High anion conductivity in a ternary non-equilibrium phase of BaF(2) and CaF(2) with mixed cations.

A highly conductive ternary fluoride with mixed cations is prepared by joint high-energy ball milling of cubic BaF(2) and CaF(2) in the ratio 0.4 : 0.6. The sample produced at room temperature consists of a nanocrystalline, defect-rich mixed (Ba,Ca)F(2) phase with retained cubic symmetry as well as of single-phase CaF(2) particles. The anion conductivity of the mixed phase, which decomposes at higher temperature (770 K) into BaF(2) and CaF(2), exceeds that of single-phase nanocrystalline BaF(2) by two and that of CaF(2) by four orders of magnitude. In turn, these conductivities are each greater by about two orders than those of the respective microcrystalline counterparts. Structural features of the samples are characterized by X-ray diffraction, TEM and (19)F MAS NMR spectroscopy. Static (19)F NMR spectra confirm the unexpectedly high anion conductivity probed by impedance spectroscopy.

93Nb and 17O NMR chemical shifts of niobiophosphate compounds.

Niobiophosphate compounds with a large range of niobium and oxygen environments were studied with (93)Nb and (17)O solid-state NMR. (93)Nb isotropic chemical shift of pure niobate Nb(ONb)(6), pure phosphate Nb(OP)(6) and mixed phosphate-niobate Nb(OP)(x)(ONb)((6-x)) (1

Increasing the sensitivity of 2D high-resolution NMR methods applied to quadrupolar nuclei.

Gan and Kwak recently proposed a soft-pulse added mixing (SPAM) idea in the classical two-pulse multiple-quantum magic-angle spinning scheme. In the SPAM method, a soft pi/2 pulse is added after the second hard-pulse (conversion pulse) and all coherence orders in between them are constructively used to obtain the signal. We, here, further extend this idea to distributed samples where the signal mainly results from echo pathways and that from anti-echo pathways dies out after a few t1 increments. We show that, with a combination of SPAM and collection of fewer anti-echoes, an enhancement of the signal to noise ratio by a factor of ca. 3 may be obtained over the z-filtered version. This may prove to be useful even for samples with long T2' relaxation times.

Residual 31P, 35,37Cl dipolar coupling in 31P MAS spectra of chlorocyclophosphazenes.

In 31P MAS NMR spectra of chlorocyclophosphazenes, characteristic splittings have been observed for PCI or PCl2 groups. At different applied magnetic fields, the fine structure and total width of the patterns change in a characteristic way, demonstrating that the splittings are due to indirect spin-spin and residual dipolar interactions with the chlorine nuclei directly bonded to phosphorus. For trans-nongeminal N3P3Cl3(NMe2)3 and N3P3Cl6 as examples, the spectra have been analyzed to obtain information on chlorine nuclear quadrupole coupling constants and 35,37Cl, 3P indirect spin-spin coupling constants. Neglect of these interactions may result in misinterpretations of the multiplicity in 3P MAS spectra of chlorophosphazenes.

Sample restriction using magnetic field gradients in high-resolution solid-state NMR

Many solid-state NMR experiments are sensitive to inhomogeneity in the radiofrequency field. We propose a method to restrict the sample volume, in magic angle spinning experiments, using a static magnetic field gradient and a selective pulse. The position of the gradient is calculated for our experimental configuration and we have simulated the effects of selective pulses to determine the excited volume. The resulting sequences are applied to a sample of sodium acetate using frequency-switched Lee-Goldburg proton-proton homonuclear dipolar decoupling. A gain of a factor of 2 on the carbon resolution is experimentally observed. Copyright 2000 Academic Press.

The nature of oxygen in sporopollenin from the pollen of Typha angustifolia L.

Native and peracetylated sporopollenin from the pollen of Typha angustifolia L. was investigated using several spectroscopic methods, inducing Fourier transform infrared spectroscopy (FTIR), solid-state 13C-nuclear magnetic resonance spectroscopy (13C-NMR) and X-ray photoelectron spectrometry (XPS). Interpretation of the experimental data shows that the greater part of oxygen found in sporopollenin originates from hydroxyl groups and must be derived from aliphatics and not from aromatics. This result indicates that not only aromatics and long unbranched aliphatics but also poly-hydroxyl aliphatic components are involved in the complex structure of the polymer. Furthermore, it is most probable that the monomers of the sporopollenin skeleton are linked by ether- and not by ester-linkage. Two possible approaches are suggested for the characterisation of sporopollenin structure.

Very fast MAS and MQMAS NMR studies of the spectroscopically challenging minerals kyanite and andalusite on 400, 500, and 800 MHz spectrometers.

The well-characterized minerals kyanite and andalusite have long presented great challenges in using solid state 27Al NMR to determine the isotropic chemical shift deltaCS, quadrupole coupling constant e2qQ/h, and asymmetry parameter eta for each of the inequivalent aluminum sites in these minerals. Indeed, these minerals have frequently been used to test advances in instrumentation. Recent advances in magnet technology (up to 18.8 T = 800 MHz 1H) and in MAS probe technology (spinning up to 35 kHz and considerably stronger rf) and refinements of the two-dimensional, multiple quantum magic angle spinning (MQMAS) technique suggested that these developments could be profitably used to study kyanite and andalusite by solid state 27Al NMR. The benefit of being able to study kyanite both by MAS and MQMAS techniques on 400, 500, and 800 MHz spectrometers is demonstrated. The two octahedral aluminum sites with the largest (and nearly equal) e2qQ/h values give overlapping 1D MAS or 2D 3QMAS signals at all three field strengths. Nevertheless, quantitatively accurate 3Q signal intensities at 9.4 T for all four octahedral aluminum sites (with e2qQ/h values up to 10 MHz) allow more detailed analysis. Even if the 3Q signal intensities are not quantitative, their isotropic shifts provide an approach (if accurate e2qQ/h and eta values are available) other than deconvolution of the MAS spectrum for calculating deltaCS values. For andalusite, 34 kHz MAS on the 800 MHz spectrometer significantly narrows the extremely broad signal for the octahedral aluminum, and only slight difficulties are encountered in quantitating the relative amounts of AlO5 and AlO6 present. Even with e2qQ/h = 15.3 MHz, the octahedral aluminum in andalusite gives a signal in a MQMAS experiment, albeit of reduced intensity. As appropriate, we discuss some of the benefits and limitations of these advances in instrumentation and of different experimental approaches for studying non-integral spin quadrupolar nuclei in solids.

A new sequence for the MQMAS/HETCOR experiment.

Two-dimensional (2D) multiple quantum MAS (magic angle spinning) spectroscopy has been combined with cross-polarisation to obtain a heteronuclear correlation spectrum between a quadrupolar spin-3/2 and a spin-1/2 nucleus. The advantage over the conventional correlation experiment is the increased resolution obtained in the multiple quantum dimension. Pure absorption 2D spectra can be obtained by implementing a zero quantum filter between the evolution of multiple quanta and the subsequent cross-polarisation step. The current experiment shows a considerable improvement in sensitivity compared to a previously introduced sequence.

17O magic-angle spinning nuclear magnetic resonance of CaCO3.

The first 17O magic-angle spinning nuclear magnetic resonance (MAS-NMR) from a carbonate ion in an inorganic compound is reported. The 17O MAS centreband of CaCO3 can be simulated with parameters CQ = 6.97 MHz, eta approximately 1 and an isotropic chemical shift of 204 ppm.

Quantitative structural analysis of mullite by 27Al nuclear magnetic resonance satellite transition spectroscopy.

27Al Nuclear magnetic resonance (NMR) satellite transition spectroscopy has been used for the quantitative investigation of the microstructure of polycrystalline 2/1-type mullite (approximately 76 wt.-% Al2O3 and 24 wt.-% SiO2). Using this method the chemical shifts, quadrupole interaction parameters and relative amount of Al in the various coordination polyhedra have been obtained with high accuracy. The additional suppression of second-order quadrupolar broadening of the 27Al magic-angle spinning (MAS) spinning sidebands of the inner satellite transitions allows three different AlO4 units to be detected. The three AlO4 resonances with isotropic chemical shifts at 68, 53 and 45 ppm are associated with AlO4 polyhedra in the tetrahedral double chains [AlO4(T)], the tetrahedra adjacent to the oxygen vacancies [AlO4(T*)] and the tetrahedra AlO4(T') linked by Oc* with AlO4(T*), respectively. Our NMR study of a polycrystalline sample supports the results of previous single-crystal studies of Angel et al. [Am. Mineral., 76 (1991) 332], especially the exclusion of silicon from the tetrahedral sites linked by Oc*.

27Al satellite transition spectroscopy (SATRAS) of polycrystalline aluminum borate 9Al2O3.2B2O3.

27Al NMR spectra of polycrystalline aluminum borate 9Al2O3.2B2O3 have been measured at 104, 130 and 156 MHz. The parameters of the quadrupole interaction and the isotropic chemical shifts have been obtained by fitting the CT/MAS pattern and consideration of the inner satellite transitions m = 3/2<-->1/2 and m = - 1/2<-->- 3/2. The gain in spectral resolution concerned with the observation of the MAS lines of the inner satellites leads to complete separation of the signals of AlO6, AlO5 and AlO4 polyhedra. Also signals of structural groups of one and the same coordination number can be distinguished. Experimental and theoretical lineshape calculations are compared.

A multinuclear magnetic resonance examination of the mineral grandidierite. Identification of a 27Al resonance from a well-defined AlO5 site.

11B, 27Al and 29Si magic angle spinning NMR results are reported for the boroaluminosilicate mineral grandidierite (Mg, Fe)Al3SiBO9. Three distinct aluminium sites are identified, two AlO6 and one AlO5. Despite overlap of the centrebands from these sites the use of three magnetic fields (9.4, 11.7 and 14.1 T) allows unambiguous values for the isotropic chemical shift (delta iso), quadrupolar coupling constant (Cq) and quadrupolar asymmetry parameter (eta) to be deduced for each site. The NMR spectrum from the AlO5 site is simulated with parameters Cq = 8.7 +/- 0.1 MHz, eta = 0.95 +/- 0.05 and delta iso = 41 +/- 1 ppm which are compared to values from other well-defined AlO5 units.

The structure of Ro 09-0198 in different environments.

The constitution and configuration of Ro 09-0198 (cinnamycin) have been determined in DMSO. Further investigations in aqueous solution, in SDS micelles and in a lipid bilayer have been done to study the influence of different environments on the conformation of the peptide. It turned out that in spite of the polycyclic structure of the molecule, the conformation is drastically changed going from water to SDS micelles. Ro 09-0198 orients itself in lipid bilayers as expected from its amphiphilic structure. According to a nuclear Overhauser effect spectroscopy experiment under magic angle spinning (MAS) conditions, the molecule is incorporated into the membrane with its hydrophobic part inside the bilayer.