Paramagnetic Hyperfine Structure Research Articles

Theoretical Studies of the Local Structures and EPR Parameters for the Rhombic Cu2+ Center in Cu0.5Zr2(PO4)3 Phosphate

Abstract The local structure of the rhombic Cu2+ center in Cu0.5Zr2(PO4)3 phosphate is investigated by using the high-order perturbation formulas of electron paramagnetic resonance (EPR) parameters, g-factors g i (i=x, y, z), and hyperfine structure constants A i for 3d9 ions in rhombically elongated octahedral symmetry. According to the studies, the local axial distortion angle Δα (≈ 5.1°) and the planar bond angle θ (≈ 83.8°) in [CuO6]10- cluster was obtained. The theoretical EPR parameters based on the aforementioned local structure parameters show good agreement with the observed values, and some improvement have been made as compared with the previous studies.

Investigations of EPR parameters and defect models for three Yb3+ impurity centers in ThO2 and CeO2 crystals

The electron paramagnetic resonance (EPR) parameters (g factors g(parallel to), g(perpendicular to) and hyperfine structure constants (171)A(parallel to), (171)A(perpendicular to), (173)A(parallel to) and (173)A(perpendicular to)) for three Yb(3+) centers, one cubic (center I) and two trigonal (centers II and III). in ThO(2) and CeO(2) crystals are studied from a complete diagonalization (of energy matrix) method. In the method, the Zeeman and hyperfine interaction terms are added to the classical Hamiltonian and the crystal-field parameters are calculated from the empirical superposition model. From the studies, the EPR parameters for these Yb(3+) centers in ThO(2) and CeO(2) crystals are reasonably explained, the defect model for the trigonal Yb(3+) center II suggested in the previous paper is confirmed and the defect structure of center II are also obtained. The theoretical hyperfine structure constants A of two Yb isotopes for centers II and III are suggested and remain to be checked by further experiments. The results are discussed. (C) 2010 Elsevier B.V. All rights reserved.

Analysis of hyperfine structure of Rare‐Earth Ions in Single Crystals by Electron Paramagnetic Resonance Spectroscopy

Electron paramagnetic spectroscopy of rare‐earth ions in single crystals is an interesting tool to analyze the hyperfine structure of the ground state of the rare‐earth. This can be useful for coherent spectroscopy and quantum information applications where the hyperfine structure of the electronic levels is used. Moreover, in some cases, the electron paramagnetic resonance hyperfine structure of interacting rare‐earth ions allows us to retrieve the isotropic exchange interaction between the two interacting ions. We illustrate these points with the hyperfine structure of Yb3+ ions in vanadate crystals, the hyperfine structure of Er3+ ions in Y2SiO5, and the hyperfine structure of Yb3+ pairs in CsCdBr3.

Mössbauer and EPR Study of Recombinant Acetyl-CoA Synthase from Moorella thermoacetica

Mössbauer and EPR spectroscopies were used to study the electronic structure of the A-cluster from recombinant acetyl-CoA synthase (the alpha subunit of the alpha2beta2 acetyl-CoA synthase/CO dehydrogenase). Once activated with Ni, these subunits have properties mimicking those associated with the alpha2beta2 tetramer, including structural heterogeneities. The Fe4S4 portion of the A-cluster in oxidized, methylated, and acetylated states was in the 2+ core oxidation state. Upon reduction with dithionite or Ti3+ citrate, samples of Ni-activated alpha developed the ability to accept a methyl group. Corresponding Mössbauer spectra exhibited two populations of A-clusters; roughly, 70% contained [Fe4S4]1+ cubanes, while approximately 30% contained [Fe4S4]2+ cubanes, suggesting an extremely low [Fe4S4](1+/2+) reduction potential for the 30% portion (perhaps <-800 mV vs NHE). The same population ratio was observed when Ni-free unactivated alpha was used. The 70% fraction exhibited paramagnetic hyperfine structure in the absence of an applied magnetic field, excluding the possibility that it represents an [Fe4S4]1+ cluster coupled to a (proximal) Ni(p)1+. EPR spectra of dithionite-reduced, Ni-activated alpha exhibited features at g = 5.8 and g(ave) approximately 1.93, consistent with a physical mixture of {S = 3/2; S = 1/2} spin-states for A-clusters containing [Fe4S4]1+ clusters. Incubation of Ni-activated alpha with dithionite and CO converted 25% of alpha subunits into the S = 1/2 A(red)-CO state. Previous correlation of this state to functional A-clusters suggests that only the 30% fraction not reduced by dithionite or Ti3+ citrate represents functional A-clusters. Comparison of spin states in oxidized and methylated states suggests that two electrons are required for reductive activation, starting from the oxidized state containing Ni(p)2+. Refitting published activity-vs-potential data supports an n = 2 reductive activation. Enzyme starting in the methylated state exhibited catalytic activity in the absence of an external reductant, suggesting that the two electrons used in reductive activation are retained by the enzyme after each catalytic cycle and that the enzyme does not have to pass through the A(red)-CO state during catalysis. Taken together, our results suggest that a Ni(p)0 state may form upon reductive activation and reform after each catalytic cycle.

Calculated geometry and paramagnetic hyperfine structure of the Cu 7 cluster

All-electron spin polarized DFT calculations have been performed to optimize the pentagonal bipyramidal (D 5h) geometry of the Cu 7 cluster by using the B3LYP and the B3PW1 functionals with different basis sets. Dirac scattered-wave and its non-relativistic limit calculations are used to calculate the 63Cu hyperfine coupling constants using a first order perturbational procedure. Our calculations for the Cu 7 cluster predict the 2 A 2 ″ as its ground state. The calculated hyperfine coupling constants are in reasonable agreement with those experimentally determined for Cu 7 in a matrix isolated ESR study by Van Zee and Weltner [J. Chem. Phys. 92 (1990) 6976].

Calculated paramagnetic hyperfine structure of pentagonal bipyramid Ag7 cluster

Symmetry-adapted angular momentum basis functions have been generated for the D5h* molecular double point group to obtain the self-consistent Dirac cluster wave function Φ and the Dirac cluster orbitals. Once Φ is obtained, we proceed throughout a relativistic first-order perturbation procedure to calculate the magnetic hyperfine tensors of the Ag7 cluster. The calculated spin distribution and magnetic hyperfine tensors fully support the ESR assignment made by Weltner et al. of a cluster composed of seven silver atoms with a pentagonal bipyramid structure. The single unpaired electron spin spend 40.3% of its time on each axial silver atom.

Electronic structure and magnetic properties of LaFeO3 at high pressure.

The electronic ground state and magnetic properties of the wide-gapped antiferromagnetic insulator ${\mathrm{LaFeO}}_{3}$ have been investigated by $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy at pressures up to 63 GPa and temperatures of 7--300 K. Two separate magnetic-electronic phase transitions have been identified in the measured pressure range. At 300 K a new nonmagnetic phase begins to evolve at \ensuremath{\sim}30 GPa and coexists in ever increasing abundance with the original magnetic phase at pressures up to \ensuremath{\sim}45 GPa. In the range 45--55 GPa and 300 K the spectrum is comprised solely of a nonmagnetic phase having a single Fe site. Spectra recorded at 48 GPa and temperatures down to 7 K exhibit features of paramagnetic hyperfine structure. The results are consistent with a progressive transition from a magnetically ordered state to that of a spin-disordered state in the range 30--45 GPa. At 300 K and higher pressures of 55--63 GPa the nonmagnetic spectra show features of two sites with similar isomer shifts but different quadrupole splittings. Pressure evolution of the hyperfine interaction parameters and the magnetic transition at 30--45 GPa may be explained by a change of the original Fe high-spin state, namely, spin crossover to a low-spin configuration. The two sites at 55--63 GPa have been attributed to the coexistence of different charge states at crystallographically equivalent sites. These distinct charge states are supposed to represent both low-spin Fe(III) and Fe(II) as a result of fluctuations across a ligand-to-metal charge-transfer gap \ensuremath{\Delta}\ensuremath{\sim}${\mathit{k}}_{\mathit{B}}$T that has been reduced under high pressure.

Spin-lattice relaxation and intramolecular motions in transferrin on the Mössbauer spectroscopy data

A new approach for the investigation of intramolecular motions in proteins, based on the study of spin-lattice relaxation of the iron atom of the protein active centres, has been developed. The gamma-resonance spectra have been obtained for the globules of transferrin enriched with57Fe. They possessed a paramagnetic hyperfine structure up to room temperature. This afforded the opportunity to experimentally investigate high temperature conformational motions in proteins and to study their influence on the spin-lattice relaxation. A theoretical model of relaxational spectra calculations has been developed, which enabled us to extract the frequency distribution of the spin-lattice relaxation from the shape of the spectra. The approach developed for the study of intramolecular dynamics of proteins based on the examination of spin-lattice relaxation in gamma-resonance spectra increases the frequency scale by at least one order in comparison with conventional methods.

Calculated Paramagnetic Hyperfine Structure of the C2v Isomers of Ag3

Calculated Paramagnetic Hyperfine Structure of the C2v Isomers of Ag3

Mössbauer study of spin-lattice relaxation and intramolecular motions in transferrin

A new approach for the investigation of intramolecular motions in proteins, based on the study of spin-lattice relaxation of ferric iron in the protein active centres, has been developed. The Mössbauer spectra of dry and hydrated transferrin enriched with 57Fe isotope have been obtained. They revealed a paramagnetic hyperfine structure with the magnetic splitting of lines up to room temperature. This allowed one to investigate experimentally high temperature conformational motions in the protein and to study their influence on spin-lattice relaxation. A theoretical model for the fitting of the relaxation spectra has been developed that made it possible to restore the frequency distribution of spin-lattice relaxation rates from experimental data. Two peaks in these distributions at 10 8 s −1 and 10 10 s −1 have been discovered in the temperature interval 78–300 K. It proves that there is a difference in the dynamic states of two iron atoms in the active centres of the transferrin molecule for N- and C-lobes. The approach developed for the study of intramolecular dynamics of proteins based on the analysis of spin-lattice relaxation increases the frequency range of the molecular motions under investigation at least one order in comparison with the traditional methods, based on the study of the Mössbauer effect probabilities and mean-square displacements of atoms.

A new paramagnetic hyperfine structure effect in manganese tetramers. The origin of “multiline” EPR signals from an S 2 state of a photosynthetic water-splitting enzyme

A new theory of EPR spectra for singlet states of Mn tetramers, allowing spin-hybridizations for component composite spins, is developed to elucidate the origin of the “multiline” EPR signals from the Mn clusters in an S 2 state of photosystem II. By computer simulation it is shown that the Mn cluster most probably exists in the Mn(III, IV, IV, IV) oxidation state with a strong-antiferromagnetic exchange interaction ( J ab S a· S b) between a pair of Mn(III) and Mn(IV) ions and the other weak or intermediate ones satisfying the relations, (3/2) p 01 (2 J ac−2 J ad+ J bd− J bc) cos 2θ=[ J cd− J ac− J ad+ 1 2 ( J bc+ J bd)] sin 2θ⪢0, where p 01=1.291 and θ, a polar angle of the spin—state vector in the x– y plane of S cd=0 and S cd=1 spin configurations, is ≈ 38°–39° in control samples.

A new probe for local structure: Paramagnetic hyperfine structure in Nd3+ Mössbauer spectra

The4I9/2 ground state of Nd3+ (4f3) is split by a crystal field of lower than cubic symmetry into five Kramer doublets. The magnetic hyperfine interactions can be calculated by using an effective magnetic hyperfine tensor A, which is obtained from the linear combination coefficients of the ground state doublet eigenvector. The hyperfine tensor A and the line shape depend strongly on the local structure of the system. Nondiagonal magnetic hyperfine interactions produce nonadiabatic relaxation. Corresponding lineshapes are calculated by means of the Clauser-Blume model and the eigenvalue treatment of superoperators. We found for Nd3+ in the investigated laser phosphate glass a network-forming function consistent with aC3h orD3h point symmetry.

Paramagnetic hyperfine structure of193Ir in K2IrCl6 diluted into K2PtCl6

The paramagnetic hyperfine splitting of Ir4+ (5d5) in K2IrCl6 diluted into diamagnetic K2PtCl6 has been observed at 4.2 K and Ir∶Pt ratios of 1∶10 and 1∶25. In the latter case a narrow paramagnetic pattern with a hyperfine coupling constant ofA=−13.1(2) mm/s was observed, but both samples also exhibit a single Ir4+ line typical for fast relaxation, either because of macroscopic inhomogeneities in the Ir distribution or because part of the Ir spins are still coupled to nearest Ir neighbours.

Relaxation phenomena in Cr3+ doped Al2O3−Fe 2 57 O3 systems

Mossbauer absorption spectra have been measured for Cr3+ doped Al2O3−Fe257O3 systems for different values of the Cr3+ concentration at room temperature. The cross relaxation between Fe3+ and Cr3+ ions, which destroys the paramagnetic hyperfine structure of Fe57 observed in undoped Al2O3−Fe257O3, is thoroughly studied. The experimental results suggest a new kind of cross-relaxation process involving three spins, i.e. two Fe3+.ions and one Cr3+. The process, though it is a higher-order one, is highly effective because it is energy-conserving.

Evidence for the multiphase nature of bentonites from Mössbauer and EPR spectroscopy

AbstractMössbauer spectra of several smectites demonstrate the existence of at least three phases with distinct Fe populations: (i) a component with very low Fe content (&lt; 1%), which shows slowly-relaxing paramagnetic hyperfine structure at both 4·2 K and 77 K; (ii) a component with intermediate Fe content (∼ 1–10%) which is seen as doublets in the spectra at 4·2 K, 77 K and ambient temperature; (iii) an Fe-rich phase (&gt; 30% Fe), which shows magnetic ordering at 4·2 K and 77 K. These data are consistent with components (i) and (ii) corresponding to Fe incorporated in aluminosilicate structures from distinct phases, whereas (iii) is characteristic of an iron oxide phase, probably goethite in most cases. These conclusions are supported by EPR measurements which show magnetically-dilute Fe in more than one type of structural environment plus an additional component with magnetically-interacting ions.

Paramagnetic resonance hyperfine structure of hexachloroprotactinate(IV).

Dirac scattered-wave calculations are presented for the ${\mathrm{PaCl}}_{6}^{2\mathrm{\ensuremath{-}}}$ cluster that models the ${\mathrm{Pa}}^{4+}$ impurity site in the octahedral ${\mathrm{Cs}}_{2}$${\mathrm{ZrCl}}_{6}$ lattice. The calculations predict a ${\ensuremath{\Gamma}}_{7u}$ ground state, and show fairly good agreement with the observed g tensor, $^{231}\mathrm{Pa}$ hyperfine interactions, and crystal-field splittings. The Zeeman and $^{231}\mathrm{Pa}$ hyperfine interactions are isotropic, arising from the large orbital contributions to the magnetic moment of the ground Kramers doublet. The amount of covalency is predicted to be about 3%, and the largest contribution to covalency comes from the chlorine 3${d}_{5/2}$ spinors (\ensuremath{\sim}2%).

Paramagnetic hyperfine structure of iron (57) in V1−xFexO2

With the aid of Fe(57) Mössbauer spectroscopy the nuclear hyperfine structure corresponding to the crystal states of trivalent iron in VO2 is observed for polycrystalline paramagnetic samples of V0.9985Fe0.0015O2 and V0.9974Fe0.0026O2 at temperatures between 4.2 and 200 K. Mit Hilfe der Fe(57)-Mößbauer-Spektroskopie wird die magnetische Hyperfeinstruktur, die der Kristallfeldaufspaltung von Fe3+ in VO2 entspricht, für die polykristallinen Proben V0,9985Fe0,0015O2 und V0,9974Fe0,0026O2 im Temperaturbereich zwischen 4,2 und 200 K untersucht.

Möussbauer spectroscopy of nafion polymer membranes exchanged with Fe2+, Fe3+, and Eu3+

AbstractMössbauer spectra of “Nafion” perfluoronated acid membranes exchanged with iron or europium have been studied as a function of ion concentration, water content, temperature, and applied magnetic field in order to characterize the ionic phase in these materials. In every sample, the recoil‐free fraction falls to zero at a temperature which decreases from 280 to 220 K with increasing water content. The sudden fall in recoil‐free fraction corresponds to a glass transition in the ionic phase rather than its melting point. In Fe2+ Nafion, the spectrum is independent of water content above 6 wt%, and is typical of fully hydrated, isolated Fe2+(H2O)6 species. The isomer shift, quadrupole splitting, and linewidth all change continuously below 6 wt % water, indicating a range of environments for the iron. For Fe3+ Nafion, ions in several distinct environments can be identified on the basis of their magnetic properties. Isolated Fe3+ species (Fe–Fe distances greater than 12 Å) showing paramagnetic hyperfine structure in their 4.2‐K spectra predominate in slightly neutralized samples, and in those with high water content. The proportion of dimers, whose quadrupole splitting is about 1.7 mm/s, increases with increasing iron content or decreasing water content. A third component of the spectra has a quadrupole splitting of 0.4 mm/s. It is associated with paramagnetic iron having other iron ions in the neighborhood, and also with iron ions located in the interior of small groups or chains formed from dimers. There is no iron belonging to magnetically ordered clusters involving hundreds of ferric ions of the type found in some other ionomers. All the exchanged ions belong to the ionic phase, and none are associated with the polymer backbone.

Mössbauer spectroscopic studies of [Fe(H 2O) 6] 3+ in amorphous frozen solutions at weak applied magnetic fields

Abstract The ferric hexaquo complex, [Fe(H 2 O) 6 ] 3+ , has been studied by Mossbauer spectroscopy of amorphous aqueous frozen solutions at weak applied magnetic fields. Spectra of well resolved paramagnetic hyperfine structure have been interpreted in terms of a spin Hamiltonian model for the crystal field interaction proposed in a previous work. Reasonable fits could be obtained only by the addition of a random magnetic field of a few Gauss which is attributed partly to the dipolar interaction with neighbouring iron ions, and partly to the ligand hyperfine interaction.

Mössbauer spectroscopic studies of [Fe(H 2O) 6] 3+ in amorphous frozen solutions at weak applied magnetic fields

The ferric hexaquo complex, [Fe(H 2O) 6] 3+, has been studied by Mössbauer spectroscopy of amorphous aqueous frozen solutions at weak applied magnetic fields. Spectra of well resolved paramagnetic hyperfine structure have been interpreted in terms of a spin Hamiltonian model for the crystal field interaction proposed in a previous work. Reasonable fits could be obtained only by the addition of a random magnetic field of a few Gauss which is attributed partly to the dipolar interaction with neighbouring iron ions, and partly to the ligand hyperfine interaction.