Hellmut Eckert

A CALCIUM HYDROXYAPATITE PRECIPITATED FROM AN AQUEOUS-SOLUTION - AN INTERNATIONAL MULTIMETHOD ANALYSIS

Abstract In this paper a pure calcium hydroxyapatite prepared from aqueous solutions at low temperature, was analysed by a large number of techniques in six Institutes. The techniques employed (frequently in more than one laboratory) were X-ray diffraction, IR analysis, BET measurement, chemical analysis, differential thermal analysis, magic angle spinning NMR, TEM, size distribution measurements, crystal growth and crystal dissolution measurements. Several data were obtained not only at room temperature but in the range up to 900 °C. The results show that the apatite is very pure. The impurities CO 3 and NO 3 are both below 0.1%. The important ion HPO 2- 4 is present in amounts of ≈ 2 mol% of total phosphate and covers most likely the outer surface of the crystals. Five different techniques were employed to determine the HPO 2- 4 content. H 2 O is present in adsorbed form in about 2.0 wt%. The Ca/P ratio is between 1.63 and 1.66. The OH - content is about 10% by weight lower than the stoichiometric value. Crystal sizes have mean values of 33 nm width and 133 nm length. BET surface is 37 m 2 g -1 . X-ray powder diffraction yields a and c lattice parameters of 9.428 and 6.882 A at RT and after a brief heating to 900 °C in N 2 . If the apatite is heated at 850 °C for 18 h in air, partial decomposition takes place to form α and β Ca 3 (PO 4 ) 2 . 1 H MAS-NMR shows that the H 2 O is present in a very mobile form and is presumably adsorbed in several monolayers. The absolute quantitation of the structural hydroxyl content by 1 H MAS-NMR reveals a slight deficiency, as do IR results. Kinetic data suggest that the dissolution mechanism is controlled by polynuclear surface nucleation catalysed by hydrogen ions, the interfacial surface free energy σ being 50 mJ m -2 at pH = 7. Growth kinetics analysed for a polynuclear mechanism lead to the value σ = 87 mJ m -2 at pH = 7.

Dephasing of spin echoes by multiple heteronuclear dipolar interactions in rotational echo double resonance NMR experiments.

The application of rotational echo double resonance (REDOR) nuclear magnetic resonance (NMR) for accurate distance measurements has thus far been largely restricted to isolated heteronuclear two-spin systems. In the present paper, the informational content of REDOR curves is explored for systems characterized by multi-spin interactions. To this end, numerical REDOR simulations are presented for cases in which single observe spins S are dipolarly coupled to groups of spins I in distinct geometries. To develop the utility of REDOR for characterizing dipolar couplings in unknown and/or ill-defined geometries, the validity ranges and systematic errors of certain analytical approximations are studied. In the limit of short dipolar evolution times where 0 < deltaS/S0 < or = 0.2 to 0.3, the REDOR difference signal intensity increases approximately proportional to the square of the dipolar evolution time. Here, the curvature depends simply on the second moment M2 characterizing the overall strength of the heterodipolar coupling, irrespective of specific molecular geometries. Fitting experimental REDOR data in this manner produces slight systematic underestimates of M2. However, these errors tend to be counterbalanced by additional systematic errors made by neglecting weak couplings to more remote spins and distribution effects caused by disorder. Based on these findings, the results suggest a convenient method of obtaining site-resolved second moment information in disordered materials.

Physical and chemical characterization of surface vanadium oxide supported on titania: influence of the titania phase (anatase, rutile, brookite and B)

Abstract Different phases of titania were prepared and used to support ca. 1 wt.-% V 2 O 5 . The different titania phases prepared were: anatase (A22), rutile (R28), brookite (BT110) and B-phase (B18). Physical characterization of the various vanadia-titania catalysts was performed using X-ray photoelectron spectroscopy (XPS), in situ Raman and 51 V solid state nuclear magnetic resonance (NMR) spectroscopy. The XPS results reveal that the all the catalysts contain various levels of impurities. In situ dehydration Raman shows, for all the samples, the stretching vibration of the terminal VzO bond at ca. 1030 cm −1 . Solid state 51 V NMR spectra of all the samples in the dehydrated state show basically the same powder pattern with a peak maximum around −660 to −670 ppm. The combined Raman and NMR results indicate that the same surface vanadium oxide species is present on all the titania supports irrespective of the crystal structure of the bulk titania phase. Partial oxidation of methanol show similar activity and selectivity for the various vanadia-titania catalysts. The reaction selectivity was primarily to formaldehyde and methyl formate (92–96%). The turnover number for methanol oxidation was essentially the same for all the vanadia-titania catalysts and ranged from 1.4 to 2.8 s −1 . These results indicate that the type of titania phase used as the support is not critical for partial oxidation over vanadia-titania catalysts as long as other parameters (e.g. surface impurities ) are similar. Thus, the structure-reactivity studies of the different vanadia-titania catalysts suggest that the specific titania phase is not a critical parameter in determining the physical or chemical nature of the surface vanadia phase.

Reactions of phosphorus/boron frustrated Lewis pairs with SO2

The frustrated Lewis pair tBu3P/B(C6F5)3 (1) readily adds SO2 to yield the zwitterionic adduct tBu3P+–S(O)–OB−(C6F5)3 (3). A series of intramolecular vicinal P/B FLPs Mes2P–(X)–B(C6F5)2 [X = –CH2–CH2– (2a), –CHMe–CH2– (2b), cyclo-C6H10 (5)] add SO2 at −78 °C to yield the corresponding six-membered addition products 4a, 4b, 6. The adducts contain a chiral sulfur center. The [B]–O–(O)S–[P] addition products 3, 4b and 6 were characterized by X-ray diffraction.

Vanadium(V) Environments in Bismuth Vanadates: A Structural Investigation Using Raman Spectroscopy and Solid State 51V NMR

The Bi2O3V2O5 system was examined using Raman spectroscopy and solid state 51V wideline, magic-angle spinning (MAS), and nutation NMR spectroscopy. The methods are shown to be complementary in the identification of the various phases and in the characterization of their vanadium site symmetries. Most of the compositions examined (1:1 ≤ Bi:V ≤ 60:1) are multiphasic. Depending on the Bi:V ratio, the following phases have been identified: BiVO4, Bi4V2O11, a triclinic type-II phase, a cubic type-I phase, γ-Bi2O3 doped with V(V) (sillenite), and β-Bi2O3. Detailed spectroscopic characterization reveals that vanadium is tetrahedrally coordinated in all these compounds, and that the degree of symmetry increases with increasing Bi:V ratio. At the highest Bi:V ratios, the combined interpretation of the Raman and NMR data provides strong evidence for the presence of Bi5+O4 tetrahedra.

Structures and Properties of Spherical 90-Vertex Fullerene-Like Nanoballs

By applying the proper stoichiometry of 1:2 to [Cp(R)Fe(eta(5)-P(5))] and CuX (X=Cl, Br) and dilution conditions in mixtures of CH(3)CN and solvents like CH(2)Cl(2), 1,2-Cl(2)C(6)H(4), toluene, and THF, nine spherical giant molecules having the simplified general formula [Cp(R)Fe(eta(5)-P(5))]@[{Cp(R)Fe(eta(5)-P(5))}(12){CuX}(25)(CH(3)CN)(10)] (Cp(R)=eta(5)-C(5)Me(5) (Cp*); eta(5)-C(5)Me(4)Et (Cp(Et)); X=Cl, Br) have been synthesized and structurally characterized. The products consist of 90-vertex frameworks consisting of non-carbon atoms and forming fullerene-like structural motifs. Besides the mostly neutral products, some charged derivatives have been isolated. These spherical giant molecules show an outer diameter of 2.24 (X=Cl) to 2.26 nm (X=Br) and have inner cavities of 1.28 (X=Cl) and 1.20 nm (X=Br) in size. In most instances the inner voids of these nanoballs encapsulate one molecule of [Cp*Fe(eta(5)-P(5))], which reveals preferred orientations of pi-pi stacking between the cyclo-P(5) rings of the guest and those of the host molecules. Moreover, pi-pi and sigma-pi interactions are also found in the packing motifs of the balls in the crystal lattice. Electrochemical investigations of these soluble molecules reveal one irreversible multi-electron oxidation at E(p)=0.615 V and two reduction steps (-1.10 and -2.0 V), the first of which corresponds to about 12 electrons. Density functional calculations reveal that during oxidation and reduction the electrons are withdrawn or added to the surface of the spherical nanomolecules, and no Cu(2+) species are involved.

Reversible switching between p- and n-type conduction in the semiconductor Ag10Te4Br3

Semiconductors are key materials in modern electronics and are widely used to build, for instance, transistors in integrated circuits as well as thermoelectric materials for energy conversion, and there is a tremendous interest in the development and improvement of novel materials and technologies to increase the performance of electronic devices and thermoelectrics. Tetramorphic Ag(10)Te(4)Br(3) is a semiconductor capable of switching its electrical properties by a simple change of temperature. The combination of high silver mobility, a small non-stoichiometry range and an internal redox process in the tellurium substructure causes a thermopower drop of 1,400 microV K(-1), in addition to a thermal diffusivity in the range of organic polymers. The capability to reversibly switch semiconducting properties from ionic to electronic conduction in one single compound simply by virtue of temperature enables novel electronic devices such as semiconductor switches.

Dipolar solid state NMR approaches towards medium-range structure in oxide glasses

Modern solid state nuclear magnetic resonance presents new powerful opportunities for the elucidation of medium range order in glasses in the sub-nanometer region. In contrast to standard chemical shift spectroscopy, the strategy presented here is based on the precise measurement and quantitative analysis of internuclear magnetic dipole-dipole interactions, which can be related to distance information in a straightforward manner. The review discusses the most commonly employed experimental techniques, producing dipolar coupling information in both homo- and heteronuclear spin systems. The approach is particularly powerful in combination with magic-angle sample spinning, producing site-resolved dipolar coupling information. We present new applications to oxide-based network glasses, permitting network connectivities and spatial cation distributions to be elucidated.

Spatial distributions and chemical environments of cations in single- and mixed alkali borate glasses: Evidence from solid state NMR

Abstract The structural environments and the spatial distributions of the alkali ions in single- and mixed-alkali borate glasses are studied by complementary solid state NMR techniques. Specifically, spin echo decay spectroscopy is used to extract homodipolar second moments for 23 Na and 133 Cs in binary sodium and cesium borate glasses. These values are found to be quantitatively most consistent with spatially homogeneous cation distributions, except in sodium borate glasses with cation contents ≤16 mole %. Complementary isotropic chemical shifts extracted from field-dependent magic-angle spinning (MAS)-NMR depend linearly on alkali ion content, revealing a continuous concomitant change in the oxygen environment of the alkali ions. This effect can be related to structural changes in the network, where trigonal BO 3/2 units are progressively converted to tetrahedral BO 4/2 − sites as the alkali oxide content is increased. Taken together these data argue strongly against cation clustering models previously proposed for other types of glass systems. Isotropic 7 Li and 23 Na chemical shift data measured for mixed-alkali Li,Na and K,Na-borate glasses containing 30 mole % alkali oxide indicate universal compositional trends that can be understood in terms of the site-mismatch concept of Bundes dynamic structure model: Consistent with current semi-empirical predictions, mismatching the cation of interest, e.g. Na + to a smaller Li site produces a low-frequency shift, while mismatching to a larger K site produces high-frequency 23 Na isotropic chemical shifts.

FT-IR, FT-Raman and 95Mo MAS–NMR studies on the structure of ionically conducting glasses in the system AgI–Ag2O–MoO3

Local environments of molybdenum in glasses of the system (AgI)y–[(Ag2O)x–(MoO3)1−x]1−y were investigated by use of near-infrared Fourier transform (NIR-FT) Raman spectroscopy, FT-IR spectroscopy, and 95Mo magic-angle spinning (MAS) NMR. Specifically, the following cross-sections of the composition diagram were studied: series I: x=0.5, and 0.50≤y≤0.75 and series II: y=0.50, and 0.4≤x≤0.5. Comparison of the spectroscopic results with those obtained on crystalline model compounds indicates that the structures of series I- glasses (composition ratio Ag2O/MoO3=1) are solely based on Ag+, I−, and isolated MoO42− anions. On the other hand, glasses of series II (composition ratio Ag2O/MoO3<1) have a more complex structure consisting of both tetrahedral orthomolybdate, MoO42−, and a second, polynuclear species, in which MoO6 octahedra are linked to MoO4 tetrahedra in a manner similar to the structure of crystalline Na2Mo2O7.