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About Nanocrystalline Hydroxyapatite and Apatite Binding in Bone: A Solid State-NMR Perspective
(2004)
About nanocrystalline Hydroxyapatite and the binding of apatite in bone: a novel NMR perspective
(2004)
The preparation and structural investigation of 17O-enriched xNa2O(100-x)P2O5 glasses (46.5<=x<=62.8) by nuclear magnetic resonance (NMR) is described. Enriched phosphoric acid was prepared by hydrolysis of PCl5 with 17O-enriched water and neutralized with sodium carbonate. The sodium metaphosphate was then melted at 800 °C for 15 h and quenched. Polyphosphate and ultraphosphate glass compositions were prepared by remelting the metaphosphate with sodium carbonate and phosphorus pentoxide, respectively. 31P magic angle sample spinning (MAS) NMR was used to determine the Na2O/P2O5 content in the glasses. 17O NMR spectra (quadrupole echo for non-rotating samples and multiple-quantum excitation for rotating samples (MQMAS)) show two oxygen sites in the samples with large quadrupolar coupling constants (4.7 and 7.7 MHz), in accordance with the high phosphorus electronegativity. According to the correlation of 17O quadrupolar constants with bond ionicity, these two components are attributed to bridging POP and non-bridging PO...Na oxygens. The average POP bond angle is estimated with the quadrupolar asymmetry derived from the fit of the static echo spectra. The MQMAS spectrum shows a distribution of non-bridging oxygen chemical shifts, attributed to a variation of bond length and angle.
Reaction of 2-hydroxy-para-benzoquinones with DMSO/Ac2O produced dimethylsulfonium ylides, of which crystal structures as well as solid and liquid state NMR spectra were recorded. The ylides react with tertiary methylamine N-oxides in a one-pot, multi-step process to 3-methylamino-substituted benzoquinones. The mechanism starts with a deoxygenative deprotonation of the amine N-oxides, followed by a formal electrophilic displacement of DMSO by the resulting carboniumiminium ion.
17O enriched sodium borophosphate glasses were prepared from isotopically enriched NaPO3 and H3BO3. These glasses have been studied by 17O, 11B and 31P NMR including 17O and 11B multiple quantum magic angle sample spinning (MQMAS), 11B31P heteronuclear correlation (HETCOR) NMR and 11B 31P rotational echo double resonance (REDOR). For comparison, the crystalline borophosphates BPO4 and Na5B2P3O13 were included in the investigations. The latter compound shows three sharp 31P resonances at -0.2, -2 and -8 ppm and two BO4 sites that can only be resolved by MQMAS.
The 17O NMR spectra were recorded using both the static echo method at medium magnetic field (9.4 T) as well as MAS and MQMAS methods at high field (17.6 T). In total, five oxygen sites were identified in these borophosphate glasses: POP, NaOP, POB, BOB, NaOB. However, these five sites are not present simultaneously in any of the glasses. The 17O MQMAS spectra prove that POB links play a major role in borophosphate glasses. These results are confirmed by the complementary 11B MAS spectra that show the presence of asymmetric and symmetric trigonal groups BO3a and BO3s and two tetrahedral BO4 units. 11B 31P REDOR NMR is used to give independent information to assign the 11B lines to structural units present in the glasses. These REDOR measurements reveal that BOP bonds are present for each borate unit, including the BO3 groups. Particularly, a structural proposal for the two different BO4 resonances is given in terms of a different number of bonded phosphate tetrahedra. The 31P MAS spectra are usually broad and not well resolved. It is shown by 11B31P HETCOR NMR that a possible structural assignment of a 31P signal at about -20 ppm to Q2 units as in binary sodium phosphate glasses is wrong and that the phosphate tetrahedron belonging to this resonance must be connected to borate groups
Mixed sodiumlead borophosphate glasses were prepared in two compositional series xNa2O(50 - x)PbO10B2O340P2O5 and xNa2O(50 - x)PbO20B2O330P2O5 with x = 0, 10, 20, 30, 40 and 50 mol% Na2O. Their density decreases and molar volume increases slightly with increasing Na2O content. Chemical durability decreases steeply for glasses containing 4050 mol% Na2O, whereas glass transition temperatures reveals a minimum at the glasses with 3040 mol% Na2O. The 31P MAS NMR and Raman spectra of the first series show that with increasing Na2O content the ratio of structural units Q2/Q1 slowly increases and the Q2 phosphate units and BO4 units prevail in the structure of these glasses. In the second series of glasses Q1 diphosphate units together with BO4 and small number of BO3 units form structural network of glasses. The replacement of PbO by Na2O results in higher ionicity of bonding interactions between compensating cations and the anionic network. The lower cation field strength of Na+ ions results in the observed downfield shift of ? (31P) values in Na-rich glasses and narrower vibrational bands in their Raman spectra.
Fabrication of carbon/silica hybrid materials using cationic polymerization and the sol-gel process
(2002)
Rotational echo double resonance (REDOR) of spin- nuclei is an extremely useful tool for the determination of distances in solids as well as of relative orientations of chemical shift and dipole tensors. We present the corresponding version for measuring the relative orientation of electric quadrupole and dipole tensors and demonstrate its applicability for non-bridging oxygens in phosphate glasses using 17O{31P} REDOR NMR. The orientational information is found in the changes of the second-order quadrupole patterns as a function of the echo delay. Results and numeric simulations are presented for 17O{31P} REDOR NMR of 17O-enriched sodium phosphate glasses. For non-bridging oxygens, the symmetric quadrupole tensor is found to be aligned along the phosphorusoxygen bond. The distance between P and the non-bridging oxygen is calculated for two glasses of different compositions.
Two different radical polymerization techniques have been applied to covalently graft vinylformamide (VFA) onto silica particles. Grafting by the polymerization of VFA using an immobilized azo initiator on silica has been found less effective, because monomer conversion is limited in non-aqueous solvents and grafting yields are low in water. Radical copolymerization of VFA with vinyltriethoxysilane (VTS)-functionalized silica particles is suitable to produce poly(vinylformamide) (PVFA) silica hybrid particles in respectable yield. The VFA/VTS-silica ratio determines the degree of grafting. The PVFA-VTS-co-grafted silica particles can be acidically hydrolyzed into poly(vinylamine)-grafted silica particles. Molecular structures of the surface groups and grafted polymer chains have been confirmed by means of solid state 13C{1H} cross-polarization magic-angle spinning (CP MAS) NMR spectroscopy. Zeta potential measurements show the altering of the former silica particles surface charges arising from the introduction of basic groups on the surface.
Glasses of the compositional series of 50PbOyB2O3(50-y)P2O5xTiO2 (with x=016) have been prepared and characterized by the determination of density, molar volume, glass transition temperature, dilatation softening temperature, thermal and chemical durability. Structural changes were evaluated from the studies by 31P and 11B MAS NMR, Raman and infrared spectroscopy. Molar volume of the glasses decreases and the glass transition temperature increases with increasing additions of TiO2. 31P MAS NMR spectra revealed that titanate units enter phosphate chains decreasing the number of Q2 units and increasing the number of Q1 units. This result is confirmed by the observed changes in the Raman and infrared spectra of the TiO2-doped glasses.