17O MAS and 3QMAS NMR investigation of crystalline V(2)O(5) and layered V(2)O(5).nH(2)O gels

The environments for oxygen sites in crystalline V(2)O(5) and in layered vanadia gels produced via sol-gel synthesis have been investigated using (17)O MAS and 3QMAS NMR. For crystalline V(2)O(5), three structural oxygen sites were observed: V=O (vanadyl), V(2)O (doubly coordinated), and V(3)O (triply coordinated). Line-shape parameters for these sites were determined from numerical simulations of the MAS spectra. For the vanadia gels at various stages of dehydration, assignments have been proposed for numerous vanadyl, doubly coordinated, and triply coordinated oxygen sites. In addition, by correlating the (17)O MAS and 3QMAS NMR, (51)V MAS NMR, and thermogravimetric analysis data, the coordination of water sites has been established. On the basis of these results, the gel structure and its evolution at various stages of hydration have been detailed. Upon rehydration of the layered gel, we observed a preferred site for initial water readsorption. The oxygen atoms of these readsorbed water molecules readily exchanged into all types of oxygen sites even at room temperature.     

Probing Cation and Vacancy Ordering in the Dry and Hydrated Yttrium-Substituted BaSnO(3) Perovskite by NMR Spectroscopy and First Principles Calculations: Implications for Proton Mobility

Hydrated BaSn(1-x)Y(x)O(3-x/2) is a protonic conductor that, unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy and density functional theory (total energy and GIPAW NMR calculations) investigation of BaSn(1-x)Y(x)O(3-x/2) (0.10 ≤ x ≤ 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The DFT energetics show that Y doping on the Sn site is favored over doping on the Ba site. The (119)Sn chemical shifts are sensitive to the number of neighboring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn sublattice is close to random up to x = 0.20, while at higher substitution levels, Y-O-Y linkages are avoided, leading, at x = 0.50, to strict Y-O-Sn alternation of B-site cations. These results are confirmed by the absence of a "Y-O-Y" (17)O resonance and supported by the (17)O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by (89)Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e., sites next to the vacancy) were observed by (119)Sn and (89)Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at x = 0.50, which is ascribed to the presence of residual Sn-O-Sn defects in the cation-ordered material and their ability to accommodate O vacancies. High-temperature (119)Sn NMR reveals that the O ions are mobile above 400 °C, oxygen mobility being required to hydrate these materials. The high protonic mobility, even in the high Y-content materials, is ascribed to the Y-O-Sn cation ordering, which prevents proton trapping on the more basic Y-O-Y sites.     

Electronic structure of HgBa2Ca(n-1)Cu(n)O(2n+2) (n = 1, 2, 3) superconductor parent compounds from periodic hybrid density functional theory

The electronic structure of HgBa(2)Ca(n) (-1)Cu(n)O(2n+2) (n = 1, 2, and 3) high T(c) superconductor parent compounds has been investigated by means of periodic hybrid density functional theory. Similar to other cuprates, these materials are predicted to exhibit an antiferromagnetic ground state with well localized S = 1/2 magnetic centers at the Cu(2+) sites. However, the presence of the HgO(2) structural units largely defines the nature of states dominating the energy range around Fermi energy. This results in a complex charge transfer character of the insulating gap which decreases when increasing the number of CuO(2) planes in the unit cell, to the point that in the HgBa(2)Ca(2)Cu(3)O(8) compound it becomes so small that one can claim that the resulting material is metallic. Nevertheless, the metallic character arises from the HgO(2) structural units and coexists with the antiferromagnetic order arising from the localized spins at the Cu(2+) sites.     

Ion-binding study by 17O solid-state NMR spectroscopy in the model peptide Gly-Gly-Gly at 19.6 T

Li(+) and Ca(2+) binding to the carbonyl oxygen sites of a model peptide system has been studied by (17)O solid-state NMR spectroscopy. (17)O chemical shift (CS) and quadrupole coupling (QC) tensors are determined in four Gly-(Gly-(17)O)-Gly polymorphs by a combination of stationary and fast magic-angle spinning (MAS) methods at high magnetic field, 19.6 T. In the crystal lattice, the carbonyl oxygen of the central glycyl residue in two gly-gly-gly polymorphs form intermolecular hydrogen bonds with amides, whereas the corresponding carbonyl oxygens of the other two polymorphs form interactions with Li(+) and Ca(2+) ions. This permits a comparison of perturbations on (17)O NMR properties by ion binding and intermolecular hydrogen bonding. High quality spectra are augmented by density functional theory (DFT) calculations on large molecular clusters to gain additional theoretical insights and to aid in the spectral simulations. Ion binding significantly decreases the two (17)O chemical shift tensor components in the peptide plane, delta(11) and delta(22), and, thus, a substantial change in the isotropic chemical shift. In addition, quadrupole coupling constants are decreased by up to 1 MHz. The effects of ion binding are found to be almost an order of magnitude greater than those induced by hydrogen bonding.     

Yttrium(III)-containing tungstoantimonate(III) stabilized by tetrahedral WO4(2-) capping unit, [{Y(α-SbW9O31(OH)2)(CH3COO)(H2O)}3(WO4)]17-

The yttrium(III)-containing tungstoantimonate(III) [{Y(α-SbW(9)O(31)(OH)(2))(CH(3)COO)(H(2)O)}(3)(WO(4))](17-) (1) has been synthesized in a simple one-pot reaction of Y(3+) ions with [α-SbW(9)O(33)](9-) and WO(4)(2-) in a 3:3:1 molar ratio in 1 M LiOAc/AcOH buffer at pH 5.3. Polyanion 1 is composed of three (α-SbW(9)O(33)) units linked by three Y(3+) ions and a capping, tetrahedral WO(4)(2-) capping unit, resulting in an assembly with C(3v) symmetry. The hydrated ammonium-sodium salt of 1 was investigated in the solid state by single-crystal XRD, FT-IR spectroscopy, thermogravimetric and elemental analyses, and in solution by multinuclear NMR spectroscopy.     

Understanding the synergistic catalytic effect between La(2)O(3) and CaO for the CH(4) lean De-NO(x) reaction: kinetic and mechanistic studies

Doping of La(2)O(3) crystallites with Ca(2+) ions significantly enhances the intrinsic rate of NO reduction by CH(4) in the presence of 5% O(2) at 550 degrees C compared to pure La(2)O(3) and CaO solids, while the opposite is true after doping of CaO with La(3+) ions. It was found that the 5 wt % La(2)O(3)-95 wt % CaO system has one of the highest intrinsic site reactivities (TOF = 8.5 x 10(-3) s(-1)) reported at 550 degrees C for the NO/CH(4)/O(2) reaction among metal oxide surfaces. The doping process occurred after first dispersing La(2)O(3) and CaO crystallites in deionized water heated to 60 degrees C for 90 min, while the dried material was then ground and heated slowly in air to 800 degrees C and kept at this temperature for 5 h. The doping process had the effect of creating surface oxygen vacant sites (F-type defects) in the oxide lattices the concentration of which is a function of the wt % La(2)O(3) used in the mixed oxide system as revealed by photoluminescence and O(2) chemisorption studies. According to DRIFTS (15)NO transient isotopic experiments (SSITKA), oxygen vacant sites in Ca(2+)-doped La(2)O(3) promote the formation of a more active chemisorbed NO(x) species (NO(2)(-)) that contributes to the enhancement of reaction rate as compared to pure lanthana, calcium oxide, and La(3+)-doped CaO. These results were supported by the kinetic orders of the reaction with respect to NO and O(2) obtained as a function of wt % La(2)O(3) content in the mixed oxide system. Carbon dioxide (a reaction product) competes for the same oxygen vacant sites to form stable adsorbed carbonate-like species, thus lowering the reduction rate of NO. The dependence of the reaction TOF on the wt % La(2)O(3) loading at 550 degrees C was found to follow the trend of the dependence of photoluminescence intensity on the wt % La(2)O(3) content in the La(2)O(3)-CaO oxide system.     

Y:BaZrO3 Perovskite Compounds II: Designing Protonic Conduction by using MD Models

The proton dynamics in Y‐doped BaZrO₃ derivatives, in particular the different dopant environments within a Pm${\bar 3}$ m cubic framework, were studied by using classical molecular dynamics (MD) calculations. Single‐ and double substitution of zirconium by yttrium atoms was considered. The presence of yttrium induced variations in the surrounding oxygen sites, according to their local geometrical arrangements. The differences among such distinct oxygen sites became evident when protons interacted with them and upon changes in the temperature. So, different proton transfer pathways, which had different energy barriers, characterized the topologically different oxygen sites. The experimental proton‐hopping activation energy was only reproduced in those structures in which two yttrium atoms formed a Y‐O‐Y arrangement, which also acted as multilevel protonic traps. Protonic conduction in these materials could be improved by avoiding such yttrium clustering, hence preventing the formation of the protonic traps.     

(119)Sn MAS NMR and first-principles calculations for the investigation of disorder in stannate pyrochlores

The local structure and cation disorder in Y(2)Ti(2-x)Sn(x)O(7) pyrochlores, materials proposed for the encapsulation of lanthanide- and actinide-bearing radioactive waste, is studied using (119)Sn (I = 1/2) NMR spectroscopy. NMR provides an excellent probe of disorder, as it is sensitive to the atomic scale environment without the need for any long-range periodicity. However, the complex and overlapping spectral resonances that often result can be difficult to interpret. Here, we demonstrate how (119)Sn DFT calculations can be used to aid the spectral interpretation and assignment, confirming that Sn occupies only the six-coordinate pyrochlore B site, and that the Sn chemical shift is sensitive to the number of Sn/Ti on the neighbouring B sites. Although distinct resonances are resolved experimentally when the Ti content is low, there is significant spectral overlap for Ti-rich compositions. We establish that this is a result of two competing contributions to the Sn chemical shift; an upfield shift resulting from the incorporation of the more polarizing Ti(4+) cation onto the neighbouring B sites, and a concomitant downfield shift arising from the decrease in unit cell size. Despite the considerably easier spectral acquisition, the lower resolution in the (119)Sn spectra hinders the extraction of the detailed structural information previously obtained using (89)Y NMR. However, the spectra we obtain are consistent with a random distribution of Sn/Ti on the pyrochlore B sites. Finally, we consider whether an equilibrium structure has been achieved by investigating materials that have been annealed for different durations.     

17O NMR in room temperature phase of La2Mo2O9 fast oxide ionic conductor

A room temperature (17)O NMR study of La(2)Mo(2)O(9), a fast oxide ionic conductor exhibiting a phase transition at 580 degrees C between a low-temperature alpha-phase and a high-temperature beta-phase, is presented. Four partly overlapping quasi-continuous distributions of oxygen sites are evidenced from 1D magic angle spinning (MAS) and 2D triple quantum MAS NMR experiments. They can be correlated with the three oxygen sites O1, O2 and O3 of the high-temperature crystal structure. The low-temperature phase is characterized by two distributed sites of type O1, which proves that the symmetry is lower than in the cubic high-temperature phase. Two-dimensional experiments show that there is no dynamic exchange process, on the NMR time-scale, between the different oxygen sites at room temperature, which agrees well with conductivity results.     

Ca(2+)/vacancies and O2-/F- ordering in new oxyfluoride pyrochlores Li(2x)Ca1.5-x square 0.5-xM2O6F (M = Nb,Ta) for 0 < or = x < or = 0.5

New oxyfluorides Li(2x)Ca(1.5-x) square (0.5-x)M2O6F (M = Nb, Ta), belonging to the cubic pyrochlore structural type (Z = 8, a approximately 10.5 angstroms), were synthesized by solid state reaction for 0 < or = x < or = 0.5. XRD data allowed us to determine their structures from single crystals for the two alpha and beta-Ca(1.5) square (0.5)Nb2O6F forms and from powder samples for the others. This characterisation was completed by TEM and solid state 19F NMR experiments. For the Ca(1.5) square (0.5)M2O6F (x = 0) pyrochlore phases, the presence of a double ordering phenomenon is demonstrated, involving on one hand the Ca(2+) ions and the vacancies and on the other hand the oxide and the fluoride anions which are strictly located in the 8b sites of the Fd3m aristotype space group. The Ca(2+) ions/vacancies ordering leads to a reversible phase transition, a (P4(3)32) <--> beta (Fd3m). The 19F NMR study strongly suggests that, in the beta-phases, the fluoride ions are only on average at the centre of the Ca3 square tetrahedron. It shows that slightly different Ca-F distances occuring in alpha-Ca(1.5) square (0.5)Nb2O6F may be related to a more difficult thermal ionic and vacancies diffusion process than in the tantalate compound. This may explain the hysteresis phenomenon presented by the phase transition. A solid solution Li(2x)Ca(1.5-x) square (0.5-x) Ta2O6F (0 < or = x < or = 0.5) was prepared and the order-disorder phase transition observed for Ca(1.5) square (0.5)M2MO6F compounds disappears for all the other compositions where less or no more vacancies exist in the 16d sites. In the LiCaM2O6F compounds, the 19F NMR study allows us to determine the Ca(2+) and Li+ ions distributions around the fluoride ions and shows that the [FLi2Ca2] environment is clearly favoured.     

O triclusters revisited: classical MD and quantum cluster results for glasses of composition (Al(2)O(3))2(SiO(2))

The (17)O NMR spectrum of CaAl(2)Si(2)O(8) glass shows two types of O sites that are not present in the crystalline material. One of these, with (17)O NMR parameters C(Q) = 2.3 MHz and delta = +20 ppm, has been assigned to a "tricluster" O, a local geometry in which the O is coordinated to three tetrahedrally coordinated atoms, either Al or Si. For crystalline CaAl(4)O(7), a tricluster site (with three Al linkages to O, i.e., OAl(3)) has been characterized experimentally, with a C(Q) of 2.5 MHz and a delta of about +40 ppm. Thus, a C(Q) value of 2.5 MHz or less seems to be a characteristic of such sites, although they may show a range of delta values. However, several different quantum chemical cluster calculations employing energy-optimized geometries for various tricluster species have given C(Q) values considerably larger than that seen experimentally in the CaAl(2)Si(2)O(8) glass (with minimum C(Q) values of 3.0 MHz even for all Al species). We have recently shown that for edge-sharing geometries, in which the tricluster O atoms participate in "two-membered rings" of composition Al(2)O(2), the calculated C(Q) values are considerably lower, in the range identified in the glass. However, such two-membered ring geometries had been observed only in crystalline inorganic alumoxanes. Ab initio MD calculations on related compositions, such as the calcium aluminosilicate, CAS, (CaO)(0.21)(Al(2)O(3))(0.12)(SiO(2))(0.67), show a small percentage of O triclusters, but none in two-membered rings of the Al(2)O(2) type, and the calculated C(Q) values for the triclusters that do exist are higher than seen in the original experiments on CaAl(2)Si(2)O(8) glass and not significantly different from those for two-coordinate O in Si-O-Al sites. However, a classical MD simulation of the structure of glassy aluminum silicate AS2, (Al(2)O(3))2(SiO(2)), gave a predominance of O triclusters within two-membered rings, with structures much like those seen in the alumoxanes. We have now calculated (17)O nuclear quadrupole coupling constants and NMR shielding values for clusters extracted from these simulations, using standard quantum chemical methods. The calculated C(Q) values for these O triclusters are now in the range observed experimentally in the CaAl(2)Si(2)O(8) glass (around 2.3-2.6 MHz) when the tricluster O is surrounded by three Al, two of which are part of an Al(2)O(2) ring. This supports the experimentalists' contention that such tricluster O species do exist and have been seen by (17)O NMR.     

First-principles calculations for the cooperative transitions of Yb3+ dimer clusters in Y3Al5O12 and Y2O3 crystals

A first-principles multielectron method is applied to the calculations for the cooperative transitions of trivalent ytterbium ions (Yb3+) in yttrium aluminum garnet (Y3Al5O12; YAG) and yttrium sesquioxide (Y2O3) crystals. The method is based on a molecular-orbital method, in which the overlap between the Yb 4f and the oxygen 2p orbitals is directly considered through a self-consistent procedure. A Yb2(3+) two-ion model and a (Yb2O14)22- dimer cluster embedded in the point charges of the YAG lattice are compared. The 4f-2p overlaps in the cluster model is needed to explain the cooperative transition probability originating from electric dipole transitions. A (Yb2O10)14- dimer molecule in Y2O3 lattice produces larger electric dipole transition probabilities than the case of YAG. The smaller coordination number in Y2O3 produces the larger 4f-2p overlaps, which result in the larger transition probabilities.     

Combined charge and spin density experimental study of the yttrium(III) semiquinonato complex Y(HBPz3)2(DTBSQ) and DFT calculations

High-resolution X-ray diffraction and polarized neutron diffraction experiments have been performed on the Y-semiquinonate complex, Y(HBPz3)2(DTBSQ), in order to determine the charge and spin densities in the paramagnetic ground state, S = (1/2). The aim of these combined studies is to bring new insights to the antiferromagnetic coupling mechanism between the semiquinonate radical and the rare earth ion in the isomorphous Gd(HBPz3)2(DTBSQ) complex. The experimental charge density at 106 K yields detailed information about the bonding between the Y3+ ion and the semiquinonate ligand; the topological charge of the yttrium atom indicates a transfer of about 1.5 electrons from the radical toward the Y3+ ion in the complex, in agreement with DFT calculations. The electron density deformation map reveals well-resolved oxygen lone pairs with one lobe polarized toward the yttrium atom. The determination of the induced spin density at 1.9 K under an applied magnetic field of 9.5 T permits the visualization of the delocalized magnetic orbital of the radical throughout the entire molecule. The spin is mainly distributed on the oxygen atoms [O1 (0.12(1) mu B), O2(0.11(1) mu B)] and the carbon atoms [C21 (0.24(1) mu B), C22(0.20(1) mu B), C24(0.16(1) mu B), C25(0.12(1) mu B)] of the carbonyl ring. A significant spin delocalization on the yttrium site of 0.08(2) mu B is observed, proving that a direct overlap with the radical magnetic orbital can occur at the rare earth site and lead to antiferromagnetic coupling. The DFT calculations are in good quantitative agreement with the experimental charge density results, but they underestimate the spin delocalization of the oxygen toward the yttrium and the carbon atoms of the carbonyl ring.     

Systematic study of compositional and synthetic control of vacancy and magnetic ordering in oxygen-deficient perovskites Ca2Fe(2-x)Mn(x)O(5+y)and CaSrFe(2-x)Mn(x)O(5+y) (x = 1/2, 2/3, and 1; y = 0-1/2)

Ten compounds belonging to the series of oxygen-deficient perovskite oxides Ca(2)Fe(2-x)Mn(x)O(5) and CaSrFe(2-x)Mn(x)O(5+y), where x = 1/2, 2/3, and 1 and y ≈ 0-0.5, were synthesized and investigated with respect to the ordering of oxygen vacancies on both local and long-range length scales and the effect on crystal structure and magnetic properties. For the set with y ≈ 0 the oxygen vacancies always order in the long-range sense to form the brownmillerite structure containing alternating layers of octahedrally and tetrahedrally coordinated cations. However, there is a change in symmetry from Pnma to Icmm upon substitution of Sr for one Ca for all x, indicating local T(d) chain (vacancy) disorder. In the special case of CaSrFeMnO(5) the neutron diffraction peaks broaden, indicating only short-range structural order on a length scale of ~160 ?. This reveals a systematic progression from Ca(2)FeMnO(5) (Pnma, well-ordered tetrahedral chains) to CaSrFeMnO(5) (Icmm, disordered tetrahedral chains, overall short-range order) to Sr(2)FeMnO(5) (Pm3m, destruction of tetrahedral chains in a long-range sense). Systematic changes occur in the magnetic properties as well. While long-range antiferromagnetic order is preserved, the magnetic transition temperature, T(c), decreases for the same x when Sr substitutes for one Ca. A review of the changes in T(c) for the series Ca(2)Fe(2-x)M(x)O(5), taking into account the tetrahedral/octahedral site preferences for the various M(3+) ions, leads to a partial understanding of the origin of magnetic order in these materials in terms of a layered antiferromagnetic model. While in all cases the preferred magnetic moment direction is (010) at low temperatures, there is a cross over for x = 0.5 to (100) with increasing temperature for both the Ca(2)Fe(2-x)Mn(x)O(5) and the CaSrFe(2-x)Mn(x)O(5) series. For the y > 0 phases, while a brownmillerite ordering of oxygen vacancies is preserved for the Ca(2) phases, a disordered Pm3m cubic perovskite structure is always found when Sr is substituted for one Ca. Long-range magnetic order is also lost, giving way to spin glass or cluster-glass-like behavior below ~50 K. For the x = 0.5 phase, neutron pair distribution function (NPDF) studies show a local structure related to brownmillerite ordering of oxygen vacancies. Neutron diffraction data at 3.8 K show a broad magnetic feature, incommensurate with any multiple of the chemical lattice, and with a correlation length (magnetic domain) of 6.7(4) ?.     

1H and (113)Cd NMR Investigations of Cd(2+) and Zn(2+) Binding Sites on Serum Albumin: Competition with Ca(2+), Ni(2+), Cu(2+), and Zn(2+)

1H and (113)Cd NMR studies are used to investigate the Cd(2+) binding sites on serum albumin (67 kDa) in competition with other metal ions. A wide range of mammalian serum albumins possess two similar strong Cd(2+) binding sites (site A 113-124 ppm; site B 24-28 ppm). The two strong sites are shown not to involve the free thiol at Cys34. Ca(2+) influences the binding of Cd(2+) to isolated human albumin, and similar effects due to endogenous Ca(2+) are observed for intact human blood serum. (1)H NMR studies show that the same two His residues of human serum albumin are perturbed by Zn(2+) and Cd(2+) binding alike. Zn(2+) displaces Cd(2+) from site A which leads to Cd(2+) occupation of a third site (C, 45 ppm). The N-terminus of HSA is not the locus of the two strong Cd(2+) binding sites, in contrast to Cu(2+) and Ni(2+). After saturation of the N-terminal binding site, Cu(2+) or Ni(2+) also displaces Cd(2+) from site A to site C. The effect of pH on Cd(2+) binding is described. A common Cd(2+)/Zn(2+) binding site (site A) involving interdomain His residues is discussed.     

Influence of calcium-induced aggregation on the sensitivity of aminobis(methylenephosphonate)-containing potential MRI contrast agents

A novel class of 1,4,7,10-tetraazacyclododecane-1,4,7-tris(methylenecarboxylic) acid (DO3A)-based lanthanide complexes with relaxometric response to Ca(2+) was synthesized, and their physicochemical properties were investigated. Four macrocyclic ligands containing an alkyl-aminobis(methylenephosphonate) side chain for Ca(2+)-chelation have been studied (alkyl is propyl, butyl, pentyl, and hexyl for L(1), L(2), L(3), and L(4), respectively). Upon addition of Ca(2+), the r(1) relaxivity of their Gd(3+) complexes decreased up to 61% of the initial value for the best compounds GdL(3) and GdL(4). The relaxivity of the complexes was concentration dependent (it decreases with increasing concentration). Diffusion NMR studies on the Y(3+) analogues evidenced the formation of agglomerates at higher concentrations; the aggregation becomes even more important in the presence of Ca(2+). (31)P NMR experiments on EuL(1) and EuL(4) indicated the coordination of a phosphonate to the Ln(3+) for the ligand with a propyl chain, while phosphonate coordination was not observed for the analogue bearing a hexyl linker. Potentiometric titrations yielded protonation constants of the Gd(3+) complexes. log K(H1) values for all complexes lie between 6.12 and 7.11 whereas log K(H2) values are between 4.61 and 5.87. Luminescence emission spectra recorded on the Eu(3+) complexes confirmed the coordination of a phosphonate group to the Ln(3+) center in EuL(1). Luminescence lifetime measurements showed that Ca-induced agglomeration reduces the hydration number which is the main cause for the change in r(1). Variable temperature (17)O NMR experiments evidenced high water exchange rates on GdL(1), GdL(2), and GdL(3) comparable to that of the aqua ion.     

Nanoscale fullerene compression of an yttrium carbide cluster

The nanoscale parameters of metal clusters and lattices have a crucial influence on the macroscopic properties of materials. Herein, we provide a detailed study on the size and shape of isolated yttrium carbide clusters in different fullerene cages. A family of diyttrium endohedral metallofullerenes with the general formula of Y(2)C(2n) (n = 40-59) are reported. The high field (13)C nuclear magnetic resonance (NMR) and density functional theory (DFT) methods are employed to examine this yttrium carbide cluster in certain family members, Y(2)C(2)@D(5)(450)-C(100), Y(2)C(2)@D(3)(85)-C(92), Y(2)C(2)@C(84), Y(2)C(2)@C(3v)(8)-C(82), and Y(2)C(2)@C(s)(6)-C(82). The results of this study suggest that decreasing the size of a fullerene cage with the same (Y(2)C(2))(4+) cluster results in nanoscale fullerene compression (NFC) from a nearly linear stretched geometry to a constrained "butterfly" structure. The (13)C NMR chemical shift and scalar (1)J(YC) coupling parameters provide a very sensitive measure of this NFC effect for the (Y(2)C(2))(4+) cluster. The crystal structural parameters of a previously reported metal carbide, Y(2)C(3) are directly compared to the (Y(2)C(2))(4+) cluster in the current metallofullerene study.     

The nature of oxygen exchange in ZrW2O8 revealed by two-dimensional solid-state 17O NMR

17O magic angle spinning (MAS) NMR has been used to determine the nature of oxygen exchange in ZrW(2)O(8). A highly effective isotopic labelling technique has been developed and 1D NMR and 2D exchange spectroscopy (EXSY) experiments have revealed that mutual exchange occurs between all oxygen sites, even at temperatures considerably below the alpha to beta order-disorder phase transition.     

(17)O multiple-quantum MAS NMR study of high-pressure hydrous magnesium silicates.

Two (17)O-enriched hydrous magnesium silicates, the minerals hydroxyl-chondrodite (2Mg(2)SiO(4).Mg(OH)(2)) and hydroxyl-clinohumite (4Mg(2)SiO(4).Mg(OH)(2)), were synthesized. High-resolution "isotropic" (17)O (I = (5)/(2)) NMR spectra of the powdered solids were obtained using three- and five-quantum MAS NMR at magnetic field strengths of 9.4 and 16.4 T. These multiple-quantum (MQ) MAS spectra were analyzed to yield the (17)O isotropic chemical shifts (delta(CS)) and quadrupolar parameters (C(Q), eta and their "product" P(Q)) of the distinct oxygen sites resolved in each sample. The values obtained were compared with those found previously for forsterite (Mg(2)SiO(4)). The (17)O resonances of the protonated (hydroxyl) sites were recorded and assigned with the aid of (17)O [(1)H] cross-polarization and comparison with the spectrum of (17)O-enriched brucite (Mg(OH)(2)). Using all of these data, complete assignments of the five crystallographically inequivalent oxygen sites in hydroxyl-chondrodite and of the nine such sites in hydroxyl-clinohumite are suggested. The validity of these assignments are supported by the observation of a correlation between (17)O isotropic chemical shift and Si-O bond length. The (29)Si MAS NMR spectra of the two minerals were also obtained.     

17O NMR and computational study of a tetrasiliconiobate ion, [H(2+x)Si4Nb16O56](14-x)-

Rates of oxygen-isotope exchange were measured in the tetrasiliconiobate ion [H(2+x)Si(4)Nb(16)O(56)]((14-x)-) to better understand how large oxide ions interact with water. The molecule has 19 nonequivalent oxygen sites and is sufficiently complex to evaluate hypotheses derived from our previous work on smaller clusters. We want to examine the extent to which individual oxygen atoms react independently with particular attention given to the order of protonation of the various oxygen sites as the pH decreases from 13 to 6. As in our previous work, we find that the set of oxygen sites reacts at rates that vary over approximately 10(4) across the molecule at 6相似文献