Paramagnetic centers by X-ray-irradiation of aluminium hydroxide

EPR spectra of paramagnetic centers originating from X-ray-irradiation of aluminium hydroxide at room temperature have been measured. The EPR spectrum represents a superposition of EPR spectra of ionic centers O^–, holes of type and trapped electrons. Radiation chemical yield of paramagnetic centers observed at room temperature (293 K) is G(spins.)=4.4±0.6) spins per 100 eV absorbed energy. The decay of paramagnetic centers in irradiated Al(OH)₃ was oberved at 293 K. The rate constant of the paramagnetic centers decay in irradiated Al(OH)₃ is K₂=(0.0980±0.0019) kg·mol^–1·min^–1 and their half-life is 9.43±0.18 days.Dedicated to the memory of the late Genrikh Markovis Kolyiari.     

Tunnelling electron transfer from bulk electron centers of magnesium oxide to adsorbed molecules of nitrous oxide

Tunneling electron transfer from bulk F⁺-centers of MgO to molecules of N₂O adsorbed on MgO surface has been detected and studied.

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F⁺- N₂O, .

     

Study of relaxation rates of stable paramagnetic centers in gamma-irradiated alanine

The stable L-alanine radical induced by gamma-irradiation was examined by electron paramagnetic resonance (EPR), transfer saturation EPR and electron nuclear double resonance (ENDOR) in the temperature region of fast motion of the methyl group (180-320 K). From the obtained spectral line broadening and spectral intensity the correlation time for the methyl rotation was estimated. The complex processes determining the relaxation rate were examined in the same temperature interval. It was shown that important contributions to the relaxation rate arise from non-secular and pseudo-secular types of contributions. The non-secular contribution involves intramolecular dynamics while the pseudo-secular contribution originates from intermolecular motions. The obtained values for the dynamical parameters have been compared with those obtained by pulse EPR methods and by proton nuclear magnetic resonance (NMR) on undamaged crystals.     

Paramagnetic centers of photocatalysts based on nitrogen-doped titanium dioxide

Titanium dioxide doped with nitrogen (N-TiO₂) was studied by electron paramagnetic resonance spectroscopy. Two types of paramagnetic centers were observed: N^· and NO. The concentration of these centers depends significantly on temperature and irradiation with visible and UV light. It was established that the energy levels corresponding to the N^· and NO centers are, respectively, 2.3 and 2.0 eV below the N-TiO₂ conduction band. These centers are involved in the photooxidation of organic compounds on the N-TiO₂ surface.     

Paramagnetic centers in silver oxalate in the initial stages of photolysis and radiolysis

Conditions of generation and removal are considered for one type of paramagnetic center produced in polycrystalline silver oxalate by X-rays, -rays, and UV light. The ESR spectra show that these are C₂O ₄ ^– ions trapped at cation vacancies. The mechanisms of photolysis and radiolysis are discussed.     

Paramagnetic bridging hydrides of relevance to catalytic hydrogen evolution at metallosulfur centers

Paramagnetic hydrides are likely intermediates in hydrogen-evolving enzymic and molecular systems. Herein we report the first spectroscopic characterization of well-defined paramagnetic bridging hydrides. Time-resolved FTIR spectroelectrochemical experiments on a subsecond time scale revealed that single-electron transfer to the μ-hydride di-iron dithiolate complex 1 generates a 37-electron valence-delocalized species with no gross structural reorganization of the coordination sphere. DFT calculations support and (1)H and (2)H EPR measurements confirmed the formation an S = ? paramagnetic complex (g = 2.0066) in which the unpaired spin density is essentially symmetrically distributed over the two iron atoms with strong hyperfine coupling to the bridging hydride (A(iso) = -75.8 MHz).     

Montmorillonite incorporated with magnesium oxide

MgO was incorporated into montmorillonite (MMT) though the controlled formation of Mg(OH)₂ followed by calcination. The evidence for Mg²⁺ in MMT before calcination was concluded from TG curves where two different temperature ranges for Mg(OH)2 dehydroxylation suggested that the location of Mg²⁺ at its internal and external surfaces. FTIR by diffuse reflectance technique of calcined sample showed a typical MgO band. XRD indicated a decrease in structural order and no collapse of clay structure. The organic compounds adsorption to MMT modified with MgO as suggested by increased degradation temperatures occurred at internal and external clay surfaces besides in an area formed probably by MgO and MMT layers.     

Paramagnetic defect centers at the MgO surface. An alternative model to oxygen vacancies

On the basis of embedded cluster calculations, we propose a new model for the structure of paramagnetic color centers at the MgO surface usually denoted as F(S)(H)(+) (an electron trapped near an adsorbed proton). These centers are produced by exposing the surface of polycrystalline MgO to H(2) followed by UV irradiation. We demonstrate that properties of H atom absorbed at surface sites such as step edges (MgO(step)) and reverse corner sites (MgO(RC)), formed at the intersection of two step edges, are compatible with a number of features observed for F(S)(H)(+). Our calculations suggest that (i) H(2) dissociates at the reverse corner site heterolytically and that there is no barrier for this exothermic reaction; (ii) the calculated vibrations of the resulting MgO(RC)(H(+))(H(-)) complex are fully consistent with the measured ones; (iii) desorption of a neutral H atom from the diamagnetic precursor requires UV light and leads to the formation of stable neutral paramagnetic centers at the surface, MgO(step)(H(+))(e(-))(trapped) and MgO(RC)(H(+))(e(-))(trapped). The computed isotropic hyperfine coupling constants and optical transitions of these centers are in broad agreement with the existing experimental data. We argue that these centers, which do not belong to the class of "oxygen vacancies", are two of the many possible forms of the F(S)(H)(+) defect center.     

Hydration of medium reactive industrial magnesium oxide with magnesium acetate

Medium reactive magnesium oxide reacts incompletely with available water to form magnesium hydroxide. To enhance the hydration of medium reactive magnesium oxide, the effect of magnesium acetate as hydrating agent was studied. The extent to which different parameters (concentration of magnesium acetate, solution temperature and solid to liquid ratio of MgO to magnesium acetate) influence the hydration rate of a medium reactive industrial sample of magnesium oxide were evaluated. The degree of rehydration measured as percentage Mg(OH)₂being formed, increases from approximately 56% using 0.5 M magnesium acetate solutions at 25°C to 64% at 50°C, to more than 70% at 70°C. The major part of rehydration of the medium reactive MgO sample occurs within the first few minutes of the reaction for all three temperatures studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of nanocrystalline magnesium oxide by thermolysis of magnesium citrate

Magnesium citrate, obtained by dissolving magnesium oxide in an aqueous solution of citric acid, turned out to be a decahydrate of the composition Mg₃(C₆H₅O₇)₂·10H₂O, which was established by the results of synchronous thermal analysis, X-ray diffraction and IR spectroscopy. It is shown that the thermal decomposition of this salt proceeds in three stages in the temperature ranges of 120–250, 250–370 and 370–550 °C to form nanocrystalline magnesium oxide with grain sizes from 7 to 23 nm.     

Reaction of magnesium oxide and magnesium silicates with ammonium hydrodifluoride

The reactions of magnesium oxide and magnesium silicates (forsterite and serpentines) with ammonium hydrodifluoride is studied using DTA, X-ray powder diffraction, and IR spectroscopy. The conditions for the formation of intermediate phases are determined. The structure of the silicate mineral does not significantly affect the fluorination.     

The preparation of high-purity magnesium oxide

The results are presented of an investigation with the radioactive isotope⁴⁵Ca of processes of coprecipitation, crystallization and liquid—liquid extraction of calcium for the preparation of high-purity salts of magnesium.     

The preparation of high-purity magnesium oxide

Investigations on the separation of trace amounts of Ba, Mn, Fe, Sr+Y, Co, Ca, Zn, Ag and Na from macro-amounts of Mg are described.     

Phase transformation in the cupric magnesium oxide system

Due to the incomplete removal of OH groups, dehydration at 500°C of solid solutions of magnesium ions in Cu(OH)₂ and of copper ions in Mg(OH)₂ formed by coprecipitation leads to the formation of nonequilibrium cupric magnesium oxyhydroxide compounds with the structures of CuO and MgO, respectively. As a result of high-temperature dehydration, cupric oxide and solid solutions are formed in which up to 20 at. % of magnesium in MgO is replaced by copper ions.

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Cu(OH)₂ Mg(OH)₂ 500°C OH- CuO MgO . - 20% MgO.