Nuclear magnetic resonance (NMR) spectroscopy offers an element-selective, inherently quantitative and experimentally very flexible approach for the structural elucidation of non-crystalline materials. The present review introduces the basic concepts of this technique, highlighting the use of advanced NMR methodology for characterizing short- and intermediate range order in bioactive glass systems. The current state of the literature in this field is summarized in a comprehensive manner. NMR can give clear-cut and quantitative answers about the extent of network polymerization, the spatial distribution of the network former and network modifier species, and the structural roles of Group III elements introduced into these networks. These results facilitate our understanding of the influence of bioglass compositions upon the dissolution kinetics and bioactivities of these glasses. A particular mission of this review is to highlight the utility of non-routine, more advanced experimentation, in the hope of their increased usage and circulation in future applications.
The main six nuclear isotopes used in obtaining high-resolution magic-angle spinning NMR spectra for the structural characterization of bioactive glasses
Absolute photoabsorption cross sections for Nan+ (2≤n≤21) were measured in the visible energy range. The cluster ions were produced in a gas aggregation source and thus have a canonical distribution of internal energy corresponding to a temperature of ~ 105 K. The spectra for n≤9 and 11 exhibit between two and six absorption lines, and are in qualitative agreement with ab inito quantum chemical calculations. For n=15 and 21, the position of the resonances can be explained as excitations of a nearly free electron gas in a spheroidal container. An evolution is thus observed from molecular-like transitions to a giant collective resonance of the electron cloud. The integrated oscillator strength is 0.95 per 3s-electron in the energy range covered for n≥4, showing that the main excitations of the valence electrons have been found. 相似文献
Ende August haben Sigma‐Aldrich High‐Technology und die NRW Graduate School of Chemistry ihren zweiten gemeinsamen Doktoranden‐Workshop in Materials Chemistry veranstaltet. Mehr als 60 Doktoranden aus dem In‐ und Ausland haben drei Tage in den Münsteraner chemischen Instituten über Materialwissenschaften diskutiert. 相似文献
A new preparation route towards rare-earth (RE) doped polycrystalline lead lanthanum zirconate titanate (PLZT) ceramics (RE = Y3+, Nd3+, Yb3+), based on the use of doped lanthanum oxide or zirconia, is reported. Structural characterization by X-ray powder diffraction reveals that secondary phase formation can be substantially diminished in comparison to conventional preparation methods. The distribution of the rare-earth dopants was investigated as a function of concentration by static 207Pb spin echo NMR spectra, using Fourier Transformation of Carr–Purcell–Meiboom–Gill spin echo trains. For the Nd- and Yb-doped materials, the interaction of the 207Pb nuclei with the unpaired electron spin density results in significant broadening and shifting of the NMR signal, whereas these effects are absent in the diamagnetic Y3+ doped materials. Based on different concentration dependences of the NMR lineshape parameters, we conclude that the structural role of the Nd3+ dopants differs significantly from that of Yb3+. While the Nd3+ ions appear to be statistically distributed in the PLZT lattice, incorporation of Yb3+ into PLZT appears to be limited by the appearance of doped cubic zirconia as a secondary phase. 相似文献
The spectral theory of the Friedrichs model on the positive half line with Hilbert–Schmidt perturbations, equipped with distinguished
analytic properties, is presented. In general, the (separable) multiplicity Hilbert space is assumed to be infinite-dimensional.
The results include a spectral characterization of its resonances and the association of so-called Gamov vectors. Sufficient
conditions are presented such that all resonances are simple poles of the scattering matrix. The connection between their
residual terms and the associated Gamov vectors is pointed out.
Dedicated to Izrail Cudicovič Gohberg on his 80th birthday
Submitted: May 15, 2008., Accepted: October 28, 2008. 相似文献
Vitreous samples (1-x) AgPO3–x MoO3 (0 ≤ x ≤ 0.5) were prepared by conventional melt-quenching and characterized by Differential Scanning Calorimetry (DSC). The structural evolution of the vitreous network was monitored by 31P solid state nuclear magnetic resonance and Raman scattering, and assignments were aided by corresponding studies on the model compound AgMoO2PO4. The 31P MAS-NMR data differentiate between species having two, one, and zero P―O―P linkages (Q(2) Q(1), and Q(0) species), respectively. Interatomic connectivities involving these units are revealed by two-dimensional INADEQUATE data, utilizing the formation of double quantum coherences mediated by indirect 31P–31P spin–spin interactions via P―O―P linkages. As this method discriminates against isolated P atoms, it also serves as an important spectral editing tool for constraining lineshape fits. 95Mo NMR data and Raman spectra suggest that the Mo species are most likely six-coordinate, forming four P―O―Mo linkages and are otherwise invariant with composition, except at MoO3 contents ≥ 40 mole %, where some Mo―O―Mo bonding and/or clustering is observed. 相似文献
The synthesis of bifunctional mesoporous silica nanoparticles is described. Two chemically orthogonal functionalities are incorporated into mesoporous silica by co‐condensation of tetraethoxysilane with two orthogonally functionalized triethoxyalkylsilanes. Post‐functionalization is achieved by orthogonal surface chemistry. A thiol–ene reaction, Cu‐catalyzed 1,3‐dipolar alkyne/azide cycloaddition, and a radical nitroxide exchange reaction are used as orthogonal processes to install two functionalities at the surface that differ in reactivity. Preparation of mesoporous silica nanoparticles bearing acidic and basic sites by this approach is discussed. Particles are analyzed by solid state NMR spectroscopy, elemental analysis, infrared‐spectroscopy, and scanning electron microscopy. As a first application, these particles are successfully used as cooperative catalysts in the Henry reaction. 相似文献
Magic-angle-spinning NMR spectra of aprotic solids, ceramics and glasses frequently suffer from poor site resolution due to wide chemical shift distribution effects. In such cases, cross-polarization and heteronuclear double-quantum filtering experiments involving nuclei other than 1H offer unique spectral editing capabilities. The utility of such assignment techniques for examining site populations in semiconductor alloys is demonstrated for the chalcopyrite systems CdGeAs2−xPx, CdSiAs2−xPx and ZnxCd1−xGeP2. The results permit a distinction between local and non-local effects on experimental chemical shift trends and reveal that compositional dependences observed in these alloys are dominated by non-local chemical shift contributions. 相似文献