Wednesday, May 21, 2008

Aaron's Journal Update

J. Phys. Chem. C, 112 (14), 5552 -5562, 2008. 10.1021/jp7107973 S1932-7447(71)00797-0
Multinuclear Solid-State NMR Studies of Ordered Mesoporous Bioactive Glasses
Ekaterina Leonova, Isabel Izquierdo-Barba, Daniel Arcos, Adolfo López-Noriega, Niklas Hedin, Maria Vallet-Regí, and Mattias Edén*
Abstract: The local structures of highly ordered mesoporous bioactive CaO-SiO2-P2O5 glasses were investigated for variable Ca contents. 1H NMR revealed a diversity of hydrogen-bonded and "isolated" surface silanols as well as adsorbed water molecules. The structural roles of Si and P were explored using a combination of 29Si and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) techniques; the proximities of Si and P to protons were studied through cross-polarization-based experiments, including 1H-29Si and 1H-31P hetero-nuclear two-dimensional correlation spectroscopy. The results are consistent with SiO2 being the main pore-wall component, whereas P is present as a separate amorphous calcium orthophosphate phase, which is dispersed over the pore wall as nanometer-sized clusters. The excess Ca that is not consumed in the phosphate phase modifies the silica glass network where it associates at/near the mesoporous surface. This biphasic structural model of the pore wall leads to the high accessibility of both Ca and P to body fluids, and its relation to the experimentally demonstrated high in vitro bioactivities of these materials is discussed.

J. Phys. Chem. C, 112 (14), 5672 -5678, 2008. 10.1021/jp7108708 S1932-7447(71)00870-7
Conductivity and Fluoride Ion Dynamics in -PbSnF4; 19F Field-Cycling NMR and Diffraction Studies
Eoin Murray, Dermot F. Brougham,* Jovan Stankovic, and Isaac Abrahams
Abstract: Fast-field cycling 19F NMR relaxometry has been applied to investigate fluoride ion dynamics in the layered anionic conductor PbSnF4. Two dynamic processes, on different timescales, were shown to drive the 19F relaxation. By considering the temperature dependencies of the NMR, conductivity, and diffraction data, a complete mechanism for fluoride transport can for the first time be proposed. The slower process is due to anion exchange between equivalent sites (F(2)) in the conducting fluoride plane, which lie between Sn and Pb layers. This is a diffusive process related directly to the mechanism of electrical conduction. The activation barriers for this motion agree closely with those determined from the temperature dependence of the DC conductivity. The faster process is due to non-diffusive exchange between the occupied sites (F(2)) and nominally vacant sites (F(1)), which lie between Sn planes. While this process does not directly limit conductivity, it generates vacancies on the F(2) sites and partial occupancy of the F(1) sites. Above 340 K, the fast process shows an increase in activation energy, as increased occupancy of F(1) sites necessitates the formation of a Frenkel defect on a significant proportion of the F(1) sites prior to an F(2)-F(1) jump. In this temperature range, the slow process shows a decrease in the activation energy, also observed in the conductivity data, due to increased numbers of vacancies in the F(2) sites which provide additional diffusive pathways through the conducting fluoride plane. The results demonstrate that -PbSnF4 is essentially a two-dimensional (anisotropic) conductor, in which nondiffusive fluoride exchange into sites normal to the conducting plane provides a high population of vacancies within the conducting plane, resulting in unusually high conductivity.

J. Phys. Chem. C, 112 (15), 6151 -6159, 2008. 10.1021/jp711438v S1932-7447(71)01438-9
Intermediate-Range Order of Alkali Disilicate Glasses and Its Relation to the Devitrification Mechanism
James G. Longstaffe, Ulrike Werner-Zwanziger, Jose F. Schneider, Marcio L. F. Nascimento, Edgar D. Zanotto, and Josef W. Zwanziger*
Abstract: There are two general mechanisms of devitrification in glass: heterogeneous nucleation of crystals from surfaces and impurities and homogeneous nucleation from the volume. It is thought that structural similarities between glass and crystal at the intermediate-range level influence the mechanism followed; however, there are scarce experimental studies to test this hypothesis. In this paper solid-state nuclear magnetic resonance spectroscopy is used to probe intermediate-range order in sodium and lithium disilicate glasses through measurement of the second moment of the distribution of dipolar couplings. These two glasses undergo heterogeneous and homogeneous nucleation, respectively. The second moments measured for the lithium glass closely follow the trend established by the layered structures of the isochemical crystalline phases, while the same measurements for the sodium glass do not. This observation supports the hypothesis that glasses capable of homogeneous nucleation are structurally more similar to the resulting crystalline phases than those glasses that exhibit only heterogeneous nucleation.

J. Phys. Chem. C, 112 (15), 6165 -6172, 2008. 10.1021/jp7114498 S1932-7447(71)01449-3
19F MAS NMR Investigation of Strontium Substitution Sites in Ca2+/Sr2+ Fluorapatite Solid Solutions
Gyunggoo Cho, # Chung-Nin Chau, and James P. Yesinowski*
Abstract: The partial replacement of Ca2+ by Sr2+ in the fluorapatite lattice results in additional peaks in the 19F MAS NMR spectra at 9.4 T other than the main resonance of Ca10F2(PO4)6 at 64.0 ppm (from hexafluorobenzene). The assignment of these peaks to specific structural configurations is possible in the sample containing the least strontium, with a composition of Ca8.97Sr1.03F2(PO4)6. The solid-solution character of this sample is established by the observation of spectral spin diffusion between various peaks in the SPARTAN (selective population anti-z and rate of transfer to adjacent nuclei) experiment. Calculations based upon modeling the 19F chemical shift tensor show that this process is facilitated for most crystallites by the close approach of two peaks' resonances during the rotor cycle. A peak and set of spinning sidebands with an isotropic chemical shift of 79.6 ppm is assigned to fluoride ions in the center of a triangle of Ca2Sr ions (the so-called Ca(2) sites occurring as Ca3F in the fluorapatite lattice). Smaller chemical shift perturbations observed by deconvolution of two shoulders at 61.2 and 58.8 ppm on the main 64.0 ppm resonance are assigned to a Ca3F configuration that has, respectively, either one or multiple Sr2+ neighbors in the adjacent Ca(2) sites. Quantitative peak intensity measurements relative to the main 64 ppm resonance of both the 79.6 ppm peak as well as the deconvoluted peaks separately indicate that Sr2+ ions preferentially occupy the Ca(2) site at a level 23% greater than that expected for random substitution in the above solid solution. A sample having the composition Ca4.95Sr5.05F2(PO4)6 has a qualitatively similar site preference and has peaks assigned to Ca2SrF at 70 ppm, CaSr2F at 87 ppm, and Sr3F at 105 ppm.

J. Phys. Chem. C, 112 (16), 6430 -6438, 2008. 10.1021/jp7119087 S1932-7447(71)01908-3
H2O and Cation Structure and Dynamics in Expandable Clays: 2H and 39K NMR Investigations of Hectorite
Geoffrey M. Bowers,* David L. Bish, and R. James Kirkpatrick
Abstract: Variable temperature 39K and 2H nuclear magnetic resonance (VT NMR) spectroscopy of K+-saturated hectorite, a prototypical smectite clay, provides new insight into the relationships between the structural and dynamical behavior of K+ and H2O in confinement and at surfaces. In d = 10 Å K-exchanged hectorite, interlayer K+ is rigidly held by the silicate rings, probably in 12-coordinate inner-sphere sites as in muscovite mica. In a 1/1.5 by weight hectorite/water paste, K+ occurs on interlayer and external surface sites that are indistinguishable by 39K NMR. The K+ environments experience changes in dynamical behavior over the temperature range from -50 to 60 C that are directly related to H2O dynamics. 39K NMR of the paste sample shows dynamic line narrowing at low temperatures due to modulation of the electric field gradient (EFG) at frequencies of the order of the static line width ( 20 kHz) and two "melting"-type dynamic transitions near -10 C, one for surface and one for confined K+. At and above 0 C, K+ remains closely associated with the clay surfaces and experiences motion at frequencies greater than 200 kHz and less than 10 MHz, as revealed by 39K T1 relaxation behavior, nutation behavior, and the 39K quadrupolar product. These data are consistent with rapid exchange between inner- and outer-sphere K+ sites reported previously for K-montmorillonite based on molecular dynamics simulations. Deuterium NMR shows the presence of two unique H2O environments in the system: one structurally and dynamically consistent with bulk water between particles and one attributable to H2O confined in the interlayer. Confined H2O experiences anisotropic motion between -50 and 0 C via fast rotation (>2 MHz) about a single axis oriented 127.5 ± 0.5 from the principal axis of the 2H EFG, potentially due to C2 rotation. This motion does not affect the 39K EFG significantly. Melting of free and confined H2O occurs between -10 and 0 C and near 0 C, respectively, similar to the melting behavior of K+ and likely reflecting the onset of molecular diffusion. At and above 10 C, all H2O environments experience motion near or in excess of 300 kHz through at least three NMR-indistinguishable mechanisms, including Brownian motion of free water, exchange of free and confined H2O near particle edges, and diffusive motion of H2O that remains confined on the experimental time scale. The correlation between the rates of 2H and 39K motion and the observed melting transitions for both spin populations strongly suggest that 39K melting and dynamics above the melting transition are linked to an increase in the motional freedom of H2O.

J. Phys. Chem. C, 112 (19), 7503 -7508, 2008. 10.1021/jp711762n S1932-7447(71)01762-X
Temperature Dependence of Spontaneous Polarization in Order-Disorder Pyridinium Periodate Extracted from 2H NMR Data
J. W sicki, A. Pajzderska,* and Z. Fojud
Abstract: On the basis of occupancy factor data for nitrogen atoms in the pyridinium cation obtained from the structural study of PyHIO4 by neutron diffraction,4 the shape of the potential for cation reorientation about the axis perpendicular to its plane was found. The 2H NMR spectra of d5PyHIO4 taken over a wide range of temperatures have allowed the determination of potential minima populations. These data were used to obtain the temperature dependence of spontaneous polarization. The agreement between the spontaneous polarization calculated in this way and the measured values is very good and indicates that the ferroelectric crystal is of the order-disorder type.

J. Phys. Chem. B, 112 (13), 3927 -3930, 2008. 10.1021/jp711433d S1520-6106(71)01433-0
Water Diffusion in Nanoporous Glass: An NMR Study at Different Hydration Levels
Domenico Majolino,* Carmelo Corsaro, Vincenza Crupi, Valentina Venuti, and Ulderico Wanderlingh
Abstract: By pulsed field gradient nuclear magnetic resonance measurements, we investigated the translational diffusion of water confined in the 200 Å diameter pores of a sol-gel silica glass. The experiments, performed as a function of the hydration level, showed an enhancement of the self-diffusion coefficient when the water content corresponds to one or fewer monolayers. An explanation for this occurrence has been given in terms of a two-phase process involving a fast molecular exchange between the liquid and the vapor phase. Moreover, in partially filled pores, the surface water diffusion coefficient was measured, and was 4 times lower than the diffusion of liquid confined water in saturated spaces.

J. Phys. Chem. B, 112 (15), 4496 -4505, 2008. 10.1021/jp709739v S1520-6106(70)09739-0
Theoretical Predictions of 31P NMR Chemical Shift Threshold of Trimethylphosphine Oxide Absorbed on Solid Acid Catalysts
Anmin Zheng, Hailu Zhang, Xin Lu, Shang-Bin Liu,* and Feng Deng*
Abstract: The 31P NMR chemical shifts of adsorbed trimethylphosphine oxide (TMPO) and the configurations of the corresponding TMPOH+ complexes on Br nsted acid sites with varying acid strengths in modeled zeolites have been predicted theoretically by means of density functional theory (DFT) quantum chemical calculations. The configuration of each TMPOH+ complex was optimized at the PW91/DNP level based on an 8T cluster model, whereas the 31P chemical shifts were calculated with the gauge including atomic orbital (GIAO) approach at both the HF/TZVP and MP2/TZVP levels. A linear correlation between the 31P chemical shift of adsorbed TMPO and the proton affinity of the solid acids was observed, and a threshold for superacidity (86 ppm) was determined. This threshold for superacidity was also confirmed by comparative investigations on other superacid systems, such as carborane acid and heteropolyoxometalate H3PW12O40. In conjunction with the strong correlation between the MP2 and the HF 31P isotropic shifts, the 8T cluster model was extended to more sophisticated models (up to 72T) that are not readily tractable at the GIAO-MP2 level, and a 31P chemical shift of 86 ppm was determined for TMPO adsorbed on zeolite H-ZSM-5, which is in good agreement with the NMR experimental data.

J. Phys. Chem. B, 112 (16), 4943 -4947, 2008. 10.1021/jp7110082 S1520-6106(71)01008-3
Preparation and Characterization of New Glassy System As2P2S8-Ga2S3
Silvia H. Santagneli, José Schneider, Igor Skripachev, Sidney J. L. Ribeiro, and Younès Messaddeq*
Abstract: Glasses having the composition (100 - x)As2P2S8-xGa2S3 with x ranging from 0 to 50% were investigated to determine the compositional effect on properties and local structure. The glass transition temperature (Tg) and the stability parameter against crystallization (Tx - Tg) increased with the addition of Ga2S3. The structure of these glasses was probed by Raman scattering, Fourier transform infrared (FT-IR) and 31P nuclear magnetic resonance. On the basis of the observed vibrations and the strength of the 31P-31P homonuclear magnetic dipolar coupling, two scenarios can be proposed for the structural evolution induced by the addition of Ga2S3. For x 20% we may have the formation of GaS4E- groups (E = nonbonding electron), and for x 30% we have depolymerization of the As2P2S8 units and the formation of a network of GaPS4 units with each PS4/2 unit (Q4) species carrying a single positive formal charge

J. Phys. Chem. B, 112 (18), 5813 -5823, 2008. 10.1021/jp800580n S1520-6106(80)00580-9
51V NMR Chemical Shifts from Quantum-Mechanical/Molecular-Mechanical Models of Vanadium Bromoperoxidase
Mark P. Waller, K. R. Geethalakshmi, and Michael Bühl*
Abstract: According to quantum-mechanical/molecular-mechanical (QM/MM) optimizations, the active-site geometries of vanadium-dependent bromoperoxidase (VBPO) and vanadium-dependent chloroperoxidase (VCPO) are very similar. 51V NMR chemical shifts calculated from QM/MM-optimized models of VBPO are critically compared to VCPO and are found to be very similar for the two related proteins. The primary difference between these related structures, the presence of a His411 in VBPO whereas Phe397 is located at that position in VCPO, is studied via analysis of the respective theoretical 51V NMR spectra. The long-range electrostatic effects from more distal residues are also studied to establish their effect. Similar results are obtained for the two active sites of the VBPO homodimer. The experimentally observed shielding of the isotropic 51V NMR chemical shift on going from VCPO to VBPO is somewhat underestimated in the QM/MM models studied. NMR and NQC tensors of both enzymes are predicted to show noticeable differences, suggesting that precise solid-state 51V NMR data, when they become available, can be a sensitive probe for subtle differences in structural details between these enzymes.

J. Phys. Chem. B, 112 (19), 5881 -5882, 2008. 10.1021/jp801687s S1520-6106(80)01687-2
A Tribute to Attila Szabo
There are many biological NMR papers to be found in this special issue (Issue 19) for those who are interested.

J. Phys. Chem. B, 112 (19), 6114 -6121, 2008. 10.1021/jp076808o S1520-6106(07)06808-3
Theory of Stochastic Dipolar Recoupling in Solid-State Nuclear Magnetic Resonance
Robert Tycko*
Abstract: Dipolar recoupling techniques in solid-state nuclear magnetic resonance (NMR) consist of radio frequency (rf) pulse sequences applied in synchrony with magic-angle spinning (MAS) that create nonzero average magnetic dipole-dipole couplings under MAS. Stochastic dipolar recoupling (SDR) is a variant in which randomly chosen rf carrier frequency offsets are introduced to cause random phase modulations of individual pairwise couplings in the dipolar spin Hamiltonian. Several aspects of SDR are investigated through analytical theory and numerical simulations: (1) An analytical expression for the evolution of nuclear spin polarization under SDR in a two-spin system is derived and verified through simulations, which show a continuous evolution from coherent, oscillatory polarization exchange to incoherent, exponential approach to equilibrium as the range of random carrier offsets (controlled by a parameter fmax) increases; (2) in a many-spin system, polarization transfers under SDR are shown to be described accurately by a rate matrix in the limit of large fmax, with pairwise transfer rates that are proportional to the inverse sixth power of pairwise internuclear distances; (3) quantum mechanical interferences among noncommuting pairwise dipole-dipole couplings, which are a complicating factor in solid-state NMR studies of molecular structures by traditional dipolar recoupling methods, are shown to be absent from SDR data in the limit of large fmax, provided that coupled nuclei have distinct NMR chemical shifts.

J. Phys. Chem. B, 112 (20), 6285–6287, 2008. 10.1021/jp800646k
Fluorine-19 NMR Chemical Shift Probes Molecular Binding to Lipid Membranes
Eduard Y. Chekmenev,*†‡ Siu-Kei Chow,† Daniel Tofan,‡ Daniel P. Weitekamp,‡ Brian D. Ross,† and Pratip Bhattacharya†
Abstract: The binding of amphiphilic molecules to lipid bilayers is followed by 19F NMR using chemical shift and line shape differences between the solution and membrane-tethered states of −CF3 and −CHF2 groups. A chemical shift separation of 1.6 ppm combined with a high natural abundance and high sensitivity of 19F nuclei offers an advantage of using 19F NMR spectroscopy as an efficient tool for rapid time-resolved screening of pharmaceuticals for membrane binding. We illustrate the approach with molecules containing both fluorinated tails and an acrylate moiety, resolving the signals of molecules in solution from those bound to synthetic dimyristoylphosphatidylcholine bilayers both with and without magic angle sample spinning. The potential in vitro and in vivo biomedical applications are outlined. The presented method is applicable with the conventional NMR equipment, magnetic fields of several Tesla, stationary samples, and natural abundance isotopes.

Macromolecules, 41 (7), 2514–2519, 2008. 10.1021/ma702815k
Restricted Segmental Mobility Can Facilitate Medium-Range Chain Diffusion: A NMR Study of Morphological Influence on Chain Dynamics of Polyethylene
Y.-F. Yao,† R. Graf,† H. W. Spiess,*† and S. Rastogi†‡
Abstract: The influence of the morphology on the local motion in the noncrystalline regions and chain diffusion between crystalline and noncrystalline regions is studied in ultrahigh molecular weight linear polyethylene: The behaviors of samples of the same material crystallized from the melt and from solution are compared. The geometrical restrictions of the conformational transitions are probed via anisotropic NMR interactions, i.e., 13C−1H dipole–dipole couplings and 13C chemical shift anisotropy. As they are averaged out under MAS, recoupling techniques are applied to yield sideband patterns or quasi-static NMR spectra, from which residual anisotropies are determined. Chain diffusion is probed by 13C exchange NMR. As expected, the local conformational transitions are more restricted in the solution crystallized sample compared with the melt crystallized one. Moreover, the motional narrowing observed by both techniques indicates that the local mobility in the noncrystalline regions of the solution crystallized sample consists mainly of effectively axial motion of extended trans-conformers around their local chain axes. This facilitates the chain diffusion between the crystalline and the noncrystalline regions, being significantly faster in solution crystallized compared to melt crystallized samples, where the local mobility is much more isotropic. The implications of these results for the understanding of crystal thickening and cold drawing are discussed.

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