EPR Parameters Research Articles

Chloro-substituted dipicolinate vanadium complexes: Synthesis, solution, solid-state, and insulin-enhancing properties

Three vanadium complexes of chlorodipicolinic acid (4-chloro-2,6-dipicolinic acid) in oxidation states III, IV, and V were prepared and their properties characterized across the oxidation states. In addition, the series of hydroxylamido, methylhydroxylamido, dimethylhydroxylamido, and diethylhydroxylamido complexes were prepared from the chlorodipicolinato dioxovanadium(V) complex. The vanadium(V) compounds were characterized in solution by (51)V and (1)H NMR and in the solid-state by X-ray diffraction and (51)V NMR. Density Functional Theory (DFT) calculations were performed to evaluate the experimental parameters and further describes the electronic structure of the complex. The small structural changes that do occur in bond lengths and angles and partial charges on different atoms are minor compared to the charge features that are responsible for the majority of the electric field gradient tensor. The EPR parameters of the vanadium(IV) complex were characterized and compared to the corresponding dipicolinate complex. The chemical properties of the chlorodipicolinate compounds are discussed and correlated with their insulin-enhancing activity in streptozoticin (STZ) induced diabetic Wistar rats. The effect of the chloro-substitution on lowering diabetic hyperglycemia was evaluated and differences were found depending on the compounds oxidation state similar as was observed for the vanadium III, IV and V dipicolinate complexes (P. Buglyo, D.C. Crans, E.M. Nagy, R.L. Lindo, L. Yang, J.J. Smee, W. Jin, L.-H. Chi, M.E. Godzala III, G.R. Willsky, Inorg. Chem. 44 (2005) 5416-5427). However, a linear correlation of oxidation states with efficacy was not observed, which suggests that the differences in mode of action are not simply an issue of redox equivalents. Importantly, our results contrast the previous observation with the vanadium-picolinate complexes, where the halogen substituents increased the insulin-enhancing properties of the complex (T. Takino, H. Yasui, A. Yoshitake, Y. Hamajima, R. Matsushita, J. Takada, H. Sakurai, J. Biol. Inorg. Chem. 6 (2001) 133-142).

Synthesis and characterization of new ternary Cu(II)-polyamines-ATP complexes

Four new complexes of Cu(II) of stoichiometry [Cu(ATP)(polyamine)] containing as ligands the polyamines (PA) ethylenediamine, 1,3-diaminopropane, spermidine or spermine and adenosine 5′triphosphate were prepared from aqueous solution at pH 6. The synthesis, characterization, thermogravimetric, vibrational spectroscopy, electron paramagnetic resonance analyses are described and show that these complexes have similar molecular structures. The infrared spectra and the thermal analysis are briefly discussed based on the peculiarities of the complexes. The IR spectra of the ligands and their copper complexes were used to assign the various groups and compare the shifts due to complexation. The EPR parameters values for the complexes show that Cu(II) is complexed in a similar way in the four complexes. Similarity in the coordination mode of complexes in solid state has been determined and discussed. The data obtained suggest that the four complexes present one water molecule of hydration and are complexed through two oxygen atoms from ATP and through two nitrogen atoms of each polyamine.

Structural investigation of xFe2O3 · (100 − x)[P2O5 · TeO2] glass system by FT-IR study and EPR spectroscopy

Glasses from xFe2O3 · (100 − x)[P2O5 · TeO2] system, with 0 ≤ x ≤ 50 mol%, were investigated by X-ray diffraction, FT-IR and EPR spectroscopies. The XRD patterns show a vitreous state of studied samples for x ≤ 35 mol% Fe2O3. The FT-IR spectrum of the P2O5 · TeO2 glass matrix reveals a structure formed from PO4, TeO4 and TeO3 units. The addition and the increasing of Fe2O3 content modify progressively the structure of the glass matrix. The local structure in the investigated glasses was revealed by means of EPR using Fe3+ (3d5; 6S5/2) ions as paramagnetic probes. The EPR spectra present two resonance absorption lines characteristic to Fe3+ ions centred at geff ≈ 2.0, for 0.5 ≤ x ≤ 35 mol% and geff ≈ 4.3, for 0.5 ≤ x ≤ 5 mol%. The variation of the EPR parameters, the intensity and line-width of these absorption lines, with iron ions composition has been followed.

Synthesis, Molecular Structure, and EPR Analysis of the Three-Coordinate Ni(I) Complex [Ni(PPh3)3][BF4

The compound [Ni(PPh(3))(3)][BF(4)] x BF(3) x OEt(2) was isolated in crystalline form from the olefin oligomerization catalyst system Ni(PPh(3))(4)/BF(3) x OEt(2) and structurally characterized by X-ray diffraction. The influence of vibronic coupling on the EPR parameters of three-coordinate metal complexes with a 3d(9) electronic configuration was investigated within the framework of ligand field theory. Analytical expressions for g-tensor components and isotropic hyperfine coupling constants with ligand nuclei were obtained using first-order perturbation theory. It has been shown that the account of the vibronic interaction in the excited state predicts the existence of three-axial anisotropy of the g-tensor even at the level of first-order perturbation theory; two axes of the g-tensor located in a plane of three-coordinate structure can rotate about the main z axis when a compound is distorted by motion of ligands. It has been shown that in three points of the potential energy surface minimum, for which linear and quadric constants of the vibronic interactions have an identical signs, the HFS isotropic constant from one ligand is larger than HFS constants from the other two; for different vibronic constant signs the ratio between HFS constants varies on opposite. This theoretical researches are in the quality consent with experimental data for a three-coordinate Ni(I) and Cu(II) flat complexes.

DFT Analysis of g and 13C Hyperfine Coupling Tensors for Model NiI(CO)nLm (n = 1−4, L = H2O, OH−) Complexes Epitomizing Surface Nickel(I) Carbonyls

Relativistic calculations within the spin-orbit mean-field (SOMF) approximation, the zero-order regular approximation (ZORA), and the scalar relativistic method based on the Pauli Hamiltonian were performed for the prediction and interpretation of the electronic g tensor and (13)C hyperfine tensor for a set of model polycarbonyl nickel(I) complexes with aqua or hydroxy coligands. They exhibit extensive similarities with heterogeneous [Ni(I)(CO)(n)]-surface complexes produced upon adsorption of carbon monoxide on Ni(I) ions grafted on silica or inside the zeolite channels. Benchmark calculations showing the influence of the exchange-correlation functional on the g tensor were carried out for well-defined nickel(I) complexes of known structure. On this basis, the SOMF-B3LYP scheme was chosen for calculations of the g tensor, and the obtained results were in satisfactory agreement with literature EPR data found for the [Ni(I)(CO)(n)]/SiO(2) system. The calculated g and A((13)C) tensors allowed polycarbonyl complexes of various stereochemistries to be distinguished. The nature of the Deltag(ii) shifts was assessed in terms of the molecular orbital contributions due to the magnetic-field-induced couplings and their structure sensitivity. The noncoincidence of g and (13)C hyperfine principal axes and their orientation with respect to the molecular framework was also examined. The ability of DFT calculations to follow consistently variations of the EPR parameters induced by stereochemical changes around the Ni(I) center provides an invaluable reference for the interpretation of experimental results.

The distances of Co–O band in SrLaAlO4: Co2+ crystal

Based on Zhao's semi-SCF d-orbit wave functions model of free Co2+ ions, the superposition crystal field model and Macfarlane's perturbation loop method, the distances of the Co–O band in a SrLaAlO4:Co2+ crystal were determined by calculating the experimental optical spectra and EPR parameter. The agreement between the calculated result and the experimental data showed quantitatively that Co–O distances perpendicular to and parallel with the C4 axis were 0.1892 and 0.2139 nm, respectively.

Relativistic two-component calculations of electronic g-tensor for oxo-molybdenum(V) and oxo-tungsten(V) complexes: The important role of higher-order spin-orbit contributions

In an effort towards the reliable calculation of EPR parameters of molybdenum and tungsten enzyme active sites, the effect of higher-order spin-orbit effects and of exchange–correlation functional on the electronic g-tensors of small [MVOCl5]2− and medium-sized [MVO(bdt)2]− (bdt=benzene-1,2-dithiolate), [MVOCl3(bpy)] (bpy=bipyridine) complexes (M=Mo or W) has been evaluated. Comparison of one-component calculations with perturbational treatment of spin-orbit coupling and two-component non-collinear spin density functional calculations indicate the substantial importance of higher-order spin-orbit effects, in particular for the tungsten species. Pseudopotential calculations tend to underestimate the spin-orbit effects compared to all-electron calculations based on the Douglas–Kroll–Hess Hamiltonian. Hybrid functionals with somewhat enhanced exact-exchange admixture tend to provide better agreement with experiment than pure gradient-corrected functionals.

Comment on “Local Lattice Structure Study of the Octahedral (CrO6)9− Clusters for Cr3+ Ion Doping in a Variety of Oxide Crystals by Simulating the Corresponding EPR and Optical Spectra”

(and also of the smalldistortion undergone by the oxygen octahedron), can be obtainedfrom an analysis of experimental values of optical transitionsand EPR parameters through a parametrized crystal-field model.In essence such an analysis is founded on two main assumptions:(1) Electronic properties due to Cr

Is Allred's [Hg(cyclam)]3+a True Mercury(III) Complex?

Relativistic density functional calculations are used to examine the spectroscopic parameters of [Hg(cyclam)]3+ (cyclam=1,4,8,11-tetraazacyclotetradecane; see picture), which is thought to contain Hg3+, and to analyze its electronic structure. Although the computed EPR parameters and excitation energies are consistent with the experimental data, a detailed analysis of the electronic structure contradicts the assignment as an HgIII complex.

Crystal structure, optical, magnetic, and photochemical properties of the complex pentakis(dimethyl sulfoxide)nitrosylchromium(2+) hexafluorophosphate

The nitrosyl complex [Cr(dmso) 5(NO)](PF 6) 2 ( 1) (dmso = dimethyl sulfoxide) has been prepared by the solvolysis of [Cr(NCCH 3) 5(NO)](PF 6) 2 in neat dmso. The optical absorption spectrum of 1 in dmso shows maxima at 734, 567, 450, 413, and 337 nm. Continuous photolysis of 1 with λ = 365–580 nm light in dmso solution results in a release of NO with quantum yield, Φ, in the range 0.034–0.108 mol Einstein −1. Irradiation of a deoxygenated CH 3CN solution of [Cr(NCCH 3) 5(NO)](PF 6) 2 in the presence of excess of [Fe(S 2CNEt 2) 2] results in a transfer of NO to the iron centre as shown from the characteristic EPR spectrum of [Fe(S 2CNEt 2) 2(NO)] with A iso( 14N) = 12.2 × 10 −4 cm −1. The EPR parameters of 1 were determined: g iso, g ‖ and g ⊥ : 1.96725, 1.91881(4) and 1.992763(2); A iso( 53Cr), A ‖ ( 53Cr) and A ⊥( 53Cr): 22.8 × 10 −4, 39 × 10 −4 and 15.8 × 10 −4 cm −1; A iso( 14N), A ‖ ( 14N) and A ⊥( 14N): 5.9 × 10 −4, 2 × 10 −4 and 7.540(4) × 10 −4 cm −1.

Studies of the defect structure from the calculations of optical and electron paramagnetic resonance spectra for Ni2+ centre in α-LiIO3 crystal

By calculating the optical spectrum band positions and EPR parameters (g factors, g‖, g⊥ and zero-field splitting D) by diagonalizing the complete energy matrix of 3d8 ions in trigonal symmetry, the defect structure of Ni2+ centre in α-LiIO3 crystal is studied. It is found that to reach the good fits of optical and EPR data between calculation and experiment, the Ni2+ ion should shift by Δz ≈ 0.298 A along C3-axis and the O2− ions between the Ni2+ ion and Li+ vacancy (V Li) should be displaced away from the V Li by Δx ≈ 0.097 A because of the electrostatic interaction. The results are discussed.

One‐Electron‐Reduced and ‐Oxidized Stages of Donor‐Substituted 1,1,4,4‐Tetracyanobuta‐1,3‐dienes of Different Molecular Architectures

A series of monomeric and oligomeric donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) with various topologies have been synthesized by means of thermal [2+2] cycloaddition between tetracyanoethylene (TCNE) and donor-substituted alkynes, followed by retro-electrocyclization. One-electron-reduced and -oxidized stages of the donor-substituted TCBDs were generated by chemical methods. The obtained radical anions and radical cations were studied by using electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy, supported by density functional theory (DFT) calculations. The extent of pi-electron delocalization in the paramagnetic species was investigated in terms of the EPR parameters. Despite favorable molecular orbital (MO) coefficients, the EPR results suggest that in radical anions the spin and charge are confined to the electron-withdrawing TCBD moieties on the hyperfine EPR timescale. The observed spin localization is presumably caused by an interplay between the nonplanarity of the studied pi systems, limited pi-electron conjugation, and very likely counterion effects. In radical cations, an analogous spin and charge localization confined to the electron-donating N,N-dialkylaniline moieties was found. In this case, an efficient electron delocalization is disabled by small MO coefficients at the joints between the donor and acceptor portions of the studied TCBDs.

Effect on the EPR and local lattice structure distortion of V3+ ion doping corundum crystal: three models studied

This paper reports on the application of conventional models (the displacement model and the elongation model) and the four-parameter model in detecting the local molecular structure of a coordination complex. By diagonalizing the complete energy matrices, the EPR parameters and optical spectra as well as the local distortion structure for V3+ ion in the Al2O3 : V3+ system have been studied using the three models. The four-parameter model is shown to provide a significant improvement over the conventional models. Based on these calculations, it was found that when V3+ is doped in Al2O3, the distance between upper and lower ligand oxygen plane increases by 0.16097 Å and the shifts of O2− ions in the upper and lower ligand oxygen planes expand by 0.07076 Å and 0.004066 Å, respectively. To understand the detailed physical and chemical properties of the doped Al2O3, the contributions of the spin-orbit coupling to the zero-field splitting for V3+ ion are investigated.

INVESTIGATIONS ON THE LOCAL STRUCTURE AND THE EPR PARAMETERS FORCu2+-DOPEDGaN

The local structure and the EPR parameters (g factors and the hyperfine structure constants) for Cu2+in GaN are theoretically studied from the perturbation formulas of these parameters for a 3d9ion in trigonally distorted tetrahedra. In these formulas, the ligand orbital and spin-orbit coupling contributions are taken into account from the cluster approach, in view of the strong covalency effect of the system. Based on the studies, the impurity Cu2+is found not to occupy exactly the host Ga3+site but to suffer a slight displacement (≈ 0.004 Å ) towards the ligand triangle along C3axis due to charge and size mismatching substitution. The theoretical EPR parameters show good agreement with the experimental data. The validity of the impurity displacement is also discussed.

Investigation of the EPR and local defect structures for (FeO 6) 9− and (MnO 6) 10− clusters in TiO 2 crystal at different temperature

Based on the 252 × 252 complete energy matrices for a d 5 configuration ion in tetragonal ligand–field, the relationship between the defect structure and the EPR parameters b 2 0 , b 4 0 has been investigated. From our calculation, the local structure around impurity ion has been determined. Meanwhile, the temperature influence which varies with the kinds of doping ions on distortion has been discussed and the change curves of the zero-field splitting energies Δ E 1, Δ E 2 as functions of the local structure distortion parameter Δ R for TiO 2:Fe 3+ and TiO 2:Mn 2+ systems have been plotted.

A Perimidine-Derived Non-Kekulé Triplet Diradical

6,9-Di(tert-butyl)-1-methyltetrazolo[1,5-a]perimidine (1) has been synthesized from naphthalene in seven steps. The EPR spectra, recorded after irradiation of 1 in a butyronitrile matrix at 77 K (lambda = 351 nm) and in Ar and Xe matrixes at 4.6 K (lambda > or = 345 nm), showed a six-line, high-field signal (Delta m(S) = +/- 1), centered at 3350 G in butyronitrile, along with a half-field signal (Delta m(S) = +/- 2), which is characteristic for triplets. Simulation of the observed EPR spectra gave values for the zero-field splitting parameters of |D/hc|/cm(-1) = 0.0105, |E/hc|/cm(-1) = 0.0014 in butyronitrile and |D/hc|/cm(-1) = 0.0107, |E/hc|/cm(-1) = 0.0016 in Ar. These EPR parameters are consistent with the diradical 5,8-di(tert-butyl)-2-(N-methylimino)perimidine-1,3-diyl ((3)2) as source of the EPR spectra. Linearity of the Curie-Weiss plot and UB3LYP and (14/14)CASPT2 calculations of the singlet-triplet energy difference (DeltaE(ST) approximately 8-10 kcal/mol) indicate that the triplet is the ground state of 2, as predicted for such a nondisjoint diradical.

Characterization of heme-coordinating histidyl residues of an engineered six-coordinated myoglobin mutant based on the reactivity with diethylpyrocarbonate, mass spectrometry, and electron paramagnetic resonance spectroscopy

A genetically engineered porcine myoglobin triple mutant (H64V/V68H/H93A) (VHA-Mb) contains 6 non-axial His residues (His24, His36, His48, His81, His82, and His119) besides two candidate axial His residues (His68 and His97). Although previous resonance Raman study on the ferric VHA-Mb were not conclusive for its coordination structure, present EPR parameters of the ferric VHA-Mb were consistent with bis-imidazole coordination of His68/His97. We further investigated the reactivity of these possible His ligands with diethylpyrocarbonate (DEPC) to clarify the coordination structure and their protonation states in ferric form. We found that the non-axial His residues were easily modified with a low concentration of DEPC based on UV spectral changes and MALDI-TOF-MS analyses. On the other hand, the two candidate axial His ligands were protected from the modification due to a limited steric exposure of their imidazoles to solvent, the Fe(3+)-N(epsilon2) coordination bond, and the protonation of N(delta1) by forming a hydrogen bond with their immediate surroundings. However, once N-carbethoxylation occurred at N(epsilon2) of His97, resulting in a disruption of the heme Fe(3+)-N(epsilon2) coordination bond, it facilitated the second N-carbethoxylation to take place at N(delta1) of the same imidazole ring, leading to a bis-N-carbethoxylated derivative and further to a ring-opened derivative. These phenomena were consistent with the bis-His68/His97 coordination. Further, these were not observed at all for cytochrome b(561), a transmembrane di-heme containing protein responsible for the ascorbate-specific transmembrane electron transfer, where only a specific N(delta1)-carbethoxylation of axial His occurred at a low concentration of DEPC, leading to an inhibition of the electron acceptance from ascorbate without a release of the heme. These distinct results might be related to a specific physiological mechanism being operative at the cytosolic heme center of cytochrome b(561).

Studies of EPR theory and trigonal distortion of the (NiX 6) 4− clusters (X = halide ion) in the series of perovskite crystals AMX 3 (A = Rb, Cs; M = Cd, Mg; X = halide ion)

A theoretical method is shown to yield a detailed explanation of numerous EPR parameters for d 8 configuration ion in trigonal ligand field. On the basis of the complete energy matrix, the local lattice structures of the octahedral (NiX 6) 4− clusters in the series of perovskite crystals AMX 3 (A = Rb, Cs; M = Cd, Mg; X = Cl, Br, I) with D 3 d site symmetry have been studied by simulating the EPR and optical spectra. Moreover, the influences of the local lattice structure parameters R and θ as well as the spin-orbit coupling coefficient ζ on the ZFS parameter D from different systems are discussed.

EPR investigation of the methyl radical, the hydrogen atom and carbon oxide radicals in Maxixe-type beryl

The EPR spectra of Maxixe-type beryl contain a large number of overlapping signals. The angular dependence of the 1:3:3:1 signal typical for the CH3 radical shows that this radical is located at the center of the channel cavity with its symmetry axis parallel to the crystal c-axis and is rotating around this axis. Its EPR spectrum is axially symmetric with g// = 2.00263, g⊥ = 2.00249 and A// = 2.288 mT, A⊥ = 2.256 mT. These anisotropies have the opposite signs of those found for surface-adsorbed methyl radicals. Hydrogen atoms are located at position 2a at the center of the beryl cavity and the EPR parameters of the narrow doublet signal are A0 = 1,407 MHz and g = 2.00230. Another doublet signal, which is broader and has axial symmetry with g// = 2.00265, g⊥ = 2.00625 and A// = 0.895 mT, A⊥ = 0.885 mT, could come from a HCO3 radical. One narrow and easily saturated signal with g// = 2.00227 and g⊥ = 2.00386 is interpreted to arise from a carbon monoxide radical in the beryl channel, oriented with its axis parallel to the crystal c-axis. Additional weak doublet lines, which have similar g values as the carbon monoxide radical, are created by nearby hydrogens. A powder spectrum with g// = 2.0017 and g⊥ = 2.0004 appears upon UV irradiation of the single crystal and is easily saturated. This spectrum is interpreted to arise from a carbon dioxide radical, which rotates around its symmetry axis.

Copper−Alumina−Organic Matter Mixed Systems: Alumina Transformation and Copper Speciation As Revealed by EPR Spectroscopy

The chemical forms and solubility of Cu in alumina-organic matter systems were studied separately (Cu/Al and Cu/OM) and in mixtures (Cu/Al/OM) during long-term (up to 8 years) equilibrations at pH 6 and 7.5. The transformation of alumina was monitored by XRD, while the chemical forms of Cu were probed by EPR spectroscopy. Total dissolved Cu was determined by voltammetry. Alumina transformation to gibbsite was more rapid and complete in the Cu/Al system equilibrated at pH 7.5 than at pH 6. The presence of colloidal organic matter (Cu/Al/OM) retarded the transformation of alumina. This effect was more pronounced in the system aged at pH 7.5, likely due to the higher pH that promotes formation of Al3+--organic matter coordination complexes. As expected, the systems at pH 7.5 resulted in lower dissolved Cu concentrations than corresponding systems at pH 6. After long-term equilibrations (8 and 5 years) at pH 6 and 7.5, however, the alumina-containing coprecipitates resulted in the lowest concentrations of Cu in solution (Cu/Al < Cu/Al/OM < Cu/OM). Analyses by EPR spectroscopy indicated that Cu forms inner-sphere complexes in all systems at both pH values. Changes in the chemical forms of coprecipitated Cu (Cu/Al and Cu/Al/OM systems) occurred with time and included Cu occupying discrete sites where Cu-O-Al bond formation was dominant followed by formation of clusters (Cu-O-Cu associations) and in some cases precipitates. The anisotropic EPR parameters of the Cu/OM systems suggested that stronger interactions exist between Cu and organic matter functional groups as compared to Cu interactions with alumina-containing coprecipitates; yet, Cu solubility was highest in the Cu/OM systems. The geochemical processes described in this investigation may be effective in forest soils and wastewater treatment plants where Al and Fe salts are used as flocculation agents and to remove metal contaminants from solution.