ESR study of some Cu(II)-oxazepam complexes

The CuL₂X₂ (L = 7-chlor-l,3-dihydro-3-hydroxi-5-phenyl-2H-l,4-benzodiazepin-2-one, also known as oxazepam, X = Cl, Br) complexes were prepared and investigated by ESR (electron spin resonance) method. Powder ESR spectra of CuL₂X₂ compounds at room temperature show the presence of monomeric species having an axial symmetry with a small rhombic distortion. In case of compounds with Cl there is a superposition of two nonequivalent mononuclear species, one with ad_{x² - y²} d_{x^2 - y^2 } ground state and another with ad_z² d_{z^2 } ground state. In pyridine (Py) and dimethylformamide (DMF) solutions the monomeric species prevail. Two different monomeric species, one with pseudotetrahedral (T_d) and the other with elongated tetrahedral-octahedral symmetry, were evidenced in DMF solutions adsorbed on NaY zeolite. In Py-DMF solutions two monomeric species were also identified. Dimeric species appear in DMF and Py solutions adsorbed on NaY zeolite through the coordination of two Cu(II) ions at the same keto-oxygen from one oxazepam molecule.     

ESR study of some solvent effects of the Cu(II)-aspirinate complex

Powder ESR spectra of the [Cu₂(Asp)₄](H₂O)₂ complex show the existence of Cu(II) acetate like dimers characterized by a strong antiferromagnetic exchange interaction (J=?288 cm^?1). In pyridine-dimethylformamide solutions the monomeric species prevail. Two different monomeric species, the Cu(DMF)₄ of planar-distorted tetrahedral (T_d) symmetry and the Cu(Asp)₂(DMF)₂ with elongated tetrahedral-octahedral symmetry were evidenced in DMF solution adsorbed on NaY zeolite. In 40 % Py+60 % DMF solution two monomeric species were also identified. These are Cu(Asp)₂(DMF)₂ and Cu(Asp)₂(Py)₂, the last species having a CuN₂O₂ chromophore in atrans squareplanar arrangement. In pyridine Cu(II)-aspirinate solution only the Cu(Py)₄ monomeric species of CuN₄ chromophore was evidenced.     

ESR study of calix[4]arene by spin-trapping method

Electron spin resonance (ESR) measurements are introduced to investigate the interaction between 5,11,17,23-tetrabromo-25,26,27,28-tetramethoxycalix[4]arene (II ₄) and 2-methyl-2-nitrosopropane (MNP) or nitrosodurene (ND) which are used as spin-trapping reagents under ultraviolet irradiation. In most cases, the complicated ESR spectra were observed because of the overlap of ESR signals attributed to several spin adducts in different conformations or in an inclusion state. However, after the optimum amount of ND was added into theII ₄ solution, the ESR spectrum showed that each nitrogen hyperfine line was further split to 1:2:1 triplet lines, and this is a typical spectrum ascribed to the spin adduct of ND at one para position ofII ₄.     

Spin-Probe ESR Studies on Nanocomposite Polymer Electrolytes

The possibility of using spin-probe electron spin resonance (ESR) as a tool to study glass transition temperature, T _g, of polymer electrolytes is explored in 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL)-doped composite polymer electrolyte (PEG)₄₆LiClO₄ dispersed with nanoparticles of hydrotalcite. The T _g is estimated from the measured values of T _50G, the temperature at which the extrema separation 2A _zz of the broad powder spectrum decreases to 50 G. In another method, the correlation time τ_c for the spin probe dynamics was determined by computer simulation of the ESR spectra and T _g has been identified as the temperature at which τ_c begins to show temperature dependence. While both methods give values of T _g close to those obtained from differential scanning calorimetry, it is concluded that more work is required to establish spin-probe ESR as a reliable technique for the determination of T _g.     

Investigation of photochromic cluster systems based on molybdenum oxides by ESR spectroscopy

The electron-spin resonance (ESR) spectra of cluster polyoxometalate systems—a finely dispersed powder of the (NH₄)₆[Mo₇O₂₄]-citrie acid complex, molybdic acid, and molybdenum(VI) oxide—are investigated. The initial samples are colored under exposure to ultraviolet (UV) irradiation (photochromic effect) and thermal annealing. The ESR signal (g _⊥ = 1.94, g _‖ = 1.92) which is observed for the (NH₄)₆[Mo₇O₂₄]-citric acid photocolored samples corresponds to an electron of the molybdenum atom. This is in agreement with the data derived from the electronic spectrum. In addition, the (NH₄)₆[Mo₇O₂₄]-citric acid colored system exhibits an ESR signal (g=2.02) which corresponds to a hole at the organic ligand. This confirms the previously advanced model of intramolecular electron transfer under UV irradiation. The thermally colored molybdic acid has a similar ESR spectrum (g=1.88, 1.92, 1.93, and 1.98). For the other samples, the ESR signal is not observed. It is demonstrated that an unpaired electron of molybdenum atoms is substantially delocalized over all metal atoms in the cluster.     

Numerical study on K/S/Cl release during devolatilization of pulverized biomass at high temperature

In this paper, the interaction between different organic and inorganic K/S/Cl compounds in the solid structure of biomass is studied and a model is presented to predict the temporal release of K_g, HCl, CH₃Cl, KCl, KOH, K₂SO₄ and SO₂ from biomass devolatilization. Four types of pulverized biomass are chosen from literature, two of which have no chlorine content and two with chlorine content in lower stoichiometry to potassium. The results of the model are compared with the experimental measurements. In the presence of chlorine, KCl, HCl and K_g were found to be the dominant chlorine and potassium species. In the absence of chlorine, K_g dominates the release of potassium. KOH and K₂SO₄ release into the gas phase towards the end of devolatilization due to the overlapping with char combustion. SO₂ is the main sulfur species released into the gas phase. The model is coupled with a CFD solver where the gas phase chemistry of the K/S/Cl system can be studied using available chemical mechanisms for these species.     

Electron transfer at lanthanum endohedral fullerenes

The aim of this contribution is to study lanthanum containing fullerenes produced and treated under different conditions. The work was done using solid soot extract giving poorly resolved ESR spectra or separated lanthanofullerenes in solution showing well resolved ESR spectra. ESR experiments in connection with the mass spectrometry give information on the electronic states due to the electron transfer from the endohedral metal to the fullerene cage and the influence of oxygen on endohedral lanthanofullerene. The analysis of ESR spectra indicates the presence of up to nine octets of lanthanum containing fullerenes. These species have different stabilities. Furthermore, a very narrow single ESR line in the lanthanofullerene spectrum was detected having a linewidth ΔB=0.008 mT and ag-factor 2.0025. The electron transfer to the endohedral La@C₈₂ molecule studied by voltammetry and electrochemicalin situ ESR experiments is described. The interaction of the lanthanofullerene with oxygen in solution is also studied. In different solvents the influence of oxygen on line broadening in the ESR spectra of La@C₈₂ was observed.     

High field ESR measurements of spin gap system MCu2(PO4)2

Submillimeter and millimeter wave ESR measurements of spin gap systems SrCu₂(PO₄)₂ and PbCu₂(PO₄)₂, which have four kinds of dimers, have been performed to investigate the magnetic properties of spin gap systems using the pulsed magnetic field up to 35T. The observed ESR spectra of powder sample SrCu₂(PO₄)₂ show sharp and single peak in the temperature range from 4.2 to 80 K. The anisotropy of the g-values turned out to be very small compared to the usual anisotropic powder spectra of copper compounds. The dynamical properties will be discussed from the temperature dependence measurements.     

Electron spin resonance in carbon-doped boron nitride

It is shown that carbon impurity is responsible for the yellow color and paramagnetism in boron nitride which has been heated to 1800–2300°C in graphite furnaces. The yellow BN powders exhibited 10-line ESR spectra with spin concentrations up to 10¹⁹/g. Annealing the yellow BN at 1800–2300°C in a carbon-free environment removed the yellow color and reduced the spin concentration below the observable limit of 10¹⁶/g. Such pure white BN was unaffected by prolonged irradiation with u.v., 50 kV X-rays or 1 MeV γ-rays, whereas other less pure types of BN showed a large enhancement of ESR signal after relatively short exposures to such radiation. The ESR spectra from pure white BN doped with C¹³-enriched carbon were identical to those from BN doped with normal carbon. These results suggest that the yellow color in BN is due to an F-center produced by a carbon impurity ionization mechanism. The anisotropy of the ESR was studied in carbon-doped specimens of highly oriented BN. The room temperature hyperfine splitting at 9·151 GHz was found to vary as (a + b cos²θ), where a = 7·85 ± 0·07 G and b = −1·27 ± 0·07 G. The g-value obeyed a similar relation: g = g_⊥ + Δg cos²θ, where g_⊥ = = 2·00321 ± 0·00002 and Δg = g_∥ − g_⊥ = (−9·5 ± 0·2) × 10⁻⁴.     

Light induced charge transfer processes in Cr doped Bi12GeO20 and Bi12SiO20 single crystals

Optical absorption and ESR spectra of Bi₁₂GeO₂₀ doped with Cr were measured before and after illumination with visible light. It was found that Cr⁴⁺ ions in tetrahedral position are responsible for light induced ESR and optical spectra. The g-factor of the center is 1.945 ± 0.002. Crystal field parameters for the Cr⁴⁺ center are D_q = 820 cm^?1 and B = 429cm^?1. The photochromic effect is explained in terms of a Cr⁵⁺?Cr⁴⁺ charge transfer process.     

Theoretical explanation of absorption spectra and ESR parameters of Cu in shattuckite

The optical absorption spectra and electronic spin resonance parameters (ESR g factors g_‖, g_⊥ and hyperfine structure constants A_‖, A_⊥) for Cu²⁺ in shattuckite crystal are calculated from the two spin–orbital coupling parameters model, high-order perturbation formulas and complete diagonalization (of energy matrix) method (CDM) of 3d⁹ ion in tetragonal symmetry. The calculated results are in good agreement with the observed values. Since the ESR parameters are sensitive to the local structure of a paramagnetic impurity center, the defect structure of Cu²⁺ center in shattuckite crystal is estimated. The results are discussed.     

Oxygen hole centers in coesite,a high pressure phase of SiO2

Electron spin resonance (ESR) spectra have been measured for coesite synthesized under high pressure condition from natural quartz and irradiated by γ-rays at 77K. The prominent signals detected at 77 K have an anisotropicg-factor of orthorhombic symmetry,g ₁=2.0015,g ₂=2.0055 andg ₃=2.065 and are ascribed to an oxygen related hole center in coesite. The observed signal atg _‖=2.0033 andg _⊥=2.0085 might be related with an O- or O₂-type hole center.     

Etude par resonance paramagnetique electronique de sulfures de molybdene et de tungstene

Chalcogenides MeS_3−x (Me = Mo, W; 0 ⩽ x ⩽ 1) have been prepared by the thermal decomposition of tetrathiometallates. These compounds are largely amorphous; desulfurization as well as crystalline organization occur when the heat treatment temperature (TT) increases. At 1000°C a perfect hexagonal arrangement can be obtained (x = 1). ESR spectra of amorphous species having x in the range 0–1 have been recorded. A part of the ESR signals is ascribed to sulfur chains whilst the other part is attributed to Me (V) species. The intensity of both series of ESR lines is a decreasing function of TT. The spectra are characteristic of Me (V) in an axial symmetry environment. At TT ≅ 1000°C a narrow ESR signal remains, which is assigned to electron spin centers of impurities (g-value close to that of the free electron). The results are explained either in terms of continuous structural evolution of the solid during heat treatment or in terms of a biphasic mixture of amorphous MoS₃ and MoS₂. The variation of the spin concentration as a function of TT is reported and compared with the results previously obtained from a study of the transport properties of these solids. In all cases the values obtained for Me = W are smaller than those observed on Me = Mo.     

Electron spin resonance studies on γ-irradiated KMnO4

Electron spin resonance (ESR) studies have been carried out on the radicals produced by the γ-irradiation of single crystals of KMnO₄.Analysis of the data yields at least two paramagnetic species. The first radical is produced by room temperature irradiation and gives a single line for each of two inequivalent magnetic sites. There is no resolvable hyperfine structure and the g tensor for the species is given by, g_b = g_μ′ = 2·0176, g_x′ = 2·0020, g_z′ = 2·0112. The spectrum is tentatively assigned to the O₃^? radical although MnO₂ and MnO₄ are also possibilities.The second radical is produced by irradiation at 77°K and the spectrum disappears irreversibly at slightly higher temperatures. Analysis of the very complicated spectra yields, g_b = 1·9617, g_x = 1·9452, g_u = 1·9940, A_b = 386 MHz, A₂ = 87 MHz, A₃ = 20 MHz. This spectrum is assigned to the MnO₄^?2 radical. The relationship of the radicals observed to the effect of high energy radiation on the decomposition of KMnO₄ in the light of current solid state theory is discussed.     

Low temperature ESR spectra of nickel — doped NaCl crystals

Paramagnetic resonance of Bridgman-grown NaCl crystals nominally doped with divalent nickel was studied over the temperature range from 4 K to 300 K. The broad isotropic EPR line with g = 2.12 ÷ 2.23 and ΔH _pp = 280 ÷ 780 G was detected at temperatures higher than 130 K for samples of various thermal history. The origin of these spectra was attributed to aggregated and precipitated Ni²⁺ ions. At temperatures lower than 40 K another ESR signal was registered. It was an anisotropic one, the intensity of which strongly depends upon the temperature. This spectrum results from substitutional Ni⁺ ions with tetragonally distorted surrounding along a S100T direction. The angular dependence of the ESR line positions was fitted to an appropriate spin-Hamiltonian with the g-factor components g _∥ and g _⊥ equal to 2.86 and 2.10, respectively. The temperature-dependence of the signal intensity at temperatures between 4 and 110 K has been interpreted in terms of a transition from the static to dynamic Jahn-Teller effect.     

ESR study of color centers in yttria stabilized zirconia

The ESR of single crystals of yttria stabilized zirconia (YSZ), containing color centers due to reduction in a flowing H₂ atmosphere, has been measured at room temperature. The spin density is about 3 × 10¹⁷ cm^?3 and the strong ESR line displays axial symmetry with principal values g₆ = 1.989, g_⊥ = 1.860.The behaviour of the ESR spectra when the magnetic field is rotated about the 〈1 0 0〉 and 〈1 1 0〉 directions shows that the centers have axial symmetry, the symmetry axis being along the 〈111〉 and equivalent directions. The derivative peak to peak envelope width is g-dependent and varies from 18 Gauss at g = 1.989 to 65 Gauss at g = 1.860. The results are interpretable in terms of an electron trapped at an oxygen vacancy adjacent to an yttrium ion.     

Synthesis and characterization of hexakis(acetonitrile)chromium(III) tetrafluoroborate, [Cr(NCMe)6][BF4]3. A nonaqueous Cr source

Oxidation of [Cr^II(NCMe)₄][BF₄]₂ with thianthrinium tetrafluoroborate forms [Cr^III(NCMe)₆][BF₄]₃ exhibiting two ν_CN absorptions at 2331 and 2301 cm⁻¹, and has been structurally characterized with an average Cr-N distance of 1.999 Å. From the electronic absorption spectra the ligand field splitting, Δ₀, is 20,160 cm⁻¹, which is slightly larger than [Cr^III(OH₂)₆]³⁺ in accord with the divalent chromium analogues. The 298 K ESR has a resonance at g=1.9884, and the magnetic susceptibility has a 300 K moment of 3.85μ_B characteristic of S=3/2 Cr(III). The field dependence of the magnetization can be fit to the Brillouin function also characteristic of S=3/2.     

Reaktion von Pyridin-2-Carbonsäure Mit Tetrabutylammonium-Tetrachloro-Nitridotechnetat(VI) Sowie Tetrabutylammonium-Tetrachlorooxotechnetat (V)

Abstract

The reaction of 2-pyridine carboxylic acid (Hpic) with (n-Bu₄N)[TcNCl₄] and (n-Bu₄N)[TcOCl₄] in ethanol and methanol, respectively, yields the dinuclear μ-oxo complex [(pic)₂NTc-O-TcN(pic)(Hpic)Cl] and the monomeric complex [TcO(pic)₂Cl].

Visible and infrared spectroscopy, ESR, ¹H-NMR and ⁹⁹Tc-NMR have been used to characterize the new compounds. The most important field of application for the new compounds synthesized is radiodiagnostics.     

Site-specific oxidation of the [Fe4S4] cubanes in high-potential iron sulfur proteins as probed by EPR and orientation-selective proton ENDOR spectroscopy:Ectothiorhodospira halophila I versusRhodocyclus tenuis

Electron paramagnetic resonance (EPR) and proton ENDOR (electron-nuclear double resonance) spectroscopies were used to analyze the structural and electronic parameters of the oxidized [Fe₄S₄] cubane clusters in high-potential iron sulfur proteins (HiPIPs) fromEctothiorhodospira halophila (HiPIP I) andRhodocyclus tenuis. TheE. halophila HiPIP I EPR spectra at X- and Q-band revealed a dominant species (simulated withg _max=2.1425,g _int=2.0315,g _min=2.0296) and a minor species (ca. 5–10% contribution) which was not analyzed further. ForR. tenuis HiPIP the EPR spectrum contained a single species only (g _max=2.1140,g _int=2.0392,g _min=2.0215), i.e., withg _max significantly smaller than that of theE. halophila protein. Orientation-selected proton ENDOR spectra of HiPIP I ofE. halophila were reconstructed by simulation with slight modifications of the crystal structure data. ENDOR from two mutants, F36S and F36G, ofE. halophila HiPIP I gave evidence for a common assignment of a HB2 proton of a phenylalanine residue (36 and 44, respectively, in isoenzymes I and II as reported earlier) interacting with the mixed-valence pair iron ions Fe2 and Fe3. ForR. tenuis HiPIP, the ENDOR spectra were assigned to arise from Fe3 and Fe4 as mixedvalence pair under the assumption of an unchanged intrinsicg-tensor symmetry. The resulting site specificity of the cubane oxidation was discussed in relation to structural requirements and redox potentials of the two HiPIPs.     

ESR investigation of some copper(II)-dioxime-dichloride compounds

The following copper(II)-dioxime-dichloride compounds: [Cu(G)Cl₂]₂, [Cu(P)Cl₂]₂, [Cu(DMG)Cl₂]₂ and [Cu(O)Cl₂]₂ (where G=C₂H₄O₂N₂, P=C₃H₆O₂N₂, DMG=C₄H₈O₂N₂, O=C₈H₁₄O₂N₂) were investigated by ESR method. Spectra of powder samples obtained in the g?2 region suggest the presence of triplet ground state (S=1) realized by a weak ferromagnetic exchange coupling. In liquid and frozen solutions the monomeric species (S=1/2) prevail. Some delicate changes in the coordination polyhedra symmetry in terms of ligand molecules and solvents nature were evidence. A 4p-admixture degree of 2% in the d_xy ground state was estimated for pseudotetrahedral (Td) species in frozen solutions.