Tuesday, September 07, 2010

J. Phys. Chem. B and C, v114, Issues 32 - 35

Structure and Disorder in Amorphous Alumina Thin Films: Insights from High-Resolution Solid-State NMR

Sung Keun Lee*†, Sun Young Park†, Yoo Soo Yi† and Jaehyun Moon‡

J. Phys. Chem. C, 2010, 114 (32), pp 13890–13894
Publication Date (Web): July 28, 2010

Abstract:Revealing the extent of disorder in amorphous oxides is one of the remaining puzzles in physical chemistry, glass sciences, and geochemistry. Here, we report the 27Al NMR results for amorphous Al2O3 thin films obtained from two different deposition methods (i.e., physical vapor-deposition and atomic layer-deposition), revealing two distinct amorphous states defined by a fraction of five-coordinated Al ([5]Al). The fractions of [4]Al and [5]Al are dominant (92−95%) in both films. While the overall similarity between these two states suggests a narrow stability of available amorphous states, the fraction of [5]Al in atomic layer-deposited thin films is apparently larger and thus more disordered than that in physical vapor-deposited films. Such results require that varying extents of disorder exist in the amorphous oxides prepared under different processing conditions. As the [5]Al site (<1%) in crystalline Al2O3 is known to control its catalytic ability over [4]Al and [6]Al, the significant fractions (40%) of [5]Al in our amorphous thin films suggest that amorphous Al2O3 may be potentially useful as a new class of catalysts.

X-ray Diffraction, FT-IR, and 13C CP/MAS NMR Structural Studies of Solvated and Desolvated C-Methylcalix[4]resorcinarene

Rafal Kuzmicz†, Violetta Kowalska†, Sławomir Domagała‡, Marcin Stachowicz‡, Krzysztof Woniak‡ and Waclaw Kolodziejski*†

J. Phys. Chem. B, 2010, 114 (32), pp 10311–10320
DOI: 10.1021/jp1015565
Publication Date (Web): July 26, 2010

Abstract: Solid C-methylcalix[4]resorcinarene solvated by acetonitrile and water (CAL-Me) and then modified by slow solvent evaporation (CAL-Me*) was studied using single-crystal and powder X-ray diffraction, FT-IR, and 13C CP/MAS NMR. The CAL-Me solvate crystallizes in the monoclinic P21/n space group with three CH3CN and two H2O molecules in the asymmetric part of the unit cell. The CAL-Me molecules adopt a typical crown conformation with all of the hydroxyl groups of the aryl rings oriented up and all of the methyl groups disposed down (the rccc isomeric form). The crystalline network is formed by resorcinarene, CH3CN, and H2O molecules and assembled by intermolecular hydrogen bonds and weak C−H···A or C−H···π interactions. The desolvated CAL-Me* loses its crystalline character and becomes partly amorphous. It is devoid of CH3CN and deficient in water. However, the resorcinarene molecules still remain in the crown conformation supported by intramolecular hydrogen bonds, while intermolecular hydrogen bonds are considerably disintegrated. The work directs general attention to the problem of stability and polymorphism of resorcinarene solvates. It shows that the joint use of diffractometric and spectroscopic methods is advantageous in the structural studies of complex crystalline macromolecular systems. On the other hand, the solid-state IR and NMR spectroscopic analyses applied in tandem have been found highly beneficial to elucidate the disordered structure of poorly crystalline, desolvated resorcinarene

Conformational Changes at Mesophase Transitions in a Ferroelectric Liquid Crystal by Comparative DFT Computational and 13C NMR Study

Alberto Marini* and Valentina Domenici

J. Phys. Chem. B, 2010, 114 (32), pp 10391–10400
DOI: 10.1021/jp105095m
Publication Date (Web): July 26, 2010

Abstract: In this work, we report a detailed investigation on both the conformational and the orientational ordering properties of a ferroelectric liquid crystal mesogen, namely, M10/**, through the combination of high resolution solid state 13C NMR and density functional theory (DFT) computational methods. The trends of the observed 13C chemical shift in the blue, cholesteric, and ferroelectric SmC* phases of M10/** were analyzed in terms of conformational changes occurring in the flexible parts of the molecule. In particular, we focused on the aliphatic alpha methylenoxy carbons because of their high sensitivity to mesophase environment, as evidenced by experimental 13C chemical shift anisotropy (CSA). DFT computation of the chemical shift tensors as a function of geometrical parameters, such as dihedral angles, put in evidence significant changes in the average conformation at the mesophase transitions. The conformations predicted by DFT have been validated by comparing the calculated 13C chemical shifts with those experimentally observed for the alkoxylic carbons, whose relative orientation plays a key role in establishing the overall conformation of the molecule in each liquid crystalline phase. Furthermore, the orientational order parameters of the relevant flexible fragments were calculated and found to be in good agreement with those characterizing similar systems, thus validating our approach.

Glass-to-Vitroceramic Transition in the Yttrium Aluminoborate System: Structural Studies by Solid-State NMR

Heinz Deters†‡, Andrea S. S. de Camargo†§, Cristiane N. Santos§ and Hellmut Eckert*†

J. Phys. Chem. C, 2010, 114 (34), pp 14618–14626
Publication Date (Web): August 6, 2010

Abstract: The crystallization of laser glasses in the system (B2O3)0.6{(Al2O3)0.4−y(Y2O3)y} (0.1 ≤ y ≤ 0.25) doped with different levels of ytterbium oxide has been investigated by X-ray powder diffraction, differential thermal analysis, and various solid-state NMR techniques. The homogeneous glasses undergo major phase segregation processes resulting in crystalline YBO3, crystalline YAl3(BO3)4, and residual glassy B2O3 as the major products. This process can be analyzed in a quantitative fashion by solid-state 11B, 27Al, and 89Y NMR spectroscopies as well as 11B{27Al} rotational echo double resonance (REDOR) experiments. The Yb dopants end up in both of the crystalline components, producing increased line widths of the corresponding 11B, 27Al, and 89Y NMR resonances that depend linearly on the Yb/Y substitution ratio. A preliminary analysis of the composition dependence suggests that the Yb3+ dopant is not perfectly equipartitioned between both crystalline phases, suggesting a moderate preference of Yb to substitute in the crystalline YBO3 component

Chemical Degradation of Nafion Membranes under Mimic Fuel Cell Conditions as Investigated by Solid-State NMR Spectroscopy

Lida Ghassemzadeh†‡, Klaus-Dieter Kreuer†, Joachim Maier† and Klaus Mller*§

J. Phys. Chem. C, 2010, 114 (34), pp 14635–14645
Publication Date (Web): August 5, 2010

Abstract: A new ex situ method has been developed to mimic the degradation of the polymer membranes in polymer electrolyte membrane fuel cells (PEMFCs), caused by the cross-leakage of H2 and O2. In this ex situ setup, it is possible to expose membranes to flows of different gases with a controlled temperature and humidity. H+-form Nafion films with and without an electrode layer (Pt) have been treated in the presence of different gases in order to simulate the anode and cathode side of a PEMFC. The changes of the chemical structure occurring during the degradation tests were primarily examined by solid-state 19F NMR spectroscopy. For completion, liquid-state NMR studies and ion-exchange capacity measurements were performed. The molecular mobility changes of the ionomer membrane upon degradation were examined for the first time by variable-temperature 19F NMR line-shape, T1 and T1ρ relaxation experiments. It was found that degradation occurs only when both H2 and O2 are present (condition of gas cross-leakage) and when the membrane is coated with a Pt catalyst. The chemical degradation rate is found to be highest for H2-rich mixtures of H2 and O2, which corresponds to the anode under OCV conditions. It is further shown that side-chain disintegration is very important for chemical degradation, although backbone decomposition also takes place. The temperature-dependent line-width and spectral anisotropy alterations were explained by the reduction of static disorder in the Nafion membrane. From the relaxation data, there is evidence for structural annealing, which is independent of the chemical degradation. Chemical degradation is considered to reduce the chain flexibility, as expressed by smaller motional amplitudes, most probably due to chain cross-linking.

Activation of Ammonia Borane Hybridized with Alkaline−Metal Hydrides: A Low-Temperature and High-Purity Hydrogen Generation Material

Yu Zhang, Keiji Shimoda, Takayuki Ichikawa* and Yoshitsugu Kojima
J. Phys. Chem. C, 2010, 114 (34), pp 14662–14664
Publication Date (Web): August 5, 2010

Abstract: Recently, alkali−metal amidoborane complexes have been highlighted as materials that satisfy many of the criteria required to make hydrogen-storage media. In this paper, ammonia borane was successfully activated by the existence of hybrid alkaline−metal hydrides. The desorption results showed that this activation strategy can significantly decrease the dehydrogenation temperature and, furthermore, can successfully suppress ammonia gas release and volume expansion. These results will be helpful for the design of future hydrogen-storage media.

Solid-State 2H NMR and MD Simulations of Positional Isomers of a Monounsaturated Phospholipid Membrane: Structural Implications of Double Bond Location

Stephen R. Wassall*†, M. Alan McCabe†, Cynthia D. Wassall†, Richard O. Adlof‡ and Scott E. Feller§

J. Phys. Chem. B, 2010, 114 (35), pp 11474–11483
DOI: 10.1021/jp105068g
Publication Date (Web): August 13, 2010
Copyright © 2010 American Chemical Society

Abstract: The impact that the position of double bonds has upon the properties of membranes is investigated using solid-state 2H NMR and MD simulations to compare positional isomers of 1-palmitoyl-2-octadecenoylphosphatidylcholine (16:0-18:1PC) bilayers that are otherwise identical apart from the location of a single cis double bond at the Δ6, Δ9, Δ12, or Δ15 position in the 18:1 sn-2 chain. Moment analysis of 2H NMR spectra recorded for isomers perdeuterated in the 16:0 sn-1 chain reveals that average order parameters CD change by more than 35% and that the temperature for chain melting Tm varies by 40 °C. At equal temperature, the CD values exhibit a minimum, as do Tm values, when the double bond is in the middle of the 18:1 sn-2 chain and increase as it is shifted toward each end. Order parameter profiles generated from depaked (“dePaked”) spectra for the 16:0 sn-1 chain all possess the same shape with a characteristic “plateau” region of slowly decreasing order in the upper portion before progressively decreasing more in the lower portion. The NMR results are interpreted on the basis of MD simulation results obtained on each of the four systems. The simulations support the idea that the order parameter changes reflect differences in molecular surface areas, and furthermore that the molecular areas are a function of the strength of the acyl chain attractions.

A Solid-State 17O NMR Study of l-Tyrosine in Different Ionization States: Implications for Probing Tyrosine Side Chains in Proteins

Jianfeng Zhu, Justin Y. C. Lau and Gang Wu*
J. Phys. Chem. B, 2010, 114 (35), pp 11681–11688
DOI: 10.1021/jp1055123
Publication Date (Web): August 16, 2010
Copyright © 2010 American Chemical Society

Abstract: We report experimental characterization of 17O quadrupole coupling (QC) and chemical shift (CS) tensors for the phenolic oxygen in three l-tyrosine (l-Tyr) compounds: l-Tyr, l-Tyr·HCl, and Na2(l-Tyr). This is the first time that these fundamental 17O NMR tensors are completely determined for phenolic oxygens in different ionization states. We find that, while the 17O QC tensor changes very little upon phenol ionization, the 17O CS tensor displays a remarkable sensitivity. In particular, the isotropic 17O chemical shift increases by approximately 60 ppm upon phenol ionization, which is 6 times larger than the corresponding change in the isotropic 13C chemical shift for the Cζ nucleus of the same phenol group. By examining the CS tensor orientation in the molecular frame of reference, we discover a “cross-over” effect between δ11 and δ22 components for both 17O and 13C CS tensors. We demonstrate that the knowledge of such “cross-over” effects is crucial for understanding the relationship between the observed CS tensor components and chemical bonding. Our results suggest that solid-state 17O NMR can potentially be used to probe the ionization state of tyrosine side chains in proteins.

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