearest cation neighbors of a reference Fe atom; (2) this contribution by itself largely accounts for |Δ_obs|; (3) the contribution from cations beyond the seventh-nearest neighbors is marginal; (4) displacing the oxygen atoms from their lattice sites toward adjacent Fe atoms produces a significant effect on the distribution of EFG values at a reference Fe atom, while incipient cation ordering appears to have a relatively small effect; and (5) the contribution of the EFG = 0 component to model |EFG| distributions will be overemphasized unless cations beyond the first-nearest neighbors are included in the EFG summation. The 144 distinct (up to rotation and reflection) Li¹⁺_12?kFe³⁺_k configurations on the coordination cuboctahedron of nearest cation neighbors (required for the examination of model A) are listed, together with their symmetries and multiplicities, and it is shown that the 144 configurations engender only 17 distinct |EFG| values. Observations are also made on various geometric aspects of calculating EFG at ⁵⁷Fe³⁺_HS on cubic lattices.     

An 57Fe Mössbauer effect study of magnetic ordering in the Fe2O3Cr2O3 system

⁵⁷Fe Mössbauer effect spectra have been obtained as a function of temperature for a series of Fe₂O₃Cr₂O₃ compositions in the range 0.2–75% Fe₂O₃. From 30% of Fe₂O₃ upwards, the dependence of the reduced internal magnetic field $\text{H}{}_{\mathrm{<mtext>eff</mtext>}}\text{(T)}\text{H}{}_{\mathrm{<mtext>eff</mtext>}}\text{(T = 0)}$ on reduced temperature, $\text{T}\text{T}{}_{\mathrm{N}}$, was found to be identical with that for α-Fe₂O₃, indicating the persistence of Fe₂O₃-type ordering over this composition range. For 0.2% of ⁵⁷Fe₂O₃ in Cr₂O₃, the reduced field values fall far below those for α-Fe₂O₃ or Cr₂O₃, and weak coupling of Fe₃⁺ with Cr³⁺ has been inferred.A Morin transition similar to that for α-Fe₂O₃ was not present for samples containing 75% of Fe₂O₃. Quadrupole shifts ? were found to be negative and diminishing between 75 and 30% Fe₂O₃, and positive between 20 and 0.2%. The limiting value at lowest dilution of 0.113 ± 0.010 mm/sec corresponds to a nuclear quadrupole coupling constant e²qQ of 0.45 ± 0.04, approximately half that for α-Fe₂O₃. Cone angles for Fe³⁺ spin vectors in the spin-spiral arrangements for intermediate compositions have been derived, and are similar to, but less extreme than those deduced from neutron diffraction data.     

13C Hyperfine interaction in the hexafluorobenzene radical anion

Naturally abundant ¹³C satellites have been detected in the isotropic EPR spectrum of C₆F^?₆, the ¹³C coupling being 12.1 G The magnitude of this coupling does not support a nonplanar structure for C₆F^?₆. Also, the value is more consistent with a σ¹ than a π¹ configuration in D_6h, symmetry.     

Mössbauer spectroscopy of 57Fe in NH4MnCl3, NH4FeCl3 and NH4CoCl3

The Mössbauer spectra of ⁵⁷Fe doped NH₄MnCl₃ reveal that this compound undergoes a crystallographic phase transition at 258°K and becomes magnetic at 105°K. No crystallographic transition appeared either in NH₄FeCl₃ or in ⁵⁷Fe:NH₄CoCl₃; the latter orders magnetically around 27°K.     

The simultaneous determination of Th,Pu, Ra isotopes, 210Pb, 55Fe, 32Si and 14C in marine suspended phases

An efficient and systematic chemical procedure for the simultaneous measurement of Th, Pu, Ra isotopes, ²¹⁰Pb, ⁵⁵Fe, ³²Si and ¹⁴C in the same sample has been developed. This method has been applied successfully to the measurement of these isotopes in oceanic particulate samples collected from several tons of sea water. This procedure should find application in the analysis of geochemical samples.     

A study of magnetic ordering and phase transitions in ferrous iodide by means of high field 57Fe Mössbauer spectroscopy

High field ⁵⁷Fe Mössbauer experiments on single crystals of ferrous iodide (FeI₂) are presented. The spectrometer setup used for these measurements operates in external fields up to 15 T and is described in a detailed experimental section. In this study of the magnetic phase diagram of FeI₂ we found that the electron spin on the Fe²⁺-ion is always aligned along the crystallographic c-axis, in the several magnetic phases. These phases are characterized by the ratio of the numbers of antiparallel (_N↓) and parallel (N↑) spins. Phases I, II, IV and V are given by N↓?N↑ = 1.0, 0.60, 0.33 and 0, respectively. In phase III _N↓?_N↑ drops continuously from 1.0 to 0.33 for T > 3 K and from 0.60 to 0.33 for T < 3 K. This phase information and the critical fields, at which phase transitions occur, give the opportunity to calculate exchange interaction parameters. This has been done assuming an eight-sublattice model. We conclude that the exchange interactions with the nearest and next nearest iron-neighbours in the ab-plane are very weak (J₁ = J₂ ≈ 0). For the interaction with the third nearest neighbour we deduce J₃ < —0.8 K. The complex magnetic structure is further determined by exchange interactions with the nearest neighbours in subsequent _ab-planes, but the corresponding parameters J′₁ and J′₂ cannot be determined unambiguously. The positions of the iodine-ions seem to determine the exchange mechanisms in this compound.     

The structure and oscillational motion of 57Fe atoms in interstitial sites in Al as determined from interference of Mössbauer γ radiation

The first excited site of the ⁵⁷Fe atom entrapped in an interstitial site in aluminum, as reported by W. Petry, G. Vogl, and W. Mansel (Phys. Rev. Lett.45, 1862 (1980)). from a Mössbauer spectroscopic study of a single crystal, is analyzed by consideration of the value of the Hooke's law constant of the FeAl bonds obtained from the values for elemental Fe and Al. The eight wavefunctions for the eightfold nearly degenerate excited state are described as 2s1p1d1f hybrids of three-dimensional harmonic oscillator wavefunctions relative to the center of the undistorted Al₆ octahedron or as localized 1s functions relative to the center of the distorted octahedron. These considerations provide a qualitative understanding of the observations on this system.     

Hyperfine and quadrupole interactions at nitrogen in hemin

The hyperfine and quadrupole coupling constants at nitrogen in iron (III) porphyrin chloride have been calculated. The wave functions were obtained by means of a self-consistent charge extended Hückel method [2]. The spin Hamiltonian for the system is, where the double-primed axis refers to the principal axis system of the field-gradient tensor. In the calculation of the nitrogen hyperfine constant, three contributions were considered: 1) the Fermi contact term; 2) the exchange polarization contribution; 3) the many body contribution. The last contribution was included because of its large relative importance in atomic nitrogen. The Fermi contact contribution dominates the nitrogen hyperfine constant, and the contact term has a 15% contribution from non-local effects, indicating significant unpaired electron density between iron and nitrogen. The exchange polarization term is negative and smaller than the Fermi contact term. Correlation effects are quite small, in contrast to the atomic nitrogen case. The dipolar hyperfine terms and the principal axis system of the hyperfine interaction were also evaluated. The quadrupole coupling constant for nitrogen was calculated, as well as the corresponding principal axis system. The contribution of unpaired electrons to the field gradient at nitrogen is substantial. The results of the calculations suggest considerable delocalization of unpaired electrons, in contrast to what one assumes using a crystal field model.

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Zusammenfassung Die Hyperfein- und Quadrupol-Kopplungskonstanten für Stickstoff in Eisen(III) Porphyrinchlorid wurden ausgerechnet. Die Wellenfunktionen wurden mittels einer erweiterten Hückelmethode [2] — mit selbstkonsistenter Ladung — gewonnen. Der Spin-Hamiltonoperator für dies System ist:, wobei die doppelt gestrichenen Koordinaten sich auf das Hauptachsensystem des Feldgradiententensors beziehen. Bei der Berechnung der Konstanten der Hyperfeinstruktur des Stickstoff wurden drei Beiträge berücksichtigt: 1. der Fermikontaktterm; 2. der Beitrag der Austauschpolarisation; 3. der Vielkörperbeitrag. Der letzte Beitrag wurde wegen seiner relativen Wichtigkeit beim atomaren Stickstoff mit eingeschlossen. Der Fermikontakt-Beitrag beherrscht die Hyperfeinkonstante des Stickstoffs, 15% des Beitrags vom Kontaktterm rühren von nichtlokalen Effekten her und deuten offensichtlich auf eine Elektronendichte zwischen dem Eisen- und Stickstoffatom, die auf ungepaarte Elektronen hinweist. Der Term der Austauschpolarisation ist negativ und kleiner als der Fermikontaktterm. Korrelationseffekte sind ganz klein, im Gegensatz z. B. des atomaren Stickstoff. Die dipolaren Hyperfeinterme und das Hauptachsensystem der Hyperfeinwechselwirkung wurden ebenfalls ermittelt. Die Quadrupolkopplungskonstante für Stickstoff wurde ebenso errechnet wie das zugehörige Hauptachsensystem. Der Beitrag von ungepaarten Elektronen zum Feldgradienten beim Stickstoffatom ist wesentlich. Die Rechenergebnisse lassen eine bemerkenswerte Delokalisierung von ungepaarten Elektronen vermuten, im Gegensatz zu dem, was man erwartet, wenn man ein Kristallfeld-Modell benützt.Calcul des constantes de couplage hyperfin et quadrupolaire sur l'azote dans le chlorure de porphyrine Fe(III). Les fonctions d'ondes ont été obtenues à l'aide d'une méthode de Hückel étendue itérative [2]. L'hamiltonien de spin du système est: où -les axes doublement indicés sont les axes principaux du tenseur gradient de champ. Trois contributions sont envisagées dans le calcul de la constante hypérime sur l'azote: 1) le terme de contact de Fermi, 2) la polarisation d'échange, 3) le terme àN corps. Cette dernière contribution a été incluse à cause de son importance relative dans l'azote. Le terme de contact est dominant et contient 15% d'effets non locaux, ce qui indique une densité d'électron non apparié significative entre le fer et l'azote. Le terme de polarisation d'échange est négatif et plus petit que le terme de contact. Les effets de corrélation sont assez petits, contrairement au cas de l'azote atomique. Les termes hyperfins dipolaires et le système d'axe principal de l'interaction hyperfine ont aussi été évalués. La constante de couplage quadrupolaire pour l'azote a été calculée ainsi que l'axe principal correspondant. La contribution des électrons célibataires au gradient de champ sur l'azote est importante. Les résultats des calculs suggèrent une délocalisation considérable des électrons non appariés, contrairement à ce que l'on suppose lorsque l'on emploie un modèle de champ cristallin.

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Supported in part by the National Science Foundation and by the Petroleum Research Fund of the American Chemical Society.