Monday, September 20, 2010

Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization

A nice write-up in C&E News about DNP SSNMR of molecules bound to silica surfaces.

The corresponding JACS communication can be found at:

Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization

Anne Lesage†, Moreno Lelli†, David Gajan‡, Marc A. Caporini§, Veronika Vitzthum§, Pascal Miville§, Johan Alauzun, Arthur Roussey‡, Chlo Thieuleux‡, Ahmad Medhi, Geoffrey Bodenhausen§, Christophe Copret‡, and Lyndon Emsley*†

J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/ja104771z
Publication Date (Web): September 10, 2010
Copyright © 2010 American Chemical Society

Abstract: It is shown that surface NMR spectra can be greatly enhanced using dynamic nuclear polarization. Polarization is transferred from the protons of the solvent to the rare nuclei (here carbon-13 at natural isotopic abundance) at the surface, yielding at least a 50-fold signal enhancement for surface species covalently incorporated into a silica framework

J. Chem. Phys.

Internal symmetry of basic elements in symmetry-based recoupling sequences under magic-angle spinning

Fang-Chieh Chou, Hsin-Kuan Lee, and Jerry C. C. Chan

In solid-state NMR, many powerful pulse sequences under the condition of magic-angle spinning can be analyzed on the basis of the C- and R-sequences developed by Levitt and co-workers. It has been speculated for some years that the basic elements commonly used in symmetry-based recoupling pulse sequences have certain kind of internal symmetries. We show by a detailed analysis that a set of internal selection rules does exist for many basic elements. These internal selection rules may allow a more versatile design of CNnν or RNnν sequences when n is an integer or half-integer multiple of N. As an illustration, we have derived the symmetry arguments to rationalize the observation that the C-REDOR pulse sequence can suppress homonuclear dipole-dipole interaction, leading to the design of new windowed basic elements usable for heteronuclear dipolar recoupling with active suppression of homonuclear dipole-dipole interaction. Numerical simulations and experiments measured for [U–13C,15N]-L-alanine have been used to validate our approach. On a more general note, the symmetry rules discussed in this work can also be applied for the design of supercycles.

Puckering free energy of pyranoses: A NMR and metadynamics-umbrella sampling investigation

E. Autieri, M. Sega, F. Pederiva, and G. Guella

We present the results of a combined metadynamics-umbrella sampling investigation of the puckered conformers of pyranoses described using the GROMOS 45a4 force field. The free energy landscape of Cremer–Pople puckering coordinates has been calculated for the whole series of α and β aldohexoses, showing that the current force field parameters fail in reproducing proper puckering free energy differences between chair conformers. We suggest a modification to the GROMOS 45a4 parameter set which improves considerably the agreement of simulation results with theoretical and experimental estimates of puckering free energies. We also report on the experimental measurement of altrose conformer populations by means of NMR spectroscopy, which show good agreement with the predictions of current theoretical models.

Tuesday, September 14, 2010

J. Phys. Chem. B and C, vol. 114, Issues 36

Combined Solid-State NMR and Theoretical Calculation Studies of Brønsted Acid Properties in Anhydrous 12-Molybdophosphoric Acid

Ningdong Feng†, Anmin Zheng*†, Shing-Jong Huang‡, Hailu Zhang§, Ningya Yu‡, Chih-Yi Yang‡, Shang-Bin Liu*‡, and Feng Deng*†

J. Phys. Chem. C, 2010, 114 (36), pp 15464–15472
DOI: 10.1021/jp105683y
Publication Date (Web): August 20, 2010
Copyright © 2010 American Chemical Society

Abstract: The strength and distribution of Brønsted acidic protons in anhydrous phosphomolybdic acid (H3PMo12O40, HPMo) have been studied by solid-state magic-angle-spinning (MAS) NMR, using trimethylphosphine oxide (TMPO) as the probe molecule in conjunction with density functional theory (DFT) calculations. Brønsted acid sties with strengths exceeding the threshold of superacidity (Zheng, A. et al. J. Phys. Chem. B 2008, 112, 4496) were observed for HPMo. In addition, the locations and adsorption structures of Brønsted protons on various oxygen sites in HPMo were also identified. The preferred location of the acidic proton was found to follow the trend: corner-sharing (Ob) > edge-sharing (Oc) terminal (Od) sites. Moreover, a tendency of hybridization among Brønsted protons residing at Ob and Oc sites of HPMo was inferred by experimental as well as theoretical 31P chemical shifts of the adsorbed TMPO.

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.

J. Chem. Phys.

Self-diffusion of poly(propylene glycol) in nanoporous glasses studied by pulsed field gradient NMR: A study of molecular dynamics and surface interactions
A. Schonhals, F. Rittig, and J. Karger
Pulsed field gradient NMR is applied to investigate the self-diffusion of poly(proypylene glycol) in nanoporous glasses (nominal pore sizes of 2.5–7.5 nm). In general, the diffusion is slowed down by the confinement compared to the bulk. For native pore surfaces covered by hydroxyl groups the spin echo attenuation Ψ displays a bimodal behavior versus q2t (q-norm of a generalized scattering vector). This was explained assuming spatial regions of different diffusivities in a two-phase model. The slow component is assigned to segments forming a surface layer close to the pore walls in which the segments have a lower mobility than those located in the center of the pores. By variation of observation time it was concluded that time constant for the dynamic exchange of segments between these two regions is around 100 ms at room temperature. For silanized pores, the bimodal behavior in the spin echo attenuation Ψ shows a stretched exponential decay versus q2t. The estimated diffusion coefficients decrease strongly with decreasing pore size. The temperature dependence of the diffusion coefficient can be approximated by an Arrhenius law where the activation energy increases with decreasing pore size. The observed pore size dependence for the diffusion of poly(propylene glycol) in silanized nanoporous glasses can be discussed assuming interaction and confining size effects.

Dynamical effects in ab initio NMR calculations: Classical force fields fitted to quantum forces

Mark Robinson and Peter D. Haynes
NMR chemical shifts for an L-alanine molecular crystal are calculated using ab initio plane wave density functional theory. Dynamical effects including anharmonicity may be included by averaging chemical shifts over an ensemble of structural configurations generated using molecular dynamics (MD). The time scales required mean that ab initio MD is prohibitively expensive. Yet the sensitivity of chemical shifts to structural details requires that the methodologies for performing MD and calculating NMR shifts be consistent. This work resolves these previously competing requirements by fitting classical force fields to reproduce ab initio forces. This methodology is first validated by reproducing the averaged chemical shifts found using ab initio molecular dynamics. Study of a supercell of L-alanine demonstrates that finite size effects can be significant when accounting for dynamics.