Biomedical subjects

Ernst R H van Eck

Publications and source records attributed to Ernst R H van Eck.

7 recordsLinked to original sources

PubMed2007

The influence of size on phase morphology and Li-ion mobility in nanosized lithiated anatase TiO2.

Sustainable energy storage in the form of Li-ion batteries requires new and advanced materials in particular with a higher power density. Nanostructuring appears to be a promising strategy, in which the higher power density in nanosized materials is related to the dramatically shortened Li-ion diffusion paths. However, nanosizing materials also changes intrinsic material properties, which influence both ionic and electronic conductivity. In this work neutron diffraction is used to show that in addition to these two aspects, nanostructuring changes the phase behavior and morphology. Lithiated 40-nm TiO(2) anatase crystallites become single phase, either having the Li-poor original anatase phase, or the Li-rich Li-titanate phase, in contrast to microsized crystallites where these two phases coexist in equilibrium within one crystal particle. In addition, Li(x)TiO(2) compositions occur with stoichiometries that are not stable in micron-sized crystallites, indicating enhanced solid solution behavior. Reduced conduction electron densities at the sites of the Li ions are observed by NMR spectroscopy. This is accompanied by reduced spontaneous Li-ion mobility, suggesting a correlation between the electron density at the Li-ion site and the Li-ion mobility. The present results show that in the case of lithiated anatase TiO(2), significant effects on phase composition, morphology, and electronic configurations are induced, as well as slower intracrystallite Li diffusion.

Chemistry, Physical↗

PubMed2006

Rhodium-mediated stereoselective polymerization of "carbenes".

Unprecedented rhodium-catalyzed stereoselective polymerization of "carbenes" from ethyl diazoacetate (EDA) to give high molecular mass poly(ethyl 2-ylidene-acetate) is described. The mononuclear, neutral [(N,O-ligand)M(I)(cod)] (M = Rh, Ir) catalytic precursors for this reaction are characterized by (among others) single-crystal X-ray diffraction. These species mediate formation of a new type of polymers from EDA: carbon-chain polymers functionalized with a polar substituent at each carbon of the polymer backbone. The polymers are obtained as white powders with surprisingly sharp NMR resonances. Solution and solid state NMR data for these new polymers reveal a highly stereoregular polymer, with a high degree of crystallinity. The polymer is likely syndiotactic. Material properties are very different from those of atactic poly(diethyl fumarate) polymer obtained by radical polymerization of diethyl fumarate. Other diazoacetates are also polymerized. Further studies are underway to reveal possible applications of these new materials.

Journal Article↗

PubMed2006

Microcoil high-resolution magic angle spinning NMR spectroscopy.

We report the construction of a dual-channel microcoil nuclear magnetic resonance probehead allowing magic-angle spinning for mass-limited samples. With coils down to 235 mum inner diameter, this allows high-resolution solid-state NMR spectra to be obtained for amounts of materials of a few nanoliters. This is demonstrated by the carbon-13 spectrum of a tripeptide and a single silk rod, prepared from the silk gland of the Bombyx mori silkworm. Furthermore, the microcoil allows for radio frequency field strengths well beyond current probe technology, aiding in getting the highest possible resolution by efficiently decoupling the observed nuclei from the abundantly present proton nuclei.

Alanine↗

PubMed2006

Multinuclear solid-state high-resolution and 13C -{27Al} double-resonance magic-angle spinning NMR studies on aluminum alkoxides.

A combination of 27Al magic-angle spinning (MAS)/multiple quantum (MQ)-MAS, 13C-1H CPMAS, and 13C-{27Al} transfer of population in double-resonance (TRAPDOR) nuclear magnetic resonance (NMR) were used for the structural elucidation of the aluminum alkoxides aluminum ethoxide, aluminum isopropoxide, and aluminum tertiarybutoxide. Aluminum alkoxides exist as oligomers with aluminum in different coordinations. High-resolution 27Al MAS NMR experiments with high-spinning speed distinguished the aluminum atoms in different environments. The 27Al MAS NMR spectrum gave well-resolved powder patterns with different coordinations. Z-filter MQ-MAS was performed to obtain the number and types of aluminum environments in the oligomeric structure. 13C-1H CPMAS chemical shifts resolved the different carbon species (-CH3, =CH2, =CH-, and =C=) in the structures. 13C-{27Al} TRAPDOR experiments were employed to obtain relative Al-C dipolar interactions and to distinguish between terminal and bridging alkoxides in the crystallographic structures. The complete characterization of selected aluminum alkoxides using advanced NMR methods has evidenced the tetrameric structure for aluminum isopropoxide and the dimeric structure for aluminum tertiary-butoxide, as reported in the literature, and proposed a polymeric structure for aluminum ethoxide.

Journal Article↗

PubMed2005

New opportunities for double rotation NMR of half-integer quadrupolar nuclei.

A combined approach is presented which expands the applicability of double rotation (DOR) by overcoming its most prominent disadvantages: spinning stability and sensitivity. A new design using air-bearings for the inner rotor and a computer-assisted start-up procedure allows DOR operation over in principle unlimited time at outer rotor speeds of up to 2000Hz. Sensitivity enhancement of the DOR experiment is achieved by applying amplitude-modulated adiabatic pulses such as the double frequency sweep (DFS) before pulse excitation. Repeating the DFS enhancement and signal readout several times without allowing for spin-lattice relaxation leads to sensitivity enhancements of a factor 3 for (27)Al in various minerals. As a result, it becomes possible to study low sensitivity quadrupolar nuclei and various long duration 2D measurements can be performed routinely. Spinning is adequate to suppress residual homonuclear dipolar couplings in the spectral dimension of typical quadrupolar spin systems. In 2D-exchange spectroscopy, however, homonuclear correlation can still be established through dipolar-quadrupolar cross-terms.

Journal Article↗

PubMed2005

Structural analysis of a melaminium polyphosphate from X-ray powder diffraction and solid-state NMR data.

The crystal structure of the environmentally friendly flame retardant melaminium polyphosphate (MPoly) (2,4,6-triamino-1,3,5-triazinium x PO(3))(n)was determined by a direct-space global optimization technique from X-ray powder diffraction data. Solid-state NMR was used to corroborate the proposed hydrogen-bonding model and to determine the average degree of polymerization (n > 100). An analysis of the crystal structure of MPoly reveals aspects of molecular geometry and packing that are characteristic for melamine-containing compounds and polyphosphate salts. A comparison of MPoly with the crystal structures of its precursors melaminium orthophosphate (MP) and melaminium dihydrogenpyrophosphate (MPy) provides insight in the mechanism of the endothermic dehydration processes that takes place in the reaction path MP --> MPy --> MPoly. Solid-state NMR characterization of various samples of the same batch showed inhomogeneities in the MPoly composition. Various quantities of orthophosphates were found, which cannot be assigned to be MP.

Journal Article↗

PubMedDate not supplied

Multiple quantum spin counting techniques with quadrupolar nuclei.

Phase incremented and continuous irradiation multiple spin correlation methods are applied to spin [Formula: see text] nuclei with small quadrupole couplings such as (7)Li in LiCl and are shown to successfully produce a coherently coupled dipolar spin network. Application to the analogous Na salt shows successful spin correlation evolving at a slower rate due to the weaker homonuclear dipolar coupling strength between Na nuclei. The results are analysed using a statistical approach. Spin counting is non-trivial as not only multiple quantum coherences between spins are generated but also within the quadrupolar spin levels. Na(2)C(2)O(4) is investigated as a material with non-negligible quadrupole coupling and it is in this limit that the spin correlation techniques are found to break down.

Journal Article↗