Apparent Chemical Shift Research Articles

Chemical shifts of metallic and non-metallic Al–Re–Si approximant crystals studied by EELS and SXES

Chemical shifts of approximant crystals of 1/0-Al12Re (1/0-metallic), 1/1-Al73Re15Si12 (1/1-metallic) and 1/1-Al73Re17Si10 (1/1-non-metallic) were examined by using electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Al L-shell excitation EELS spectra of these alloys showed an apparent chemical shift only for the 1/1-non-metallic alloy to the larger binding energy side by 0.2 eV. Al-Kα, Re-Mα and Si-Kα emission SXES spectra also showed a shift to the larger binding energy side only for 1/1-non-metallic alloy. 1/0-metallic and 1/1-metallic alloys did not show any chemical shift in EELS and SXES experiments. Chemical shifts were observed only in larger binding energy side compared with pure materials. This implies the decrease of valence charge at constituent atomic sites of 1/1-non-metallic alloy compared with 1/0-metallic, 1/1-metallic and pure materials. The decreased charges should distribute intermetallic sites, which should be related to a formation of covalent bonding among Al atomic sites reported by maximum-entropy method (MEM)/Rietveld analysis on this material. This relation between chemical shift and covalent bonding nature of this approximant alloy may support the presence of covalent bonding in Al-based quasicrystals.

Apparent Chemical Shifts at X-ray Absorption Edges of 3d-, 4f-, 5d- and 5p-elements for Empirical Chemical State Analysis

Apparent Chemical Shifts at X-ray Absorption Edges of 3d-, 4f-, 5d- and 5p-elements for Empirical Chemical State Analysis

Association of paramagnetic species with formation of LiF at the surface of LiCoO2

To examine formation of LiF at the surface of LiCoO2, 7Li and 19F double-resonance NMR was applied to LiCoO2 electrode samples without any charge/discharge cycles. The two-dimensional (2D) 7Li–19F heteronuclear correlation (HETCOR) experiment using cross polarization (CP) shows a large cross peak at (7Li, 19F)=(ca. 3ppm, ca. −145ppm) for the sample without charge/discharge cycles. Since the magnetization build-up curve during 19F to 7Li CP shows a characteristic dipolar oscillation observed in pure LiF, the observed cross peak was assigned to LiF formed in the LiCoO2 electrode. For LiF in the LiCoO2 electrode, not only the apparent chemical shift differences from those of pure LiF (ca. −1ppm, −203ppm), but also the significant reduction of the spin-lattice relaxation times (T1 and T1ρ) were observed. These anomalies are ascribed to the paramagnetic effects. Creation of certain paramagnetic species in associate with the LiF formation on LiCoO2 or the LiF formation at paramagnetic sites at the surface of LiCoO2 was invoked.

Characteristic chemical shift of quasicrystalline alloy Al53Si27Mn20 studied by EELS and SXES

Chemical shifts of all constituent atoms for amorphous (Am), quasicrystalline (QC) and crystalline (Cryst) alloys of Al53Si27Mn20 were investigated for the first time by high energy-resolution electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Among Al L-shell excitation EELS spectra of Am, QC and Cryst alloys, only QC alloy showed an apparent chemical shift to the larger binding energy side by 0.4 eV. In Al-Kα and Si-Kα emission SXES spectra of these alloys, only QC alloy showed a chemical shift to the larger binding energy side by 4 eV for Al-Kα and 6 eV for Si-Kα. These chemical shift values are comparable to those of corresponding metal oxides. This indicates a smaller amount of valence charge at Al and Si atomic sites in QC alloy. On the other hand, Mn-L SXES spectra did not show any chemical shift. Therefore, the decreased charge from Al and Si sites should be distributed between atomic sites, indicating the presence of covalent bonding nature for QC ordered alloy.

Conduction Band Edge of TiO2-SnO2 Solid Mixtures Tuning for Photoelctrochemical Applications

AbstractWe report investigation of effect of conduction band edge on the dye injection and transport by preparation of (Ti,Sn)O2 solid mixtures in ratios of 80:20 and 90:10 as possible applications in dye sensitized solar cells. SEM micrographs showed highly porous with nanometer sized particles of around 6 - 10μm diameter. X-ray diffraction patterns showed strong TiO2 anatase peaks with crystal orientation directions (101) being the strongest in both the solid mixtures and in pure TiO2. XPS studies have shown an apparent chemical shift for Ti 2p and O1s core level spectra with an energy difference between the unmodified and the solid mixture being 0.65eV. Initial I-V studies have shown high Voc but low short circuit photocurrent, showing a possible unfavorable band edge shift between the semiconductor and the dye LUMO level.

High-field 19.6 T 27Al solid-state MAS NMR of in vitro aluminated brain tissue

The combination of 27Al high-field solid-state NMR (19.6 T) with rapid spinning speeds (17.8 kHz) is used to acquire 27Al NMR spectra of total RNA human brain temporal lobe tissues exposed to 0.10 mM Al 3+ (as AlCl 3) and of human retinal pigment epithelial cells (ARPE-19), grown in 0.10 mM AlCl 3. The spectra of these model systems show multiple Al 3+ binding sites, good signal/noise ratios and apparent chemical shift dispersions. A single broad peak (−3 to 11 ppm) is seen for the aluminated ARPE-19 cells, consistent with reported solution-state NMR chemical shifts of Al-transferrin. The aluminated brain tissue has a considerably different 27Al MAS NMR spectrum. In addition to the transferrin-type resonance, additional peaks are seen. Tentative assignments include: −9 to −3 ppm, octahedral AlO 6 (phosphate and water); 9 ppm, condensed AlO 6 units (Al–O–Al bridges); 24 ppm, tetrahedral AlO 3N and/or octahedral Al–carbonate; and 35 ppm, more N-substituted aluminum and /or tetrahedral AlO 4. Thus, brain tissue is susceptible to a broad range of coordination by aluminum. Furthermore, the moderate 27Al C Q values (all less than 10 MHz) suggest future NMR studies may be performed at 9.4 T and a spin rate of 20 kHz.

Effect of extraframework species on [formula omitted]O NMR chemical shifts in zeolite A

Zeolite A provides a unique opportunity to explore the relationship between structural parameters and 17 O nuclear magnetic resonance (NMR) isotropic chemical shifts ( δ ISO) because differences in site multiplicity for framework oxygens often permit unambiguous site assignments. Strontium exchanged zeolite A and hydrated and dehydrated K- and Na-A were investigated using 17 O triple quantum and five quantum magic angle spinning (3QMAS and 5QMAS) NMR. The results concur with previous observations that δ ISO is influenced by T–O–T bond angle, but also suggest that interactions between framework oxygens and extraframework species (water molecules and cations) affect δ ISO as well. Apparent chemical shifts for zeolitic water sites also exhibit relative chemical shifts as a function of extraframework cation contents consistent with shifts observed for water sites and non-bridging oxygens in glasses. Abundances of Si–O–Al sites in dehydrated forms indicate that oxygen isotope exchange rates can vary between crystallographic sites, as well as between Si–O–Si and Si–O–Al sites.

A Molecular Basis for the Selective Recognition of 2-Hydroxy-dATP and 8-Oxo-dGTP by Human MTH1

MTH1 hydrolyzes oxidized purine nucleoside triphosphates such as 8-oxo-dGTP, 8-oxo-dATP, 2-hydroxy-dATP, and 2-hydroxy rATP to monophosphates, and thus avoids errors caused by their misincorporation during DNA replication or transcription, which may result in carcinogenesis or neurodegeneration. This substrate specificity for oxidized purine nucleoside triphosphates was investigated by mutation analyses based on the sequence comparison with the Escherichia coli homolog, MutT, which hydrolyzes only 8-oxo-dGTP and 8-oxo-rGTP but not oxidized forms of dATP or ATP. Neither a replacement of the phosphohydrolase module of MTH1 with that of MutT nor deletions of the C-terminal region of MTH1, which is unique for MTH1, altered the substrate specificity of MTH1. In contrast, the substitution of residues at position Trp-117 and Asp-119 of MTH1, which showed apparent chemical shift perturbations with 8-oxo-dGDP in NMR analyses but are not conserved in MutT, affected the substrate specificity. Trp-117 is essential for MTH1 to recognize both 8-oxo-dGTP and 2-hydroxy-dATP, whereas Asp-119 is only essential for recognizing 2-hydroxy-dATP, thus suggesting that origins of the substrate-binding pockets for MTH1 and MutT are different.

A multinuclear solid-state NMR study of phospholipid-cholesterol interactions. Dipalmitoylphosphatidylcholine-cholesterol binary system.

Multinuclear (1H, 13C, 31P) MASNMR and static solid-state 31P NMR were used to study the molecular interactions between dipalmitoylphosphatidylcholine (DPPC) and free cholesterol (CHOL) in multilayers of DPPC containing 0-65 mol % CHOL with respect to total lipid at temperatures between 25 and 55 degrees C. 13C chemical shifts and line shapes for DPPC and CHOL resonances were measured in 13C MASNMR spectra. The apparent chemical shift anisotropy (CSA) of the DPPC acyl methylene resonances [(CH2)n] was calculated from the 1H MASNMR spectra. CSA and line shape changes were recorded as a function of CHOL content by 31P static solids and MASNMR. The presence of CHOL significantly changed the 13C chemical shifts and line shapes of DPPC carbonyl carbons below or above the main transition temperature of pure DPPC. Chemical shift changes were also observed for CHOL carbons as a function of the mixing ratio, signifying a changing local environment of CHOL. For mixtures with CHOL > 50 mol %, 13C MASNMR detected crystalline CHOL in the monohydrate form. When the excess CHOL was in a submicroscopic crystalline form that was not readily detected by differential scanning calorimetry, or optical microscopy (but readily observed by 13C MASNMR), the 31P powder pattern was affected, suggesting interaction of the excess CHOL with the aqueous interface of the bilayer. These results suggest the potential of 13C MASNMR for detection of crystalline CHOL in biological samples.

Spin label ESR and 31P-NMR studies of the cubic and inverted hexagonal phases of dimyristoylphosphatidylcholine/myristic acid (1:2, mol/mol) mixtures

The thermotropic phase behaviour and chain dynamics of the dimyristoylphosphatidylcholine/myristic acid (1:2, mol/mol) mixture at pH 4.0 have been studied with 31P-NMR and spin-label ESR. Broadline proton-dipolar decoupled 31P-NMR spectra show that the system undergoes a transition from a lamellar phase to a non-lamellar phase at 47° C. The 31P-NMR spectrum collapses, from a lamellar powder pattern with apparent chemical shift anisotropy Δσ = − 38 ppm below 47° C, to an isotropic spectrum with superimposed hexagonal powder pattern of Δσ = + 13 ppm above 47° C. With increasing temperature, the hexagonal component grows at the expense of the isotropic component, finally transforming to a single hexagonal phase at approximately 70° C. The ESR spectra from both fatty acid and phosphatidylcholine probes spin-labelled in the hydrocarbon chain have been used to study the ordering and mobility of the lipid chains through the lamellar-non lamellar transition. An abrupt increase in the chain mobility observed at approx. 47° C demonstrates that the lamellar-non lamellar phase transition coincides with chain melting. The apparent order parameter and polarity profiles, deduced from the ESR spectra of 11 different chain-labelled positional isomers, demonstrate flexibility and polarity gradients characteristic of a lyotropic liquid crystalline phase. The temperature dependence of the order parameters does not show any changes in the fluid phase, indicating that the conversion from the isotropic (cubic) phase to the inverted hexagonal phase is not accompanied by large changes in chain order. A comparison of the apparent order parameters and polarity profiles of the phosphatidylcholine and fatty acid spin labels indicates that the fatty acid spin labels are intercalated approximately one CH 2 unit more deeply in the hydrocarbon region than are the positionally isomeric phosphatidylcholine spin labels.

Bias-reference X-Ray photoelectron spectroscopy of sapphire and yttrium aluminum garnet crystals

An XPS technique which uses a biased gold dot as a reference marker was used to measure the A12p, the 01s, and the Y 3d bond energies in single crystals of sapphire and in Ce-doped and Ca-doped crystals of yttrium aluminum garnet. The observed A12p and the Ols binding energies were shifted in the various specimens, but the aluminum and the oxygen peaks in each sample were shifted in the same direction by the same voltages. It is postulated that these apparent chemical shifts were caused by changes in the Fermi energy levels rather than by changes in the binding energies. The Fermi energy of one of the materials was used as a standard, and the Fermi levels of all of the crystals were determined relative to it. The Fermi energy level of the sapphire crystal was measured at 3.7 eV above the top of the valence band, and that of a YAG crystal annealed in an oxidizing atmosphere at 1.9 eV.

Surface properties of silicon and aluminum oxide powders

A bias-reference XPS technique has been used to relate the apparent chemical shifts of the Ols oxide lines in a series of silica, alumina, magnesia, and phosphate powders to variations in the Fermi levels of these materials. The Fermi energies, E F , are shown to be linearly related to the aqueous point of zero charge (pzc) of these surfaces, following the relationship: pzc = −2.9 E F + 16.8. These results conform to a model of the water/oxide surface reaction which follows the generalized Lewis theory of acid/base adduct formation.

Interpretation of apparent chemical shifts in XP spectra from oxide—GaP interfaces

The cause of the apparent chemical shifts (ACS's) of XPS peaks between oxide and substrate GaP has been studied. The ACS's for Ga 3 d, P 2 p and Ga LMM change significantly depending on the conditions of oxidation. The changes in the ACS's are considered in terms of not only the chemical structure of the oxide, but also other possible causes, such as charging effects, electric dipole moment, extra-atomic relaxation energy, Fermi-level pinning position, etc. The change of the Fermi-level position at the surface of substrate GaP is revealed to be a dominant factor causing the changes in the ACS's.

Photoelectron Spectroscopy of Solids

This paper is concerned with information about electronic structure and bonding in solids which can be derived from valence band and core level photoelectron spectroscopy. The role, in research of this kind, of the excitation processes discussed elsewhere in this symposium is of concern. A number of factors contribute to the apparent chemical shifts of core levels and valence bands in solids. These factors are considered and then results for gold alloys are taken as an example of the bonding information which can be derived. Angle resolved photoelectron spectroscopy provides information about wave function symmetry as well as energy. This is reviewed and one-electron energy vs. k results for Ni metal are inspected. Such results provide the best experimental test yet devised of energy band theory and of the potentials employed.

Spin-Echo NMR Studies of Internal Rotation in 1,1-Difluoro-1,2-Dibromodichloroethane

A spin-echo study of internal rotation in liquid 1,1-difluoro-1,2-dibromodichloroethane is reported. Carr-Pucrell spin-echo (CPSE) trains were observed from −115° to +100°C with rf pulse spacings of 0.4 to 32 msec. Modulation of the echo trains was observed at temperatures below −60°C, and the modulation frequencies were found to agree quantitatively with predictions of a general theory for the combined effects of scalar coupling and chemical exchange in spin-echo experiments. At −100°C, effects of the internal rotation are negligible and dipole-dipole interactions give different relaxation rates for fluorines in different rotamers. Above −60°C the echo trains decay exponentially. The dependence of the observed decay rates on pulse spacing was used to obtain values of the free energies of activation for exchange between rotamers and values of the fluorine chemical shifts in the different rotamers. The free energies of activation are in agreement with values obtained by steady-state NMR, and by analysis of CPSE trains at lower temperatures, but the apparent chemical shift values are about 25% too large.