Rhombic Zero-field Splitting Parameters Research Articles

Zero-Field Splitting in Hexacoordinate Co(II) Complexes

A collection of 24 hexacoordinate Co(II) complexes was investigated by ab initio CASSCF + NEVPT2 + SOC calculations. In addition to the energies of spin–orbit multiplets (Kramers doublets, KD) their composition of the spins is also analyzed, along with the projection norm to the effective Hamiltonian. The latter served as the evaluation of the axial and rhombic zero-field splitting parameters and the g-tensor components. The fulfilment of spin-Hamiltonian (SH) formalism was assessed by critical indicators: the projection norm for the first Kramers doublet N(KD1) > 0.7, the lowest g-tensor component g1 > 1.9, the composition of KDs from the spin states |±1/2> and |±3/2> with the dominating percentage p > 70%, and the first transition energy at the NEVPT2 level 4Δ1. Just the latter quantity causes a possible divergence of the second-order perturbation theory and a failure of the spin Hamiltonian. The data set was enriched by the structural axiality Dstr and rhombicity Estr, respectively, evaluated from the metal–ligand distances Co-O, Co-N and Co-Cl corrected to the mean values. The magnetic data (temperature dependence of the molar magnetic susceptibility, and the field dependence of the magnetization per formula unit) were fitted simultaneously, either to the Griffith–Figgis model working with 12 spin–orbit kets, or the SH-zero field splitting model that utilizes only four (fictitious) spin functions. The calculated data were analyzed using statistical methods such as Cluster Analysis and the Principal Component Analysis.

Structural Behavior and Spin-State Features of BaAl2O4 Scaled through Tuned Co3+ Doping.

Pure and Co3+-doped BaAl2O4 [Ba(Al1-xCox)2O4, x = 0, 0.0077, 0.0379] powder samples were prepared by a facile hydrothermal route. Elemental analyses by static secondary ion mass spectrometry (SIMS), X-ray absorption spectroscopy (XAS) measurements at the Co K-edge, and X-ray diffraction studies were fully correlated, thus addressing a complete description of the structural complexity of Co3+-doped BaAl2O4 powder. Powder X-ray diffraction (PXRD) patterns indicated that prepared samples were nanocrystalline with a hexagonal P63 symmetry. The X-ray absorption near-edge structure (XANES) measurements revealed the presence of cobalt in a +3 oxidation state, while the rarely documented, tetrahedral symmetry around Co3+ was extracted from the extended X-ray absorption fine structure (EXAFS) oscillation patterns. Rietveld structure refinements showed that Co3+ preferentially substitutes Al3+ at tetrahedral Al3 sites of the BaAl2O4 host lattice, whereas the (Al3)O4 tetrahedra remain rather regular with Co3+-O distances ranging from 1.73(9) to 1.74(9) Å. The underlying magneto-structural features were unraveled through axial and rhombic zero-field splitting (ZFS) terms. The increased substitution of Al3+ by Co3+ at Al3 sites leads to an increase of the axial ZFS terms in Co3+-doped BaAl2O4 powder from 10.8 to 26.3 K, whereas the rhombic ZFS parameters across the series change in the range from 2.7 to 10.4 K, showing a considerable increase of anisotropy together with the values of the anisotropic g-tensor components flowing from 1.7 to 2.5. We defined the line between the Co3+ doping limit and influenced magneto-structural characteristics, thus enabling the design of strategy to control the ZFS terms' contributions to magnetic anisotropy within Co3+-doped BaAl2O4 powder.

EPR and Optical Absorption Study of Fe3+-Doped Mono Hydrated Dipotassium Stannic Chloride

Electron paramagnetic resonance (EPR) study of Fe3+ ion-doped dipotassium stannic chloride (K2SnCl4·H2O, DPSC) single crystal is performed at X band frequency and liquid nitrogen temperature. The Zeeman g-factor, axial zero-field splitting parameters and rhombic zero-field splitting parameters of Fe3+ ion in DPSC are determined with the help angular dependence of EPR spectra. Fe3+ ion enters the crystal lattice substitutionally replacing the K+ ion. The optical absorption spectra are recorded at room temperature in the wavelength range 195–1,100 nm. The transitions are assigned from the ground state \({}^{6}{\text{A}}_{{1{\text{g}}}} \left( {\text{S}} \right)\) to different excited states of Fe3+ ion. The observed band positions fitted by four parameters, Racah inter-electronic repulsion parameters (B and C), cubic splitting parameters (Dq) and Trees correction (α) yield: B = 831 cm−1, C = 2,198 cm−1, Dq = 820 cm−1, and α = 90 cm−1.

A Theoretical Analysis of Zero Field Splitting Parameters of Mn^(2+) Doped Diglycine Calcium Chloride Tetrahydrate

The crystal field parameters (CFPs) of Mn^(2+) in Diglycine calcium chloride tetrahydrate (DGCCT) are calculated using the superposition model. The zero field splitting parameters (ZFSPs) D and E are then evaluated using perturbation and microscopic spin Hamiltonian (SH) theory. The calculated second-rank axial and rhombic ZFSPs are compared with the experimental ones. Both the ZFSPs D and E evaluated theoretically are in good agreement with the experimental values obtained by electron paramagnetic resonance. The results suggest that the Mn^(2+) ion occupies the Ca^(2+) substitutional site in DGCCT.

Unusual field-induced transitions in exactly solved mixed spin-(1/2, 1) Ising chain with axial and rhombic zero-field splitting parameters

The mixed spin-(1/2, 1) Ising chain with axial and rhombic zero-field splitting parameters in the presence of the longitudinal magnetic field is exactly solved within the framework of decoration-iteration transformation and transfer-matrix method. Our particular emphasis is laid on an investigation of the influence of the rhombic term, which is responsible for an onset of quantum entanglement between two magnetic states S k z =±1 of the spin-1 atoms. It is shown that the rhombic term gradually destroys a classical ferrimagnetic order in the ground state and simultaneously causes diversity in magnetization curves including intermediate plateau regions, regions with a continuous change in the magnetization as well as several unusual field-induced transitions accompanied with magnetization jumps. Another interesting findings concern with an appearance of the round minimum in the temperature dependence of susceptibility times temperature data, the double-peak zero-field specific heat curves and the enhanced magnetocaloric effect. The temperature dependence of the specific heat with three separate maxima may also be detected when driving the system through the axial and rhombic zero-field splitting parameters close enough to a phase boundary between the ferrimagnetic and disordered states and applying sufficiently small longitudinal magnetic field.

Investigation of the zero-field splitting parameters using crystal-field parameters from the superposition model for an [formula omitted] centre in single-crystal diammonium indium pentachloride monohydrate (DIPM)

The zero-field splitting parameters (ZFSPs) for an Fe 3 + centre in single-crystal diammonium indium pentachloride monohydrate ( ( NH 4 ) 2 InCl 5 ⋅ H 2 O ; DIPM) are calculated using perturbation formulae and crystal-field parameters from the superposition model. The calculated second-order axial and rhombic ZFSPs at the In 3 + site turn out to be similar to those obtained from experiments. This supports the notion that the Fe 3 + impurity substitutes for the In 3 + ion in DIPM.

Local Coordination of Fe3+ in Layered LiCo1−yAlyO2 Oxides Determined by High-Frequency Electron Paramagnetic Resonance Spectroscopy

The local coordination of Fe(3+) spin probes in trigonal LiAl(y)Co(1-y)O(2) was studied using high-frequency electron paramagnetic resonance spectroscopy. This technique allows the determination of Fe(3+) ions in respect to axial and rhombic zero-field splitting parameters (ZFS). After the progressive replacement of Co by Al, the axial D parameter of Fe(3+) increases from +0.0548 to +0.2802 cm(-1). On the same order, the rhombic E parameter decreases. Structural information about the Fe(3+) site in layered LiAl(y)Co(1-y)O(2) oxides was based on modeling of the magnitude of the ZFS parameters by means of the Newman superposition model. It was found that the first metal coordination sphere including Co(3+) and Al(3+) ions gave rise to differentiation of the Fe(3+) dopants in respect to local trigonal and rhombic distortion. The maximum trigonal distortion for the FeO(6) octahedron was achieved when Fe(3+) spin probes were surrounded by Al only, while the Co environment yields a rhombic distortion of the FeO(6) octahedron.

A mononuclear MnIII/‘bis-tris’ complex and its conversion to a mixed-valence MnII/III5cluster

The use of the anion of 2,2-bis(hydroxymethyl)-2,2,2-nitrilotriethanol (bis-tris or LH5) as a chelate in Mn cluster chemistry is reported, and two products [Mn(N3)(LH3)] ( 1) and [Mn5(LH2)3(LH5)(MeOH)2.5]Cl4 ( 2) are described. The reaction of Mn(ClO4)2 x 6H2O and NaN3 with LH5 and NEt3 in a 1:1:1:2 molar ratio in DMF gave complex 1. The reaction of 1, NEt3 and MnCl2 x 4H2O in a 2:2:1 ratio in MeOH gave the pentanulear complex 2. Complex 1 contains a distorted-octahedral MnIII atom exhibiting a Jahn-Teller axial elongation. Complex 2 has a mixed-valence MnII2MnIII3 trigonal bipyramidal Mn5 topology, with the apical positions of the [Mn5(micro-OR)7](6+) core occupied by the MnII atoms. Variable-temperature, solid-state dc and ac magnetization studies are reported for 1 and 2 in the 1.8-300 K range. The data for 1 are as expected for a high-spin d4 MnIII complex with S = 2; at low temperature, the effects of zero-field splitting (ZFS) and intermolecular antiferromagnetic interactions via OHO hydrogen bonds become evident. Fitting of magnetization vs. field (H) and temperature (T) data by matrix diagonalization gave S = 2, D = -3.25 cm-1 and |E| = 0.32 cm-1, with g fixed at 2.00, where D and E are the axial and rhombic ZFS parameters, respectively. The analogous fit for 2 gave S = 3 and D = -0.68 cm-1. The combined results demonstrate the usefulness of bis-tris as a new poly-alkoxide chelate for the synthesis of new Mn complexes.

Protonation of the Binuclear Metal Center within the Active Site of Phosphotriesterase

Phosphotriesterase (PTE) is a binuclear metalloenzyme that catalyzes the hydrolysis of organophosphates, including pesticides and chemical warfare agents, at rates approaching the diffusion controlled limit. The catalytic mechanism of this enzyme features a bridging solvent molecule that is proposed to initiate nucleophilic attack at the phosphorus center of the substrate. X-band EPR spectroscopy is utilized to investigate the active site of Mn/Mn-substituted PTE. Simulation of the dominant EPR spectrum from the coupled binuclear center of Mn/Mn-PTE requires slightly rhombic zero-field splitting parameters. Assuming that the signal arises from the S = 2 manifold, an exchange coupling constant of J = -2.7 +/- 0.2 cm(-)(1) (H(ex) = -2JS(1) x S(2)) is calculated. A kinetic pK(a) of 7.1 +/- 0.1 associated with loss in activity at low pH indicates that a protonation event is responsible for inhibition of catalysis. Analysis of changes in the EPR spectrum as a function of pH provides a pK(a) of 7.3 +/- 0.1 that is assigned as the protonation of the hydroxyl bridge. From the comparison of kinetic and spectral pK(a) values, it is concluded that the loss of catalytic activity at acidic pH results from the protonation of the hydroxide that bridges the binuclear metal center.

Magnetic and optical studies on an S = 6 ground-state cluster [Cr12O9(OH)3(O2CCMe3)15]: determination of, and the relationship between, single-ion and cluster spin Hamiltonian parameters.

The dodecametallic Cr(III) cluster [Cr(12)O(9)(OH)(3)(O(2)CCMe(3))(15)] has a ground spin state of S = 6 characterized by the spin Hamiltonian parameters g(ZZ)() = 1.965, g(XX)() = g(YY)() = 1.960, D(S=)()(6) = +0.088 cm(-)(1), and E(S=)()(6) = 0 (where D and E are the axial and rhombic zero-field splitting parameters, respectively) as determined by multifrequency EPR spectroscopy and magnetization studies. Micro-SQUID magnetization studies reveal steps due to the fine structure of the ground state, with the spacing between the steps in excellent agreement with the D(S=)()(6) value determined by EPR. Analysis of high-resolution optical data (MCD) allows us to determine the single-ion g values and D value (= -1.035 cm(-)(1)) of the constituent Cr(III) ions directly. A vector coupling analysis demonstrates that the cluster ZFS is almost entirely due to the single-ion component. Thus, the relative orientations of the local and cluster magnetic axes can lead to a cluster ZFS of opposite sign to the single-ion value, even when this is the only significant contribution.

Nitric oxide binding at the mononuclear active site of reduced Pyrococcus furiosus superoxide reductase.

Nitric oxide (NO) has been used as a substrate analog to explore the structural and electronic determinants of enzymatic superoxide reduction at the mononuclear iron active site of Pyrococcus furiosus superoxide reductase (SOR) through the use of EPR, resonance Raman, Fourier transform IR, UV-visible absorption, and variable-temperature variable-field magnetic CD spectroscopies. The NO adduct of reduced SOR is shown to have a near-axial S = 32 ground state with ED = 0.06 and D = 12 +/- 2 cm(-1) (where D and E are the axial and rhombic zero-field splitting parameters, respectively) and the UV-visible absorption and magnetic CD spectra are dominated by an out-of-plane NO(-)(pi*)-to-Fe(3+)(dpi) charge-transfer transition, polarized along the zero-field splitting axis. Resonance Raman studies indicate that the NO adduct is six-coordinate with NO ligated in a bent conformation trans to the cysteinyl S, as evidenced by the identification of nu(N-O) at 1,721 cm(-1), nu(Fe-NO) at 475 cm(-1), and nu(Fe-S(Cys), at 291 cm(-1), via (34)S and (15)NO isotope shifts. The electronic and vibrational properties of the S = 32 (FeNO)(7) unit are rationalized in terms of a limiting formulation involving a high-spin (S = 52) Fe(3+) center antiferromagnetically coupled to a (S = 1) NO(-) anion, with a highly covalent Fe(3+)-NO(-) interaction. The results support a catalytic mechanism for SOR, with the first step involving oxidative addition of superoxide to form a ferric-peroxo intermediate, and indicate the important roles that the Fe spin state and the trans cysteinate ligand play in effecting superoxide reduction and peroxide release.

Why spin = 1, 2 species have no electron paramagnetic resonance signal under normal conditions: Possible detection by electron paramagnetic resonance at frequency close to D value?

A universal EPR simulation program has been created by the author, which is based on the following spin Hamiltonian equation: H = gβB · S + D{S z 2 − S(S + 1) 3 } + E(S x 2 − S y 2) where D and e are the axial and rhombic zero-field splitting parameters, respectively. The program can be used for simulation of EPR spectra with half-integer electronic spin ( S = n 2 , n = 3, 5, 7, 9) systems. In this article, the integer spin ( S = n 2 , n = 2, 4) systems are also considered. The EPR simulation results show that when D > frequency, no EPR signal can be seen from EPR simulation; when D ≈ frequency, whichever X/Q/W-band is used, the EPR signal can be seen on the basis of the simulated EPR results presented.

A theoretical investigation of the zero-field splitting parameters for an Mn2+ centre in a BiVO4 single crystal

The zero-field splitting (ZFS) parameters D, E and F for an Mn2+ centre in a BiVO4 single crystal are calculated using the point-charge electrostatic model and the superposition model. The calculated ZFS parameters at the Bi and V sites are compared with the experimental values for Mn2+. The calculated second-order axial and rhombic ZFS parameters at the Bi site turn out to be similar to those from experiment. The superposition model and the point-charge model give similar results. This supports the notion that the Mn2+ impurity substitutes for the Bi3+ ion in BiVO4.

Paramagnetic anisotropy and zero-field splitting in chloro bis (diethyldithiocarbamato) iron (III)

Results of measurements of paramagnetic anisotropy in the 80–300°K temperature range on single crystals of S = 3/2 chloro bis (diethyldithiocarbamato) iron (III) are described. The values of the axial and rhombic zero-field splitting parameters are deduced: D = 1.73 cm −1 and E = 1.19 cm −1.