Biomedical subjects

J W Zwanziger

Publications and source records attributed to J W Zwanziger.

10 recordsLinked to original sources

PubMed2006

A neutron scattering and nuclear magnetic resonance study of the structure of GeO2-P2O5 glasses.

Germanophosphate (GeO2-P2O5) glasses were studied with neutron diffraction, phosphorus, and oxygen nuclear magnetic resonance, calorimetry, viscosity measurements, and first-principles calculations. These data sets were combined to propose a structural model of GeO2-P2O5 glasses, which includes tetrahedrally coordinated phosphorus, formation of octahedrally coordinated germanium as P2O5 content increases, an absence of trigonally coordinated oxygen, and hence an absence of rutile-like GeO2 domains. The structural model was then used to propose explanations for both the observed composition dependence of the glass transition temperature and the fragility of the GeO2-P2O5 liquids.

Journal Article↗

PubMed2005

Stress, strain, and NMR.

Experimental and ab initio results that demonstrate the effect of stress on the nuclear magnetic resonance spectra of materials are shown. The design of a cell that generates uniaxial compressive stress is presented, and results on gallium phosphide and lead nitrate single crystals that illustrate the observable results of the stress are shown. Tensors that relate stress and strain to changes in the chemical shielding tensors and the electric field gradient tensors are defined formally. The elements of these tensors are then computed by a density functional theory approach that makes use of planewaves and pseudopotentials. The experimental results are interpreted with the aid of the calculations. Extensions to spinning samples and to the interpretation of optical phenomena in materials are discussed.

Algorithms↗

PubMed2005

On the spectral similarity of bridging and nonbridging oxygen in tellurites.

We show by high field (17)O solid-state nuclear magnetic resonance (NMR) and by ab initio calculations of both the NMR and the oxygen 1s photoelectron spectra that the oxygen sites in tellurite glasses show no spectroscopic distinction, even when comparing bridging and nonbridging sites. This is remarkable because two such sites differ formally by a full electronic charge, and they are readily distinguished by these same methods in silicates. We argue that this similarity arises from the symmetry breaking that occurs when the original TeO(2) crystal solid forms, due to the pseudo-Jahn-Teller distortion induced by the two additional valence electrons present in Te(IV) as compared to Si(IV).

Journal Article↗

PubMed2001

Platinum-containing hyper-cross-linked polystyrene as a modifier-free selective catalyst for L-sorbose oxidation.

Impregnation of hyper-cross-linked polystyrene (HPS) with tetrahydrofuran (THF) or methanol (ML) solutions containing platinic acid results in the formation of Pt(II) complexes within the nanocavities of HPS. Subsequent reduction of the complexes by H2 yields stable Pt nanoparticles with a mean diameter of 1.3 nm in THF and 1.4 nm in ML. The highest selectivity (98% at 100% conversion) measured during the catalytic oxidation of L-sorbose in water is obtained with the HPS-Pt-THF complex prior to H2 reduction. During an induction period of about 100 min, L-sorbose conversion is negligible while catalytic species develop in situ. The structure of the catalyst isolated after the induction period is analyzed by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. Electron micrographs reveal a broad distribution of Pt nanoparticles, 71% of which measure less than or equal to 2.0 nm in diameter. These nanoparticles are most likely responsible for the high catalytic activity and selectivity observed. The formation of nanoparticles measuring up to 5.9 nm in diameter is attributed to the facilitated intercavity transport and aggregation of smaller nanoparticles in swollen HPS. The catalytic properties of these novel Pt nanoparticles are highly robust, remaining stable even after 15 repeated uses.

Catalysis↗

PubMed2000

Anisotropy-correlated spectroscopy of quadrupolar nuclei.

The two-dimensional anisotropy-correlated NMR (2DAC) spectra of half-integer quadrupolar nuclei may be recorded by using an exchange sequence in conjunction with magic angle spinning (MAS) during evolution and detection, and off-MAS during mixing. Application of this experiment to boron oxides is described, in addition to an analysis of the spin diffusion rates in such materials.

Anisotropy↗

PubMed2000

Through-bond connectivity in solids by continuous-wave spin lock.

A simple two-dimensional correlation experiment that enables determination of through-bond connectivity in the solid state is described. The experiment is performed under fast magic angle spinning (MAS) conditions. After the initial pi/2 pulse, the magnetization develops freely under the MAS Hamiltonian. The t1-period is followed by a strong spin locking pulse used as mixing period. The dipolar coupling is averaged out by magic angle spinning, and the chemical shifts and r.f.-offsets are scaled by the applied spin locking field. Hence, for strong locking conditions, the isotropic J-coupling is the dominant interaction. The mixing Hamiltonian is thus identical to the well-known TOCSY-Hamiltonian, resulting in a net through-bond magnetization transfer. The mixing-time dependence of the exchange rates is investigated. Applications to crystalline P4S7 and MgP4O11 are shown.

Magnesium Compounds↗

PubMed1999

Off-angle correlation spectroscopy applied to spin-1/2 and quadrupolar nuclei.

A two-dimensional correlation experiment is described, in which homonuclear dipolar couplings are used to realize through-space magnetization exchange on spin-1/2 (31P) and on quadrupolar nuclei (23Na and 11B). In the detection period, Magic Angle Spinning is applied to enhance resolution, and the dipole couplings are re-introduced in the mixing period by spinning off the Magic Angle. The dependency of the exchange rates on the mixing time and the spinning angle is investigated. The influence of strong spin-locking during mixing is discussed, and shown in the spin-1/2 case to remove the dependence on chemical shift offset effects. For quadrupolar spins, the experiment yields information on the relative tensor orientations of the coupled quadrupoles. Applications to crystalline sodium aluminum diphosphate, sodium sulphite, and potassium borate glasses are shown.

Boron↗

PubMed1995

Multi-nuclear and multi-dimensional nuclear magnetic resonance investigation of silver iodide-silver phosphate fast ion conducting glasses.

The results of a multi-nuclear nuclear magnetic resonance (NMR) study of (AgI)x(Ag2O)y(P2O5)1-x-y glasses are reported. Using the two-dimensional variable-angle correlation spectroscopy experiment, the isotropic and anisotropic chemical shift interactions of phosphorus were determined as a function of silver iodide and silver oxide composition. From these measurements we determine the average conformation of the phosphate groups. In addition, the 109Ag NMR spectra were recorded, as a function of both composition and temperature. At high silver oxide concentrations, interaction between the silver and phosphate groups has been detected, but in glasses in the series the (AgPO3)x(AgI)1-x 31P NMR is strikingly independent on the silver iodide composition. The temperature dependence of the silver NMR linewidths in these systems shows clearly the silver mobility, and at lower temperatures no evidence for multiple distinct silver binding sites was observed. The silver chemical shift is strongly dependent on both composition and temperature. The former effect is interpreted in terms of the influence on the chemical shift of binding to oxygen versus iodine.

Anisotropy↗

PubMed1994

Interpreting nuclear magnetic resonance spectra of disordered materials: direct inversion of powder patterns.

The nuclear magnetic resonance (NMR) spectra of disordered materials are often interpreted by assuming distributions of the interaction parameters and fitting the spectra under these assumptions. Here we illustrate methods to extract the distributions directly from the spectra, making no such prior assumptions. The inhomogeneously broadened powder pattern observed in the NMR spectrum of a disordered solid is expressed as an integral over the powder patterns for individual sites weighted by the population distribution of the different sites. The resulting integral equation is solved for the underlying probability distribution of sites, both by singular value decomposition and by a regularization method. Results are shown for model and real one-dimensional and two-dimensional NMR experiments, with and without noise.

Data Interpretation, Statistical↗

PubMedDate not supplied

Modeling glasses using the reverse Monte Carlo algorithm: addition of nuclear magnetic resonance and expanded coordination number constraints.

In simple oxide glasses the coordination number and oxidation state of the glass-forming element can be predicted directly from the "8--n" rule. Tellurite glasses, however, are unusual in that the coordination number of oxygen around tellurium varies without a corresponding change in the oxidation state of tellurium. To model sodium tellurite glasses successfully using the reverse Monte Carlo algorithm several new constraints have been added. Changes include extending the original coordination constraint to allow multiple coordination numbers, and the addition of a new coordination constraint to keep the oxidation state of tellurium constant by limiting the number of bridging and nonbridging oxygens bonded to each tellurium atom. In addition, the second moment of the distribution of dipolar couplings for sodium atoms obtained from a spin-echo NMR experiment was added as a new constraint. The resulting real-space models are presented and the effectiveness of the new constraints is discussed.

Algorithms↗