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《Journal of Inorganic and Nuclear Chemistry》1981,43(12):3213-3215
25Mg nuclear magnetic resonance studies on 25MgCl2 in methanol containing varying amounts of water indicate that magnesium ions are preferentially solvated by water. The exchange rate between the fully and partially methanolated species is slow on the NMR time scale so that the 25Mg spectrum consists of a superposition of the two spectra at −40°C. 相似文献
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The structural analysis of three boron compounds, boron carbide (B4C), silicon tetraboride (SiB4) and hexagonal-boron nitride (h-BN), were performed using 2D 11B-triple quantum MAS (3QMAS) solid state NMR capable of averaging the second-order quadrupolar interaction of 11B that cause line broadening, splitting and low frequency shift of the central transition (−1/2, 1/2). The coordination number around the boron atom and structural symmetry of each boron compound is discussed by means of the isotropic chemical shift Δσ and the quadrupolar coupling constant CQ calculated from 3QMAS spectra. Δσ of SiB4 is quite larger than that of B4C, which is thought to be caused by its structural distortion and distribution. Δσ of h-BN was found to be higher frequency shift obviously than that of B4C and SiB4 because of the difference of the boron coordination number, three-coordinated in h-BN and six-coordinated in B4C and SiB4. h-BN has very large CQ compared to other two boron compounds since the h-BN forming a two-dimensional network has less structural symmetry than B4C and SiB4. 相似文献
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We report a solid-state (25)Mg NMR spectroscopic study of two magnesium-containing organic compounds: monopyridinated aqua(magnesium) phthalocyanine (MgPc.H(2)O.Py) and chlorophyll a (Chla). Each of these compounds contains a Mg(II) ion coordinating to four nitrogen atoms and a water molecule in a square-pyramidal geometry. Solid-state (25)Mg NMR spectra for MgPc.H(2)O.Py were obtained at 11.7 T (500 MHz for (1)H) for a (25)Mg-enriched sample (99.1% (25)Mg atom) using both Hahn-echo and quadrupole Carr-Purcell Meiboom-Gill (QCPMG) pulse sequences. Solid-state (25)Mg NMR spectra for Chla were recorded at (25)Mg natural abundance (10.1%) at 19.6 T (830 MHz for (1)H). The (25)Mg quadrupole parameters were determined from spectral analyses: MgPc.H(2)O.Py, C(Q) = 13.0 +/- 0.1 MHz and eta(Q) = 0.00 +/- 0.05; Chla, C(Q) = 12.9 +/- 0.1 MHz and eta(Q) = 1.00 +/- 0.05. This work represents the first time that Mg(II) ions in a square-pyramidal geometry have been characterized by solid-state (25)Mg NMR spectroscopy. Extensive quantum mechanical calculations for electric-field-gradient (EFG) and chemical shielding tensors were performed at restricted Hartee-Fock (RHF), density functional theory (DFT), and second-order M?ller-Plesset perturbation theory (MP2) levels for both compounds. Computed (25)Mg nuclear quadrupole coupling constants at the RHF and MP2 levels show a reasonable basis-set convergence at the cc-pV5Z basis set (within 7% of the experimental value); however, B3LYP results display a drastic divergence beyond the cc-pVTZ basis set. A new crystal structure for MgPc.H(2)O.Py is also reported. 相似文献
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The thermal expansion of some binary and ternary high lead silicate glasses was measured from room temperature to above the softening temperature. The expansion curves for all the glasses showed similar characteristics. The increase of lead oxide content increased the thermal coefficient of expansion but to a lesser extent than the alkaline fluxes. The various proposed mechanisms of thermal expansion were advanced and the experimental results could be explained by considering the bond strengths and ionic sizes of the cations. The possible effects of compactness and phase separation of the structure were also examined. 相似文献
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《Solid State Sciences》2001,3(1-2):235-243
This paper reports on the structural investigation of lithium and sodium thiosilicate crystals and glasses by means of X-ray photoelectron spectroscopy and ab initio calculation. The results are analysed in conjunction with previously reported 29Si NMR data. While NMR proved to be an effective tool for the quantitative discrimination of edge- and corner-sharing tetrahedra existing in these materials, X-ray photoelectron spectroscopy (XPS) gives information on the nature of SiS bonds, i.e. bridging and non-bridging bonds. The main result is the noticeable difference existing between the structures of lithium and sodium thiosilicate glasses, which, according to XPS data, is due to different electronic redistributions over the network when one or the other alkali is added, the sodium addition resulting in a change in the electronic distribution over the entire network. 相似文献
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Flame retardance of ethylene-vinyl acetate copolymer (EVA) can be achieved using magnesium hydroxide (Mg(OH)2) incorporated in the polymeric matrix. The adduct of small amount of zinc borate as synergistic agent in the formulation increases the fire-proofing properties. Multinuclei solid-state NMR appears as a powerful means to characterise materials before and after combustion. We show that endothermic dehydration, water vapour evolved and formation of a glassy coating provide the flame retardancy of interest to the polymeric matrix. 相似文献
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Natural abundance solid-state (25)Mg NMR measurements were made of the disodium salt of magnesium ethylenediaminetetraacetate tetrahydrate (Na(2)MgEDTA x 4 H(2)O). Both magic angle spinning (MAS) and static experiments were employed to determine the quadrupole coupling constant (C(q)) and the asymmetry parameter (eta(q)) of the electric field gradient (EFG) tensor associated with (25)Mg in this compound, giving the values C(q) = 1.675(5) MHz and eta(q) = 0.15(1). The isotropic chemical shift was determined to be delta(iso) = 0.25(10) ppm (relative to 11 M MgCl(2) aqueous solution) and a small chemical shift anisotropy (CSA) contribution (approximately -13 ppm) was detected, one of the first CSA reports in (25)Mg NMR. This compound exhibited remarkably good (25)Mg NMR sensitivity, due to its fast spin-lattice relaxation and modest quadrupole coupling, which allowed its use as a secondary shift reference and as a test sample for the implementation and optimisation of signal-enhancement methods in (25)Mg NMR spectroscopy, such as double frequency sweeps (DFS) and the use of adiabatic hyperbolic secant (HS) and WURST pulses. 相似文献
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Along with X‐ray diffraction measurements, 25Mg solid state NMR experiments were performed in natural abundance at 9.4 T on crystalline and mechanically milled samples of MgO, Mg(OH)2, MgF2, and magnesium hydroxide fluorides Mg(OH)xF2–x prepared on mechanochemical and sol‐gel syntheses routes. In addition to single pulse and spin‐echo sequences, both static 1H‐25Mg CP and 19F‐25Mg CP measurements allowed the registration of 25Mg spectra in attractive short measurement times. Although an assignment of different magnesium species in magnesium hydroxide fluorides is only hardly possible, position and line shapes of 25Mg spectra of magnesium hydroxide fluorides reflect chemically reliable trends. All samples studied here show a sixfold fluorine, oxygen, or mixed fluorine / oxygen coordination of magnesium, also in highly disordered samples. 相似文献
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The samples of dibarium magnesium orthoborate Ba2Mg(BO3)2 were synthesized by solid-state reaction. The X-ray diffraction (XRD) patterns and Raman spectra of the samples were collected. Electronic structure and vibrational spectroscopy of Ba2Mg(BO3)2 were systematically investigated by first principle calculation. A direct band gap of 4.4 eV was obtained from the calculated electronic structure results. The top valence band is constructed from O 2p states and the low conduction band mainly consists of Ba 5d states. Raman spectra for Ba2Mg(BO3)2 polycrystalline were obtained at ambient temperature. The factor group analysis results show the total lattice modes are 5Eu + 4A2u + 5Eg + 4A1g + 1A2g + 1A1u, of which 5Eg + 4A1g are Raman-active. Furthermore, we obtained the Raman active vibrational modes as well as their eigenfrequencies using first-principle calculation. With the assistance of the first-principle calculation and factor group analysis results, Raman bands of Ba2Mg(BO3)2 were assigned as Eg (42 cm−1), A1g (85 cm−1), Eg (156 cm−1), Eg (237 cm−1), A1g (286 cm−1), Eg (564 cm−1), A1g (761 cm−1), A1g (909 cm−1), Eg (1165 cm−1). The strongest band at 928 cm−1 in the experimental spectrum is assigned to totally symmetric stretching mode of the BO3 units. 相似文献
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Magnesium-25 NMR measurements were carried out on aqueous and non-aqueous solutions of magnesium salts. In the former case the25Mg resonance frequency was independent of the concentration or of the counterion. In nonaqueous solvents, however, the resonance frequency was dependent on the solvent, the concentration, and on the nature of the counterion, indicating some cation-anion interactions. Measurements on Mg2+—phosphonoacetic acid mixtures in aqueous solutions gave strong indications of complexation. Only inconclusive evidence was obtained on the complexation of Mg2+ by macro-bicyclic cryptand C211 in methanol solutions, and no evidence of complexation was obtained with macrocycle 12-crown-4 in dimethylformamide solutions. 相似文献
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J.A. Capobianco N. Raspa G. Cormier A. Monteil G. Boulon 《Chemical physics letters》1990,170(5-6):467-472
The optical absorption and emission of Cr3+ in glasses and glass-ceramics of the magnesium aluminium silicate system are reported. Glasses and glass-ceramics of high optical quality can be easily synthesized in the XMgO·Al2O3·2.5SiO2 system simply by varying X (0.5–2.0) without recourse to thermal treatment. Evidence that Cr3+ resides in both the crystalline and glass phase of the glass-ceramic is presented. Furthermore the crystalline phases are identified as Al2O3 and MgAl2O4, and the glass phases as mullite and cordierite. 相似文献
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27Al NMR study of the structure of lanthanum- and yttrium-based aluminosilicate glasses and melts 总被引:1,自引:0,他引:1
Florian P Sadiki N Massiot D Coutures JP 《The journal of physical chemistry. B》2007,111(33):9747-9757
We have investigated by (27)Al nuclear magnetic resonance spectroscopy some compositions in the Ln(2)O(3)-Al(2)O(3)-SiO2 (Ln = Y or La) ternary phase diagram containing more than 60 mol % of SiO2. One- and two-dimensional high-field (17.6 T) high-speed (30 kHz) magic angle spinning experiments have been performed along with simulations of the spectra to quantify the amount of penta-coordinated aluminum present in those glasses as a function of composition. Very high-temperature experiments have allowed to follow selected samples from 2200 degrees C down to 1700 degrees C and hence to characterize the aluminum coordination state and dynamics in those liquids. The present study re-enforces the current view that "minor" species such as penta-coordinated aluminum are actually present in a considerable amount in aluminosilicate glasses, and high-temperature liquids at and above the charge compensation join. The high-field strength of Y3+ and La3+ reveal, for the first time in glasses, a different mean electric field gradient perceived by the tetra- and penta-coordinated aluminum environments. The movements responsible for the NMR relaxation of aluminum in the high-temperature liquid are shown to be uncorrelated with the movements responsible for the macroscopic shear viscosity. Results obtained both on glasses and in situ at high-temperature suggest a preferential localization of Ln3+ nearby tetra-coordinated aluminum species, with possible formation of tricluster and/or Ln3+ coordination changes. 相似文献
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M. G. Ferreira Da Silva J. M. Fernàndez Navarro 《Journal of Sol-Gel Science and Technology》1994,2(1-3):895-901
The effect of thermal treatment on the structural incorporation of Cr2O3 in xCr2O3·(100 – x)SiO2 and 5R
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O·xCr2O3·(95 – x)SiO2 (where x=0.01–1 mol.% and R=Li, Na and Ca) gel glasses was studied by optical absorption spectrophotometry, DTA-TG, XRD and electron microscopy.Samples heat treated at 60°C have green color due to the presence of Cr3+ in octahedral coordination. The optical transmission, and color (yellow, orange or ambar), of the samples heat treated between 200 and 700°C prove that Cr3+ (octahedrally coordinated) and Cr6+ (tetrahedrally coordinated) are both present. Segregation of Cr2O3 take place at temperatures above 800°C.In reducing conditions the gel glasses were green due to the presence of a high content of Cr3+ ions. Samples containing Li or Na show crystalline phases at temperatures below 850°C. 相似文献
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The thermal conductivity, density, refractive index and viscosity of glasses with base composition (wt%): Na2O, 20; CaO, 15; SiO2 65, in which CaO is partially replaced by BaO, have been investigated.Both the thermal conductivity and the viscosity—temperature values progressively decreased by introducing BaO to replace CaO in the glasses. Meanwhile, this replacement caused an increase in other properties, i.e., density and refractive index. The experimental measurements and calculated values of either thermal conductivity or viscosity agree fairly well. The observed effects were correlated to factors such as polarization, field strength and ionic radii of the incorporated divalent cations, and their effect on the geometrical arrangement of the building units of the glassy network. 相似文献