Ligand Hyperfine Interaction Research Articles

Electron paramagnetic resonance [Cu<sup>3+</sup>–Cu<sup>2+</sup>](C<sub>2v</sub>) impurity dimers in BaF<sub>2</sub>:Cu ionic crystal

Impurity dimers [Cu3+–Cu2+] (С2v) with an antiferromagnetic bond between copper ions have been synthesized in BaF2 crystals. The study of the molecular structure of the synthesized dimers was carried out by the electron paramagnetic resonance at frequencies of 9.3 and 34.2 GHz in the temperature range from 5 to 150 K. It was shown that the 63Cu3+ and 63Cu2+ ions replaced the neighboring cations of the Ва2+ lattice, forming a dimer with a parallel bond axis of one of the 〈110〉 crystallographic axes. It has been found that the coupling between copper ions has an antiferromagnetic exchange interaction and that its repeated inverse state is a doublet, S = 1/2. There were determined parameters of Zeeman interaction, part of the parameters of the hyperfine and quadrupole interaction of 63Cu3+ and 63Cu2+ ions, as well as part of the parameters of the ligand hyperfine interaction with two equivalent F– ions.

Electron nuclear interactions in spin-3/2 color centers in silicon carbide: A high-field pulse EPR and ENDOR study

High-frequency pulsed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were used to determine electron nuclear interactions on remote ligand shells of silicon and carbon in spin-3/2 color centers with an optically induced high-temperature spin alignment in hexagonal $4H$-, $6H$-, and rhombic $15R$-silicon carbide (SiC) polytypes. The EPR and ENDOR experimental data relate unambiguously to spin-3/2 centers in which the optically induced alignment of the spin-level populations occurs. The identification is based on resolved ligand hyperfine interactions with carbon and silicon nearest, next-nearest, and the more distant neighbors and on the determination of the unpaired electron spin densities. The hyperfine interactions with $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei are unambiguously separated due to the selective population of the fine-structure levels with certain values of ${\mathrm{M}}_{\mathrm{S}}$. The signs of these interactions and, as a result, the signs of oscillating spin density at $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei, are determined. On the basis of the EPR and optically induced ENDOR measurements, signs of the fine-structure splitting for all the centers were demonstrated, which made it possible to establish the character of optically induced spin alignment, including the inverse populations of the spin levels for these centers. The values of hyperfine interaction with $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei, including those remote from the localization of the spin-3/2 center, are tabulated, which can be used by a number of algorithms in quantum information processing as long-term memory.

Very Low-Frequency Broadband Electron Paramagnetic Resonance Spectroscopy of Metalloproteins.

A previously developed spectrometer for broadband electron paramagnetic resonance (EPR) spectroscopy of dilute randomly oriented systems has been considerably modified to extend the frequency reach down to the hundred MHz range and to boost concentration sensitivity by 1 to 2 orders of magnitude. The instrument is now suitable for the study of biological systems in particular metalloproteins. As a proof of concept, examples from the class of low-spin ferric hemoproteins are studied in terms of frequency-dependent changes in their EPR spectra. Mono-heme cytochrome c EPR is determined by g-strain over a wide frequency range, whereas a combination of unresolved ligand hyperfine interaction and concentration-dependent intermolecular dipolar interaction becomes dominant at very low frequencies. In the four heme containing cytochrome c3, g-strain combines with intramolecular dipolar interaction over the full-studied frequency range of 0.23–12.0 GHz. It is concluded that the point-dipole approach is inappropriate to describe magnetic interactions between low-spin ferric heme systems and that a body of literature on redox interactions in multi-heme proteins will be affected by this conclusion.

Spin echo studies on Fe3+ ions in GaN: Spin-phonon relaxation and ligand hyperfine interactions

The electron spin echo detected inversion recovery technique at the Q-band frequency was used to characterize spin diffusion effects in spin-lattice relaxation of compensating Fe3+ impurities in n-type doped GaN crystals. It was found that the selective saturation can be achieved in the GaN:Fe3+ system due to magnetization transfer based on the spin flip-flop cross-relaxation processes. The temperature dependence of 1/T1 can be explained by direct spin-phonon processes (∼T) below 25 K and by Raman two-phonon processes (∼T9) at higher temperatures. Spin diffusion in this system is characterized by an additional cross-relaxation rate which is weakly temperature-dependent below 25 K. The transferred hyperfine interactions of Fe3+ centers with gallium and nitrogen neighbor nuclei were resolved using pulsed-electron nuclear double resonance. A comparative analysis of quadrupole interactions indicates the essential increase in the electric field gradients on the nearest nitrogen and gallium shells.

Ligand hyperfine interactions at silicon vacancies in 4H-SiC

The negative silicon vacancy () in SiC has recently emerged as a promising defect for quantum communication and room-temperature quantum sensing. However, its electronic structure is still not well characterized. While the isolated Si vacancy is expected to give rise to only two paramagnetic centers corresponding to two inequivalent lattice sites in 4H–SiC, there have been five electron paramagnetic resonance (EPR) centers assigned to in the past: the so-called isolated no-zero-field splitting (ZFS) center and another four axial configurations with small ZFS: TV1a, TV2a, TV1b, and TV2b. Due to overlapping with 29Si hyperfine (hf) structures in EPR spectra of natural 4H–SiC, hf parameters of TV1a have not been determined. Using isotopically enriched 4H-28SiC, we overcome the problems of signal overlapping and observe hf parameters of nearest C neighbors for all three components of the S = 3/2 TV1a and TV2a centers. The obtained EPR data support the conclusion that only TV1a and TV2a are related to and the two configurations of the so-called isolated no-ZFS center, (I) and (II), are actually the central lines corresponding to the transition |−1/2〉 ↔ |+1/2〉 of the TV2a and TV1a centers, respectively.

Reconciling Structural and Spectroscopic Fingerprints of the Oxygen-Evolving Complex of Photosystem II: A Computational Study of the S2 State.

The catalytic cycle of photosynthetic water oxidation occurs at the Mn4CaO5 oxygen-evolving complex (OEC) of photosystem II. Extensive spectroscopic data have been collected on the intermediates, especially the S2 (Kok) state, although the proton and electron inventories (Mn oxidation states) are still uncertain. The "high oxidation" paradigm assigns S2 Mn oxidation level (III, IV, IV, IV) or (IV, IV, IV, III), whereas a "low oxidation" paradigm posits two additional electrons. Here, we investigate the geometric (X-ray diffraction, extended X-ray absorption fine structure) and spectroscopic (electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR)) properties of the S2 state using quantum chemical density functional theory calculations, focusing on the neglected low paradigm. Two interconvertible electronic spin configurations are predicted as ground states, producing multiline ( S = 1/2) and broad ( S = 5/2) EPR signals in the low paradigm oxidation state (III, IV, III, III) and with W2 as OH- and O5 as OH-. They have "open" ( S = 5/2) and "closed" ( S = 1/2) Mn3CaO4-cubane geometries. Other energetically accessible isomers with ground spin states 1/2, 7/2, 9/2, or 11/2 can be obtained through perturbations of hydrogen-bonding networks (e.g., H+ from His337 to O3 or W2), consistent with experimental observations. Conformers with the low oxidation state configuration (III, IV, IV, II) also become energetically accessible when the protonation states are O5 (OH-), W2 (H2O), and neutral His337. The configuration with (III, IV, III, III) agrees well with earlier low-temperature EPR and ENDOR interpretations, whereas the MnII-containing configuration agrees partially with recent ENDOR data. However, the low oxidation paradigm does not yield isotropic ligand hyperfine interactions in good agreement with observed values. We conclude that the low Mn oxidation state proposal for the OEC can closely fit most of the available structural and electronic data for S2 at accessible energies.

Metal ion oxidation state assignment based on coordinating ligand hyperfine interaction.

In exchange-coupled mixed-valence spin systems, the magnitude and sign of the effective ligand hyperfine interaction (HFI) can be useful in determining the formal oxidation state of the coordinating metal ion, as well as provide information about the coordination geometry. This is due to the fact that the observed ligand HFI is a function of the projection factor (Clebsch-Gordon coefficient) that maps the site spin value S i of the local paramagnetic center onto the total spin of the exchange-coupled system, S T. Recently, this relationship has been successfully exploited in identifying the oxidation state of the Mn ion coordinated by the sole nitrogenous ligand to the oxygen-evolving complex in certain states of photosystem II. The origin and evolution of these efforts is described.

Ligand Hyperfine Structure of EPR Spectra of Eu2+ Ions in Superionic CaF2 Crystals

The electron paramagnetic resonance (EPR) spectra of the Eu 2+ ion in CaF 2 crystals is investigated. In samples with Eu 2+ concentration equal to 0.1 and 0.01 mole %, additional splitting of the hyperfine structure lines is observed. The electron paramagnetic resonance spectrum is calculated with allowance for the ligand hyperfine interactions (LHFI), and the resulting line shape of the hyperfine structure is determined for the central fine-structure transition. Comparing the calculated spectrum with the experimental one, we have determined the principal values of the LHFI tensor T|| = 18.44 MHz and T⊥ = –12.98 MHz and have carried out a comparative analysis of the values obtained with the results of other authors. Using nonlinear least squares fitting, we have determined the parameters of the partially resolved lines in the spectra.

Ligand hyperfine Yb3+ interaction in CsCaF3 and Cs2NaYF6 crystals

The operator, which determines the contribution of processes including the polarization of central ion core to the ligand hyperfine interaction (LHFI), has been obtained in the second quantization representation in the basis of partially nonorthogonal orbitals. The contributions of the impurity Yb3+ ion to the LHFI parameters have been also calculated in Cs2NaYF6 and CsCaF3 crystals, which are determined by the mechanisms studied earlier. The single-particle orbital basis has been extended as compared to the previous work. There is a sufficiently good agreement with the experiment.

Interactions of an asymmetric amine with a non-C2 symmetric Cu–salen complex: An EPR/ENDOR and HYSCORE investigation

Single enantiomers of R-/S-methylbenzylamine (MBA) were found to selectively form adducts with the chiral non-C(2) symmetric Cu-salen complex N-(3,5-di-tert-butylsalicylidene)-N'-(salicylidene)-cyclohexane-1,2-diamine copper(II), hereafter labelled [Cu(3)]. The g/A spin Hamiltonian parameters of this Cu(II) complex showed a decrease in symmetry from axial to rhombic upon formation of the [Cu(3)] + MBA adducts. The selectivity in enantiomeric discrimination was found to be only 59 ± 5% in favour of the heterochiral R,R'-[Cu(3)] + S-MBA and S,S'-[Cu(3)] + R-MBA adducts. This was directly evidenced by W-band EPR spectroscopy. The observed low selectivity for enantiomer discrimination is primarily attributed to the loss of the bulky tert-butyl groups from the 3,5 positions of [Cu(3)] compared to the parent N,N'-bis(3,5-di-tert-butylsalicylidene)-cyclohexane-1,2-diamine copper(II) ligand (labelled [Cu(1)]). The structure of the [Cu(3)] complex in the presence and absence of coordinating amine was further investigated by analysis of the ligand hyperfine interactions, as revealed through Q-band CW-ENDOR, X-band Davies ENDOR and HYSCORE. (1)H couplings from the -NH(2) group of the amine, observed by ENDOR and HYSCORE, provided direct evidence of amine coordination.

ChemInform Abstract: An EPR Investigation of Cr3+ and Ni2+ in Synthetic Pyrite Crystals.

In many synthetic pyrite single crystals of high quality and purity we could detect Cr 3+ and Ni 2+ by EPR at 78 K as unwanted impurities. The assignment was possible by means of the isotopes Cr 53 and Ni 61 , occurring in natural abundance. The paramagnetic centers exhibit unusual g-values. In case of Ni 2+ a ligand hyperfine interaction could be observed beside the normal hyperfine interaction with the Ni 61 nucleus

Structure and bonding of [VIVO(acac)2] on the surface of AlF3 as studied by pulsed electron nuclear double resonance and hyperfine sublevel correlation spectroscopy

Well defined isolated [V(IV)O(acac)(2)] surface complexes have been prepared on aluminium trifluoride, alpha-AlF(3), as solid support by equilibrium adsorption from liquid solution. The paramagnetic vanadium(iv) surface complexes were studied by pulsed electron spin resonance spectroscopy in combination with quantum chemical calculations using density functional theory. (19)F and (27)Al ligand hyperfine interactions with the nuclei of the AlF(3) support observed by pulsed electron nuclear double resonance and hyperfine sublevel correlation spectroscopy experiments verify the direct chemical bonding between the vanadium and a surface fluorine atom. Detailed analysis of the ligand hyperfine couplings supported by quantum chemical calculations indicates that the axial position of [V(IV)O(acac)(2)]trans to the oxo ligand is bound directly to a terminal fluorine atom of the support. Analysis of the (1)H ligand hyperfine interaction revealed that an acidic H(+) of the AlF(3) surface protonated a donor oxygen of the acetylacetone ligand in the surface complex.

A method for estimating the local lattice distortions near a magnetic ion from ligand hyperfine parameters: Yb3+trigonal centers in SrF2 and BaF2

An empirical relationship describing the radial dependence of the nondipole part of the anisotropic constant for the ligand hyperfine interaction in alkaline-earth fluorides is proposed. This relationship is used for calculating the distances between the magnetic ion and the fluorine ions involved in its nearest environment. The results of the calculations are in good agreement with the values obtained by other methods for both cubic and tetragonal fluorine centers in these crystals. The distances from the magnetic ion to different groups of nonequivalent fluorine ions of the nearest environment in Yb3+ trigonal centers of SrF2 and BaF2 are determined. It is found that the Yb3+ ion is slightly displaced along the trigonal axis away from the compensating fluorine ion.

Human Sco1 and Sco2 Function as Copper-binding Proteins

The function of human Sco1 and Sco2 is shown to be dependent on copper ion binding. Expression of soluble domains of human Sco1 and Sco2 either in bacteria or the yeast cytoplasm resulted in the recovery of copper-containing proteins. The metallation of human Sco1, but not Sco2, when expressed in the yeast cytoplasm is dependent on the co-expression of human Cox17. Two conserved cysteines and a histidyl residue, known to be important for both copper binding and in vivo function in yeast Sco1, are also critical for in vivo function of human Sco1 and Sco2. Human and yeast Sco proteins can bind either a single Cu(I) or Cu(II) ion. The Cu(II) site yields S-Cu(II) charge transfer transitions that are not bleached by weak reductants or chelators. The Cu(I) site exhibits trigonal geometry, whereas the Cu(II) site resembles a type II Cu(II) site with a higher coordination number. To identify additional potential ligands for the Cu(II) site, a series of mutant proteins with substitutions in conserved residues in the vicinity of the Cu(I) site were examined. Mutation of several conserved carboxylates did not alter either in vivo function or the presence of the Cu(II) chromophore. In contrast, replacement of Asp238 in human or yeast Sco1 abrogated the Cu(II) visible transitions and in yeast Sco1 attenuated Cu(II), but not Cu(I), binding. Both the mutant yeast and human proteins were nonfunctional, suggesting the importance of this aspartate for normal function. Taken together, these data suggest that both Cu(I) and Cu(II) binding are critical for normal Sco function.

Ligand electron-nuclear double resonance of T 1 Trigonal Gd3+ centers in CaF2 with a mixed oxygen-fluorine environment

This paper reports on the results of ligand electron-nuclear double resonance (ENDOR) investigations of T1 trigonal 157Gd3+ centers in the CaF2 compound. It is experimentally found that the nearest environment of an impurity center contains only one 19F ion. Anions in the other coordination shells are identical to those in the pure CaF2 crystal. However, 19F ions in these shells are displaced from their ideal positions in the lattice. The parameters of the ligand hyperfine interaction (LHFI) for 19F nuclei and their coordinates and displacements with respect to the positions in the lattice of the pure CaF2 crystal are determined. It is demonstrated that the unusual isotropic LHFI constant As>0 for Gd3+ ions in the lattice with a mixed oxygen-fluorine nearest environment can be associated with the strong polarization of impurity centers in accordance with the empirical model proposed in [1], provided the structural model of the nearest environment of impurities in the T1 centers [2] is correct. This structural model is confirmed by the analysis of the isotropic hyperfine constant A(s) for 157Gd3+ centers.

Hyperfine Interactions in Pb3+F−8F−a clusters in fluorite crystals

The parameters of hyperfine interactions in Pb3+F 8 − F a − tetragonal clusters of MeF2 crystals (Me=Ca, Sr, Ba) are interpreted. The contributions of the spin polarization to the parameters of the proper hyperfine interaction and additional (ligand) hyperfine interactions are calculated in the approximation of weak binding between a charge-compensating ion F a − and a cubic fragment in the tetragonal cluster. It is demonstrated that correct inclusion of the contributions from the spin polarization to the ligand isotropic hyperfine interaction for the F a − ion leads to anomalously large parameters of this interaction for MeF2 crystals. These results are in agreement with experimental data.

Ligand hyperfine interaction in Gd3+ tetragonal centers in CaF2 and SrF2 and the structure of the impurity nearest surroundings

ENDOR experimental spectra of Gd3+ tetragonal impurity centers in CaF2 and SrF2 crystals were used to determine the superhyperfine interaction (SHFI) constants of the impurity with 19F nuclear spins of its first coordination sphere and the compensator ion. The distances in the Cd3+F9 complex were estimated within the model of isotropic SHFI constants suggested in [1]. An analysis of the data on the SHFI and spin-Hamiltonian constants [2] in terms of the superposition model indicates significant changes in the contributions (due to the Gd3+ mixed states) to these parameters for the tetragonal centers in comparison with the corresponding contributions for the cubic and trigonal centers in the same crystals.

Ligand hyperfine interaction at the neutral silicon vacancy in 4H- and6H−SiC

The silicon vacancy in its neutral charge state ${(V}_{\mathrm{Si}})$ has been unambiguously identified in 4H- and $6H\ensuremath{-}\mathrm{SiC}.$ This was achieved by observation of ligand hyperfine interaction with the four carbon atoms in the nearest-neighbor shell and the twelve silicon atoms in the next-nearest-neighbor shell surrounding the vacancy. The complete hyperfine tensors have been determined for the ${V}_{\mathrm{Si}}^{0}$ center residing at all inequivalent lattice sites in the two polytypes. These are compared with the parameters previously obtained for the negatively charged silicon vacancy.

Method for estimating local lattice distortions near a magnetic ion based on the parameters of the ligand hyperfine interaction: Ce3+ in the fluorite homologous series

A method is suggested for estimating local lattice distortions near a paramagnetic center by using the parameters determined from experimental tensors of the ligand hyperfine interaction with surrounding nuclei through the use of a simple mathematical procedure. In general, a tensor with nine independent components can be unambiguously reduced to nine independent parameters characterizing the hyperfine interaction: the isotropic part, deviations from the symmetric and axially symmetric form, the axially symmetric component (containing dipole and pseudodipole contributions), and the angles defining the bond direction. Under certain physical assumptions, these parameters are used to estimate distortions of the first and second coordination shells of Ce3+ in the homologous series CaF2, SrF2, and BaF2.

The Neutral Silicon Vacancy in SiC: Ligand Hyperfine Interaction

The isolated silicon vacancy in its neutral charge state has unambiguously been confirmed in electron irradiated 4H and 6H SiC. This was achieved by the observation of the ligand hyperfine lines arising from interaction with C-13 atoms in the nearest-neighbor (NN) shell and With Si-29 atoms in the next-nearest-neighbor (NNN) shell in optically detected magnetic resonance (ODMR) experiments. The complete hyperfine tensors for all inequivalent lattice sites have been deduced and are compared to the known hyperfine parameters for the negatively charged silicon vacancy in the two polytypes.