Wednesday, November 21, 2007

Hiyam's Journal Update

J. Am. Chem. Soc., 129 (47), 14717 -14723, 2007. 10.1021/ja0749690 S0002-7863(07)04969-4 Web Release Date: November 7, 2007 Copyright © 2007 American Chemical Society
Membrane-Bound Conformation of Peptaibols with Methyl-Deuterated -Amino Isobutyric Acids by 2H Magic Angle Spinning Solid-State NMR Spectroscopy
Kresten Bertelsen, Jan M. Pedersen, Brian S. Rasmussen, Troels Skrydstrup, Niels Chr. Nielsen, and Thomas Vosegaard*
Contribution from the Center for Insoluble Protein Structures (inSPIN), Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark
Received July 5, 2007
We present the use of 2H magic-angle spinning (MAS) NMR on methyl-deuterated -amino isobutyric acid (Aib) as a new method to obtain fast and accurate structural constraints on peptaibols in membrane-bound environments. Using nonoriented vesicle-reconstituted samples we avoid the delicate preparation of oriented samples, and the use of MAS ensures high sensitivity and thereby very fast acquisition of experimental spectra. Furthermore, given the high content (~40%) of Aib in peptaibols and the fact that the amino acid Aib may be synthesized from cheap starting materials, even in the case of 2H, 13C, or 15N labeling, this method is ideally suited for studies of the membrane-bound conformation of peptaibols.

Thursday, November 08, 2007

Aaron's Journal Update

-Yining Huang, 23Na and 7Li NMR
J. Phys. Chem. C, 111 (36), 13427 -13436, 2007. 10.1021/jp0706275 S1932-7447(07)00627-9

Study of the Adsorption of Toluene in Zeolite LiNa-Y by Solid-State NMR Spectroscopy
Abstract: The adsorption of toluene inside partially Li+-exchanged zeolite Y (LiNa-Y) was examined by solid-state NMR spectroscopy. The environments of Li+ and Na+ ions at different sites in the framework before and after adsorption were characterized by 7Li and 23Na magic-angle-spinning (MAS) NMR. The information on the dynamic behavior of guest molecules inside the supercage of the zeolite was obtained from wide-line 2H NMR spectra. The cation-sorbate interactions were directly probed by heteronuclear dipolar coupling based double-resonance experiments such as 7Li{1H} and 23Na{1H} rotational-echo double-resonance (REDOR) experiments. Molecular modeling was also performed to assist in interpreting NMR results. The 7Li and 23Na MAS results indicate that during ion exchange the incoming Li+ ions fill the SI' sites first and then the SII sites, and the remaining Na+ ions stay mostly at SII sites. The MAS and REDOR results show that the toluene molecules are facially coordinated to the cations at SII sites in the supercage to form a -complex, resulting in a significant change in chemical shift of the Li+ ions and a dramatic reduction in the quadrupolar coupling constant (QCC) of the Na+ ions. The 2H NMR spectra indicate that at room temperature some toluene molecules reorient around the molecular long axis (two-site flip) and others undergo multiple-site jump motion, but at low temperature only two-site flip, at a much slower rate, remains. Both 2H and REDOR results show that the molecular dynamics of the guest species is also affected by the cation composition.

-Joel/Rob - M.E. Smith, 27Al, 29Si and 17O NMR

J. Phys. Chem. C, 111 (37), 13740 -13746, 2007. 10.1021/jp0739871 S1932-7447(07)03987-8

Structural Characterization of SiO2 and Al2O3 Zener-Pinned Nanocrystalline TiO2 by NMR, XRD and Electron Microscopy
Luke A. O'Dell, Shelley L.P. Savin, Alan V. Chadwick, and Mark E. Smith*
Abstract: Nanocrystalline TiO2 samples were prepared using sol-gel techniques in a pure form and also Zener pinned with either silica or alumina to reduce the growth of the crystallites during the annealing process and to stabilize the anatase phase at high temperatures. These samples were studied using 17O, 27Al, and 29Si nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and electron microscopy. The silica pinning phase was found to successfully restrict nanocrystal growth as well as stabilize the anatase phase at temperatures up to 800 C. The alumina phase had less of a pinning effect, and it reacted with the TiO2 to form tialite. 17O NMR relaxation time measurements on enriched samples showed that the presence of the pinning phases also reduced the activation energy for the oxygen ion diffusion mechanism.

Everyone - FAST MAS experiments to measure metal-carbon bond distances.

J. Phys. Chem. B, 111 (33), 9693 -9696, 2007. 10.1021/jp0727289 S1520-6106(07)02728-9

Elucidating Connectivity and Metal-Binding Structures of Unlabeled Paramagnetic Complexes by 13C and 1H Solid-State NMR under Fast Magic Angle Spinning
Nalinda P. Wickramasinghe, Medhat A. Shaibat, and Yoshitaka Ishii*
Abstract: Characterizing paramagnetic complexes in solids is an essential step toward understanding their molecular functions. However, methodologies to characterize chemical and electronic structures of paramagnetic systems at the molecular level have been notably limited, particularly for noncrystalline solids. We present an approach to obtain connectivities of chemical groups and metal-binding structures for unlabeled paramagnetic complexes by 13C and 1H high-resolution solid-state NMR (SSNMR) using very fast magic angle spinning (VFMAS, spinning speed 20 kHz). It is experimentally shown for unlabeled Cu(II)(Ala-Thr) that 2D 13C/1H correlation SSNMR under VFMAS provides the connectivity of chemical groups and assignments for the characterization of unlabeled paramagnetic systems in solids. We demonstrate that on the basis of the assignments provided by the VFMAS approach multiple 13C-metal distances can be simultaneously elucidated by a combination of measurements of 13C anisotropic hyperfine shifts and 13C T1 relaxation due to hyperfine interactions for this peptide-Cu(II) complex. It is also shown that an analysis of 1H anisotropic hyperfine shifts allows for the determination of electron-spin states in Fe(III)-chloroprotoporphyin-IX in solid states.


J. Phys. Chem. B, 111 (33), 9711 -9721, 2007. 10.1021/jp072112j S1520-6106(07)02112-8
High-Resolution Solid-State NMR Studies of Poly(vinyl phosphonic acid) Proton-Conducting Polymer: Molecular Structure and Proton Dynamics
Young Joo Lee,
* Bahar Bingöl, Tatiana Murakhtina, Daniel Sebastiani, Wolfgang H. Meyer, Gerhard Wegner, and Hans Wolfgang Spiess*
Abstract:The structure and the local proton mobility of poly(vinyl phosphonic acid) were studied by solid-state NMR under fast magic-angle spinning. At elevated temperatures, the signature of the hydrogen-bonded P-OH protons is observed in 1H magic-angle spinning (MAS) NMR as a single resonance at 10.5 ppm. Both 1H double-quantum NMR and variable-temperature experiments demonstrate that P-OH protons are mobile and thus able to contribute to proton conductivity. Below room temperature, two different types of hydrogen-bonded P-OH resonances are observed at 10.5 and 15 ppm, and 1H double-quantum NMR demonstrates that these protons are immobile on the NMR time scale. By means of first-principles calculations of a model polymer, we have assigned the additional hydrogen-bonded species at lower temperatures to phosphonic acid anhydride and charged anhydride. Also, in the 31P MAS NMR spectrum, two distinct resonances appear, arising from "normal" phosphonic acid and phosphonic acid anhydride. 31P double-quantum NMR experiments reveal that there is no phase segregation between normal and phosphonic acid anhydride and the condensation reaction occurs randomly throughout the system. The formation of acid anhydride leads to a decrease in proton conductivity through two mechanisms, (1) decrease in the number of charge carriers and (2) blockage of charge transport pathways through immobilization of charge carriers together with a hindered reorientation of the anhydride group. Our results provide strong evidence for these mechanisms by demonstrating that the conductivity is greatly influenced by the presence of phosphonic acid anhydride.


J. Phys. Chem. B, 111 (33), 9747 -9757, 2007. 10.1021/jp072061q S1520-6106(07)02061-5
27Al NMR Study of the Structure of Lanthanum- and Yttrium-Based Aluminosilicate Glasses and Melts
P. Florian,* N. Sadiki, D. Massiot, and J. P. Coutures
Abstract: We have investigated by 27Al nuclear magnetic resonance spectroscopy some compositions in the Ln2O3-Al2O3-SiO2 (Ln = Y or La) ternary phase diagram containing more than 60 mol % of SiO2. One- and two-dimensional high-field (17.6 T) high-speed (30 kHz) magic angle spinning experiments have been performed along with simulations of the spectra to quantify the amount of penta-coordinated aluminum present in those glasses as a function of composition. Very high-temperature experiments have allowed to follow selected samples from 2200 C down to 1700 C and hence to characterize the aluminum coordination state and dynamics in those liquids. The present study re-enforces the current view that "minor" species such as penta-coordinated aluminum are actually present in a considerable amount in aluminosilicate glasses, and high-temperature liquids at and above the charge compensation join. The high-field strength of Y3+ and La3+ reveal, for the first time in glasses, a different mean electric field gradient perceived by the tetra- and penta-coordinated aluminum environments. The movements responsible for the NMR relaxation of aluminum in the high-temperature liquid are shown to be uncorrelated with the movements responsible for the macroscopic shear viscosity. Results obtained both on glasses and in situ at high-temperature suggest a preferential localization of Ln3+ nearby tetra-coordinated aluminum species, with possible formation of tricluster and/or Ln3+ coordination changes.

-11B NMR

J. Phys. Chem. B, 111 (33), 9758 -9761, 2007. 10.1021/jp072223i S1520-6106(07)02223-7

11B MAS NMR Spectroscopic Study of Structural Relaxation, Aging, and Memory Effect at the Atomic S
cale in a Borosilicate Glass
Sezen Soyer Uzun and Sabyasachi Sen*
Abstract: Relaxation kinetics of boron coordination environments in a borosilicate glass in response to temperature jumps has been monitored using 11B magic-angle spinning (MAS) NMR spectroscopy. The relaxation time scale of the BO4/BO3 ratio is found to be closely comparable with those of the bulk properties, namely, refractive index and viscosity, showing a close connection with one another. Samples partially equilibrated at a low temperature, when subjected to a positive temperature jump, display an initial rapid decrease followed by an increase in the BO4/BO3 ratio. This reversal in the trend of the variation of the BO4/BO3 ratio has been interpreted to be the signature of a memory effect, and the implications are discussed within the framework of the potential energy landscape model of glassy dynamics.

- Calculation of C13 tensors in order to measure ordering in a liquid crystal

J. Phys. Chem. B, 111 (33), 9787 -9794, 2007. 10.1021/jp0730693 S1520-6106(07)03069-6

Orientational Order of a Liquid Crystal with Three Chiral Centers by a Combined 13C NMR and DFT Approach
R. Y. Dong,* M. Geppi, A. Marini, V. Hamplova, M. Kaspar, C. A. Veracini, and J. Zhang
Abstract: In this work, the liquid crystal (S)-2-methylbutyl-[4'-(4' '-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2')-benzoyl)-propionyl)]-propionate (ZLL 7/*) was investigated by means of 13C NMR spectroscopy. This compound has a very peculiar mesomorphic behavior, showing the following phases: paraelectric SmA, ferroelectric SmC*, antiferroelectric SmC*A, re-entrant ferroelectric SmC*re, and ferroelectric hexatic Sm*HEX. The structural and orientational ordering properties of ZLL 7/* have been determined by exploiting the nuclear chemical shielding properties of 13C. To this aim, solid-state NMR techniques such as CP, SPINAL-64, and SUPER have been used in combination with DFT calculations. The agreement between experimental and in vacuo DFT shielding parameters appears to be satisfactory. The orientational order parameters obtained from the 13C shielding analysis have been discussed, taking into account different data analysis approaches and comparing them to those previously obtained from an independent 2H NMR study.

-23Na and 95 Mo NMR

J. Phys. Chem. B, 111 (34), 10109 -10117, 2007. 10.1021/jp072883n S1520-6106(07)02883-0

Structural Studies of NaPO3-MoO3 Glasses by Solid-State Nuclear Magnetic Resonance and Raman Spectroscopy
Silvia H. Santagneli, Carla C. de Araujo, Wenzel Strojek, and Hellmut Eckert*
Gaël Poirier, Sidney J. L. Ribeiro, and Younes Messaddeq
Abstract: Vitreous samples were prepared in the (100 - x)% NaPO3-x% MoO3 (0 x 70) glass-forming system by a modified melt method that allowed good optical quality samples to be obtained. The structural evolution of the vitreous network was monitored as a function of composition by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), Raman scattering, and solid-state nuclear magnetic resonance (NMR) for 31P, 23Na, and 95Mo nuclei. Addition of MoO3 to the NaPO3 glass melt leads to a pronounced increase in the glass transition temperatures up to x = 45, suggesting a significant increase in network connectivity. For this same composition range, vibrational spectra suggest that the Mo6+ ions are bonded to some nonbridging oxygen atoms (Mo-O- or Mo=O bonded species). Mo-O-Mo bond formation occurs only at MoO3 contents exceeding x = 45. 31P magic-angle spinning (MAS) NMR spectra, supported by two-dimensional J-resolved spectroscopy, allow a clear distinction between species having two, one, and zero P-O-P linkages. These sites are denoted as Q(2)2Mo, Q(2)1Mo, and Q(2)0Mo, respectively. For x < n =" 0," x =" 45">

- Learn all about membrane proteins

J. Phys. Chem. B, 111 (35), 10340 -10351, 2007. 10.1021/jp073652j S1520-6106(07)03652-8

Structure, Topology, and Dynamics of Membrane Peptides and Proteins from Solid-State NMR Spectroscopy
Mei Hong
Abstract:The high-resolution structure of membrane proteins is notoriously difficult to determine due to the hydrophobic nature of the protein-membrane complexes. Solid-state NMR spectroscopy is a unique and powerful atomic-resolution probe of the structure and dynamics of these important biological molecules. A number of new solid-state NMR methods for determining the depth of insertion, orientation, oligomeric structure, and long-range (10-15 Å) distances of membrane proteins are summarized. Membrane protein depths can now be determined using several complementary techniques with varying site-specificity, distance precision, and mobility requirement on the protein. Membrane protein orientation can now be determined with or without macroscopic alignment, the latter providing a novel alternative for orientation determination of intrinsically curvature-inducing proteins. The novel analyses of -sheet membrane protein orientation are described. The quaternary structure of membrane peptide assemblies can now be elucidated using a 19F spin diffusion technique that simultaneously yields the oligomeric number and intermolecular distances up to 15 Å. Finally, long-range distances up to ~10 Å can now be measured using 1H spins with an accuracy of better than 1 Å. These methods are demonstrated on several -sheet membrane peptides with antimicrobial activities and on two -helical ion-channel proteins. Finally, we show that the nearly ubiquitous dynamics of membrane proteins can be readily examined using 2D correlation experiments. An intimate appreciation of molecular motion in these systems not only leads to important insights into the specific function of these membrane proteins but also may be exploited for other purposes such as orientation determination.

-27Al{19F} and 23Na{19F} REDOR experiments

J. Phys. Chem. B, 111 (35), 10402 -10412, 2007. 10.1021/jp072725w S1520-6106(07)02725-3

Structural Role of Fluoride in Aluminophosphate Sol-Gel Glasses: High-Resolution Double-Resonance NMR Studies
Long Zhang, Carla C. de Araujo, and Hellmut Eckert*
Abstract: The local structure of Na-Al-P-O-F glasses, prepared by a novel sol-gel route, was extensively investigated by advanced solid-state NMR techniques. 27Al{19F} rotational echo double resonance (REDOR) results indicate that the F incorporated into aluminophosphate glass is preferentially bonded to octahedral Al units and results in a significant increase in the concentration of six-coordinated aluminum. The extent of Al-F and Al-O-P connectivities are quantified consistently by analyzing 27Al{31P} and 27Al{19F} REDOR NMR data. Two distinct types of fluorine species were identified and characterized by various 19F{27Al}, 19F{23Na}, and 19F{31P} double resonance experiments, which were able to support peak assignments to bridging (Al-F-Al, -140 ppm) and terminal (Al-F, -170 ppm) units. On the basis of the detailed quantitative dipole-dipole coupling information obtained, a comprehensive structural model for these glasses is presented, detailing the structural speciation as a function of composition.

-27Al, 23Na, 19F, 29Si Solids NMR

J. Phys. Chem. B, 111 (35), 10413 -10420, 2007. 10.1021/jp073687j S1520-6106(07)03687-5

Preferential Binding of Fluorine to Aluminum in High Peralkaline Aluminosilicate Glasses
N. G. Karpukhina, U. Werner-Zwanziger, J. W. Zwanziger,* and A. A. Kiprianov
Abstract: For two series of fluoride-containing aluminosilicate glasses of high peralkaline type, we apply 27Al, 19F, 29Si, and 23Na NMR spectroscopy to understand the structural changes introduced by the addition of alkali fluorides. Adding fluoride in concentrations above the solubility limit causes crystallization of different phases in sodium and potassium glasses despite identical composition. However, the NMR spectra reveal that the structural evolution of the precrystallized states is similar in both series. In particular, fluorine coordinates exclusively to alkaline cations and aluminum. No indication of direct bonding with silicon was found from 19F 29Si cross-polarization experiments. In contrast to other glass systems, double resonance experiments in these peralkaline systems show that halide addition produces at most a minor fraction of tetrahedral aluminum containing fluorine in its coordination sphere. Instead, the fluorine addition prior to crystallization converts up to about 20% of the initial tetrahedral aluminum (1 mol % in absolute units) to 5- and 6-fold coordinated aluminum. A minor portion of five-coordinated aluminum groups is considered as the intermediate to the growing fraction of octahedral aluminum in the silicate matrix. The initialization of the crystallization process is correlated with the saturation of the silicate matrix by octahedral aluminum clusters segregating out under further doping by fluoride. It is suggested that the formation of the nonframework Al-F bonds is responsible for structural relaxation, reflected by the reduction of the glass transition temperature.

J. Phys. Chem. B, 111 (36), 10671 -10680, 2007. 10.1021/jp072001k S1520-6106(07)02001-9

Initio Calculation of Solid-State NMR Spectra for Different Triazine and Heptazine Based Structure Proposals of g-C3N4
Jan Sehnert, Kilian Baerwinkel, and Juergen Senker*
Abstract: We present a comprehensive theoretical study of the structure and NMR parameters of a large number of triazine and heptazine based structure proposals for g-C3N4 in different condensation states. This approach includes a detailed investigation of cyclic melon which tends toward the formation of densely packed hydrogen bonded meshes. In all of the investigated systems, we found planar layers to represent saddlepoints on the energy surface, whereas corrugated species were identified as minima. The corrugation source was linked to the repulsion of nitrogen lone pairs in close NN contacts. A linear dependency of the corrugation energy from the number of NN interactions in the investigated clusters was found. Heptazine based systems gain about twice as much energy per NN close contact in comparison to triazine structures which could be understood in terms of the distortion mechanism in the investigated structures. Furthermore, a full study of the 15N and 13C chemical shift tensors was performed for the different C/N layers. The description of the NMR parameters required dividing the investigated systems into subclusters for which the NMR tensors were calculated with density functional theory (DFT) methods. A statistical analysis of these entities allowed for the investigation of the change in the chemical shift upon corrugation and, in the case of the cyclic melon system, hydrogen bonding. With the here presented study, the most prominent structure models for g-C3N4 are characterized in terms of the 15N and 13C NMR parameters which now can directly be compared to experimental spectra.

-Calculation of J-couplings in DNA

J. Phys. Chem. B, 111 (36), 10813 -10824, 2007. 10.1021/jp072822p S1520-6106(07)02822-2

Theoretical Study of the Scalar Coupling Constants across the Noncovalent Contacts in RNA Base Pairs: The cis- and trans-Watson-Crick/Sugar Edge Base Pair Family
Zuzana Vok ov , Ji í poner, Judit E. poner, and Vladimír Sychrovsk *
Abstract:The structure and function of RNA molecules are substantially affected by non-Watson-Crick base pairs actively utilizing the 2'-hydroxyl group of ribose. Here we correlate scalar coupling constants across the noncovalent contacts calculated for the cis- and trans-WC/SE (Watson-Crick/sugar edge) RNA base pairs with the geometry of base to base and sugar to base hydrogen bond(s). 23 RNA base pairs from the 32 investigated were found in RNA crystal structures, and the calculated scalar couplings are therefore experimentally relevant with regard to the binding patterns occurring in this class of RNA base pairs. The intermolecular scalar couplings 1hJ(N,H), 2hJ(N,N), 2hJ(C,H), and 3hJ(C,N) were calculated for the N-H···N and N-H···O=C base to base contacts and various noncovalent links between the sugar hydroxyl and RNA base. Also, the intramolecular 1J(N,H) and 2J(C,H) couplings were calculated for the amino or imino group of RNA base and the ribose 2'-hydroxyl group involved in the noncovalent interactions. The calculated scalar couplings have implications for validation of local geometry, show specificity for the amino and imino groups of RNA base involved in the linkage, and can be used for discrimination between the cis- and trans-WC/SE base pairs. The RNA base pairs within an isosteric subclass of the WC/SE binding patterns can be further sorted according to the scalar couplings calculated across different local noncovalent contacts. The effect of explicit water inserted in the RNA base pairs on the magnitude of the scalar couplings was calculated, and the data for discrimination between the water-inserted and direct RNA base pairs are presented. The calculated NMR data are significant for structural interpretation of the scalar couplings in the noncanonical RNA base pairs.

-35/37Cl NMR of Ionic Liquids

J. Phys. Chem. B, 111 (40), 11619 -11621, 2007. 10.1021/jp0756449 S1520-6106(07)05644-1

Hydrogen Bonds in Ionic Liquids Revisited: 35/37Cl NMR Studies of Deuterium Isotope Effects in 1-n-Butyl-3-Methylimidazolium Chloride
Richard C. Remsing, Jayme L. Wildin, Ashleigh L. Rapp, and Guillermo Moyna*
Abstract: Detailed knowledge of the structure, dynamics, and interionic interactions of ionic liquids (ILs) is critical to understand their physicochemical properties. In this letter, we show that deuterium isotope effects on the chloride ion 35/37Cl NMR signal represent a useful tool in the study of interionic hydrogen bonds in imidazolium chloride ILs. Sizable 35/37Cl(H,D) values obtained for the model system 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl) upon deuteriation of the imidazolium C-2 and C-2,4,5 positions, of nearly 1 and 2 ppm, respectively, show that the approach can readily identify and differentiate Cl···H hydrogen bonds between the anion and cation. Our study is one of a few examples in which hydrogen-bonding in ILs has been investigated using deuterium isotope effects and, to our knowledge, the only one employing 35/37Cl NMR to detect these interactions. The methodology described could be easily extended to the study of other systems bearing NMR-active nuclei.

-The title says it all

J. Phys. Chem. B, 111 (42), 12088 -12096, 2007. 10.1021/jp073682m S1520-6106(07)03682-6

Provides Checklist of Generic Properties for Atomic-Scale Models of Periodic Mesoporous Silicas
Ilja G. Shenderovich,* Daniel Mauder, Dilek Akcakayiran, Gerd Buntkowsky, Hans-Heinrich Limbach,* and Gerhard H. Findenegg
Abstract:MCM-41 and SBA-15 silicas were studied by 29Si solid-state NMR and 15N NMR in the presence of 15N-pyridine with the aim to formulate generic structural parameters that may be used as a checklist for atomic-scale structural models of this class of ordered mesoporous materials. High-quality MCM-41 silica constitutes quasi-ideal arrays of uniform-size pores with thin pore walls, while SBA-15 silica has thicker pore walls with framework and surface defects. The numbers of silanol (Q3) and silicate (Q4) groups were found to be in the ratio of about 1:3 for MCM-41 and about 1:4 for our SBA-15 materials. Combined with the earlier finding that the density of surface silanol groups is about three per nm2 in MCM-41 (Shenderovich, et al. J. Phys. Chem. B 2003, 107, 11924) this allows us to discriminate between different atomic-scale models of these materials. Neither tridymite nor edingtonite meet both of these requirements. On the basis of the hexagonal pore shape model, the experimental Q3:Q4 ratio yields a wall thickness of about 0.95 nm for MCM-41 silica, corresponding to the width of ca. four silica tetrahedra. The arrangement of Q3 groups at the silica surfaces was analyzed using postsynthesis surface functionalization. It was found that the number of covalent bonds to the surface formed by the functional reagents is affected by the surface morphology. It is concluded that for high-quality MCM-41 silicas the distance between neighboring surface silanol groups is greater than 0.5 nm. As a result, di- and tripodical reagents like (CH3)2Si(OH)2 and CH3Si(OH)3 can form only one covalent bond to the surface. The residual hydroxyl groups of surface-bonded functional reagents either remain free or interact with other reagent molecules. Accordingly, the number of surface silanol groups at a given MCM-41 or SBA-15 silica may not decrease but increase after treatment with CH3Si(OH)3 reagent. On the other hand, nearly all surface silanol groups could be functionalized when HN(Si(CH3)3)2 was used.

J. Phys. Chem. B, 111 (42), 12097 -12102, 2007. 10.1021/jp074517+ S1520-6106(07)04517-8

Rotation and Diffusion of H2 in Hydrogen-Ice Clathrate by 1H NMR
Lasitha Senadheera and Mark S. Conradi*
Abstract:A recently reported hydrogen-ice clathrate carries up to four H2 in each large cage and one H2 in each small cage. We report pulsed proton NMR line shape measurements on H2-D2O clathrate formed at 1500 bar and 250 K. The behavior of the two-pulse spin-echo amplitude with respect to the nutation angle of the refocusing pulse shows that intramolecular dipolar broadening, modulated by H2 molecular reorientations, dominates the line width of the ortho-H2. Dipolar interaction between H2 guests and host D atoms explains the echo variation with the relative phases of the pulses. From 12 to 120 K, the line width varies as 1/T, demonstrating that the three sublevels of J = 1 are split by a constant energy, . The splitting arises from distortion in the otherwise high-symmetry cages from frozen-out D2O orientational disorder. Above 120 K, further line-narrowing signals the onset of H2 diffusion from cage to cage. At the lowest temperature, 1.9 K, the spectrum has Pake powder doublet-like features; the doublet is not fully developed, indicating a broad distribution of order parameters and energies .

J. Phys. Chem. B, 111 (42), 12119 -12123, 2007. 10.1021/jp076046x S1520-6106(07)06046-4

Solid-State 13C NMR Studies on Organic-Inorganic Hybrid Zeolites
Katsutoshi Yamamoto, Yasuyuki Sakata, and Takashi Tatsumi*
Abstract:A novel type of organic-inorganic hybrid zeolite with organic lattice (ZOL) is studied in detail by solid-state 13C magic angle spinning nuclear magnetic resonance (MAS NMR). The 13C MAS NMR measurements employing several pulse sequences quantitatively demonstrate that methylene groups are really incorporated in the framework, although they are partially cleaved into methyl groups. The organic species in ZOL materials are open for adsorbates, which is evidenced by the 13C MAS NMR measurements for an n-hexane-adsorbing ZOL material. This finding strongly suggests that organic moieties are incorporated as a zeolite framework, indicating that ZOL is not a physical mixture of a carbon-containing amorphous aggregate and a conventional zeolite but a true organic-inorganic hybrid zeolite.

-Massiot: 27Al NMR
J. Phys. Chem. B, 111 (44), 12707 -12714, 2007. 10.1021/jp072620t S1520-6106(07)02620-X
Cation Sublattice Disorder Induced by Swift Heavy Ions in MgAl2O4 and ZnAl2O4 Spinels: 27Al Solid-State NMR Study
Nadia Pellerin,* Catherine Dodane-Thiriet, Valérie Montouillout, Michel Beauvy, and Dominique Massiot
Abstract: MgAl2O4 and ZnAl2O4 spinels have been irradiated by swift heavy ions (86Kr and 36S) simulating the irradiation by fission products for applications in the transmutation targets. The structures of unirradiated and irradiated spinel samples have been studied by NMR spectroscopy, with 27Al magic angle spinning and multiple-quantum magic angle spinning experiments. The parameters of fluence and electronic stopping power have been compared. For 86Kr ions, the obtained spectra are modified by irradiation: we observe a rise of the amount of aluminum in tetrahedral sites and a widening of the lines associated with the different aluminum environments compared with those of the pristine samples. Site exchange in the cationic sublattice is then observed and can be quantified from NMR spectra, determining the inversion parameter. An inversion parameter of 0.77 is estimated for the MgAl2O4 spinel irradiated with 1013 Kr ions/cm2, for a value of 0.275 in the pristine samples. Moreover, a line attributed to aluminum in 5-fold coordination with oxygen is observed in irradiated spinel samples at the maximum fluence for krypton. These new aluminum environments can characterize a transition layer which could change toward an amorphous layer, increasing the electronic stopping power and/or the fluence.

- Speiss 13C NMR Studies of Polymers
Macromolecules, 40 (17), 6249 -6256, 2007. 10.1021/ma0706531 S0024-9297(07)00653-5 Chain Dynamics in Poly(n-alkyl acrylates) by Solid-State NMR, Dielectric, and Mechanical Spectroscopies
Marianne Gaborieau,* Robert Graf, Stefan Kahle, Tadeusz Pakula, and Hans W. Spiess*
Abstract:A relaxation process was detected and quantified for the first time in various poly(n-alkyl acrylates) in the melt by a solid-state NMR method, NOE with dipolar filter. By comparison with dielectric spectroscopy and dynamic mechanical measurements carried out on the same samples, the relaxation process detected by NMR occurs at a higher temperature than the simple -relaxation and on a longer time scale than the -relaxation (cooperative -relaxation). This relaxation process could be the isotropization of the main chain (by analogy with structurally similar polymethacrylates), but it is more probably a local relaxation within the alkyl side chain that is usually detected only at significantly lower temperatures. The fact that this side-chain motion is slower than the cooperative main-chain -relaxation of restricted amplitude can be rationalized in the context of restricted amplitude main-chain motions in the local nanostructure present in the samples, as NMR and dielectric spectroscopy detect motions in different frames.

Wednesday, November 07, 2007

Hiyam's Journal Update

J. Am. Chem. Soc., 129 (45), 13806 -13807, 2007.
Probing Spatially-Resolved Pt Distribution in PtRu Nanoparticles with 195Pt EC-NMR
Aaron L. Danberry, Bingchen Du, In-Su Park, Yung-Eun Sung, and YuYe Tong*

We report the first quantitative analysis of spatially resolved local Pt distribution in graphite and carbon nanofibers supported PtRu alloy nanoparticles using slow-beat measurements by in situ 195Pt electrochemical NMR. While the two samples had the same nominal 1:1 atomic ratio and similar average particle size, it was found that the former had a rather homogeneous Pt distribution but not the latter, which had Pt segregation at the core and depletion at the surface of the nanoparticles.

Friday, November 02, 2007

Hiyam's Journal Update

Phys. Rev. Lett. 99, 187003 (2007)
NMR Relaxation and Resistivity from Rattling Phonons in Pyrochlore Superconductors
Thomas Dahm1,2 and Kazuo Ueda1
(Received 13 June 2007; published 30 October 2007)

We calculate the temperature dependence of the NMR relaxation rate and electrical resistivity for coupling to a local, strongly anharmonic phonon mode. We argue that the two-phonon Raman process is dominating NMR relaxation. Due to the strong anharmonicity of the phonon an unusual temperature dependence is found having a low temperature peak and becoming constant towards higher temperatures. The electrical resistivity is found to vary like T2 at low temperatures and following a behavior at high temperatures. Both results are in qualitative agreement with recent observations on -pyrochlore oxide superconductors.

Phys. Rev. Lett. 99, 187601 (2007)

Stochastic Dipolar Recoupling in Nuclear Magnetic Resonance of Solids
Robert Tycko
(Received 1 August 2007; published 2 November 2007)

I describe a nuclear magnetic resonance (NMR) technique, called stochastic dipolar recoupling (SDR), that permits continuous experimental control of the character of spin dynamics between coherent and incoherent limits in a system of magnetic dipole-coupled nuclei. In the fully incoherent limit of SDR, spin polarization transfers occur at distance-dependent rates without the quantum mechanical interferences among pairwise dipole-dipole couplings that often limit the feasibility or precision of structural studies of solids by NMR. In addition to facilitating structural studies, SDR represents a possible route to experimental studies of effects of decoherence on the dynamics of quantum many-body systems.