Wednesday, August 08, 2007

J Phys. Chem. B Updates

-ADF calculations of 31P chemical shift tensors

Inorg. Chem., ASAP Article 10.1021/ic700514u S0020-1669(70)00514-1 Web Release Date: August 3, 2007
Ethylenebis(triphenylphosphine)platinum as a Probe for Niobium-Mediated Diphosphorus Chemistry
Nicholas A. Piro and Christopher C. Cummins*

Abstract: Ethylenebis(triphenylphosphine)platinum is used as a trap for the P2-containing molecule W(CO)5(P2), which is eliminated at room temperature from a niobium-complexed diphosphaazide ligand. The rate of W(CO)5(P2) elimination is unaffected by the presence of the platinum species. Attempts to generate and trap free P2 with the platinum ethylene complex were hindered by the direct reaction between the platinum starting material and the P2 generator, (Mes*NPP)Nb(N[Np]Ar)3. In this case, reductive cleavage of the P-P bond in the diphosphaazide ligand is induced by platinum coordination, resulting in the formation of a trimetallic system with two bridging, three-coordinate phosphorus atoms.
J Phys. Chem. B

-23Na NMR

J. Phys. Chem. B, 111 (25), 7092 -7097, 2007. 10.1021/jp070533v S1520-6106(07)00533-0 Nonergodic Arrested State in Diluted Clay Suspensions Monitored by Triple-Quantum 23Na Nuclear Magnetic Resonance

T. Gili,* S. Capuani, and B. Maraviglia

CNR-INFM CRS-SOFT, Universit di Roma "La Sapienza",P.zle A. Moro 5, I-00185 Roma, Italy, Dipartimento di Fisica, Universit di Roma "La Sapienza", P.zle A. Moro 5, I-00185 Roma, Italy, Enrico Fermi Center, Compendio Viminale, I-00184 Roma, Italy, and Fondaziona Santa Lucia IRCCS, I-00179 Roma, Italy

Abstract: The aging of water suspension of the synthetic clay Laponite has been studied by liquid-state triple-quantum filter nuclear magnetic resonance techniques, in a range of clay weight concentration (Cw = 0.012-0.028) known as the isotropic phase. Counterions dynamic parameters (rotational correlation time c and quadrupolar coupling constant e2qQ/h) have been extracted from sodium triple-quantum filtered experimental data within the multi-exponential quadrupolar relaxation theory in the fast exchange approximation. By monitoring quadrupolar sodium ions dynamical ( c and e2qQ/h) and static (counterion concentration pb) properties during the aging, we find two different mechanisms of transition toward an arrested state. Our experimental findings match with the description which states, at low concentration, the formation of clusters of Laponite disks trigger the reaching of the arrested state, while at high concentration, single disks are the basic units of the arrested phase. The procedure proposed in this paper, based on multiple quantum filtered NMR data analysis, results to be a useful means to study the routes to arrested states in aqueous colloidal dispersions.

-31P and 27Al spectroscopy

J. Phys. Chem. B, 111 (25), 7105 -7113, 2007. 10.1021/jp0710133 S1520-6106(07)01013-9

Crystallization of AlPO4-5 Aluminophosphate Molecular Sieve Prepared in Fluoride Medium: A Multinuclear Solid-State NMR Study

Jun Xu, Lei Chen, Danlin Zeng, Jun Yang, Minjin Zhang, Chaohui Ye, and Feng Deng*
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, People's Republic of China

Abstract: In the present work, multinuclear solid-state NMR techniques, combined with powder X-ray diffraction (PXRD) and infrared (IR) spectroscopy, are employed to monitor the crystallization of AlPO4-5 aluminophosphate prepared in the presence of HF under hydrothermal condition. The crystallization process is characterized by the evolution of intermediate gels, in which the long-rang ordering arrangement is probed by PXRD, revealing the threshold of the crystallization around 120 min. The appearance of 31P signals at ca. -22 and -29 ppm due to the structural P-O-Al unit and 19F signal at -120 ppm due to the structural F-Alpen-O-P unit in the NMR spectra of the series gels indicates that the crystalline framework is starting to form. The onset of the crystallization is also evidenced by the presence of the pentacoordinated Al in the structural F-Alpen-O-P unit which is considered to be associated with the ordered framework. More information about the local ordering of the gels is obtained from two-dimensional 27Al 31P heteronuclear correlation (HETCOR) and 31P/27Al double-resonance experiments. In combination with 1H 31P cross-polarization/magic-angle spinning (CP/MAS) experiments, two microdomains can be identified in the 120 min heated gel. A possible evolution mechanism of the gels consisting of three successive stages is proposed for the crystallization process

-J. Phys. Chem. B, 111 (26), 7529 -7534, 2007. 10.1021/jp070692e S1520-6106(07)00692-X

Structural Changes above the Glass Transition and Crystallization in Aluminophosphate Glasses: An in Situ High-Temperature MAS NMR Study

Leo van Wüllen,* Sebastian Wegner, and Gregory Tricot
Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30-36, D-48149 Münster, Germany

Abstract: We present an in situ high-temperature nuclear magnetic resonance study on the structural changes in aluminophosphate glasses occurring in the temperature range between the glass transition temperature Tg and the crystallization temperature Tc, Tg < T < Tc. Decisive changes in the network organization between Tg and Tc in potassium aluminophosphate glasses in the compositional range 50K2O-xAl2O3-(50 - x)P2O5 with 2.5 < x < 20 could be monitored for the first time employing 1D 31P- and 27Al-MAS NMR. Accompanying ex situ NMR experiments (31P-RFDR NMR and 31P-{27Al} CP-HETCOR NMR) on devitrified samples were performed at room temperature to further characterize the phases formed during the crystallization process. The structural role of boron-which is known to inhibit the crystallization process in these aluminophosphate glasses-on short and intermediate length scales was analyzed employing 11B-MQMAS, 11B-{27Al} TRAPDOR and 11B-{31P} REDOR NMR spectroscopy

-J. Phys. Chem. B, 111 (27), 7802 -7811, 2007. 10.1021/jp072504q S1520-6106(07)02504-7

Boltzmann Statistics Rotational-Echo Double-Resonance Analysis

John D. Gehman, Frances Separovic, Kun Lu, and Anil K. Mehta*
School of Chemistry, Bio21 Institute, University of Melbourne, Melbourne, Victoria 3010, Australia, and Department of Chemistry, Emory University, Atlanta, Georgia 30322

Abstract: A new approach to rotational-echo double-resonance (REDOR) data analysis, analogous to Boltzmann maximum entropy statistics, is reported. This Boltzmann statistics REDOR (BS-REDOR) approach is useful for reconstructing an unbiased internuclear distance distribution for multiple internuclear distances from experimentally limited REDOR data sets on isolated spin pairs. The analysis is characterized by exploring reconstructions on model data and applied to both [1-13C,15N]-glycine and a long intramolecular distance in A (16-22) peptide nanotubes. The approach also provides insight into the minimal number of REDOR data points required to allow faithful determination of dipolar couplings in systems with multiple internuclear distances.

-3Q-QCPMG MAS experiments.
J. Phys. Chem. B, 111 (28), 8014 -8019, 2007. 10.1021/jp071539n S1520-6106(07)01539-8

Order and Disorder in Titanosilicate Glass by 17O MAS, off-MAS, and 3Q-QCPMG-MAS Solid-State NMR

Flemming H. Larsen,* Stéphanie Rossano, and Ian Farnan
Department of Food Sciences, University of Copenhagen, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark, Université Paris-Est Laboratoire Géomatériaux et Géologie de l'Ingénieur (G2I), EA 4119, 5 bd Descartes, 77454 Marne la Vallée Cedex 2, France, and Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, United Kingdom

Abstract: An 17O-enriched version of the titanosilicate glass, KTS2 (K2O·TiO2·2SiO2), was analyzed by 17O MAS, off-MAS, and 3Q-QCPMG-MAS experiments. Exploiting the variations in EFG and CSA parameters for the 17O sites in KTS2 glass, we detected four types of oxygen by reduction of spinning sideband intensities in the off-MAS experiments. From the 17O off-MAS and 3Q-QCPMG-MAS experiments, the Si-O-Ti and K-O-Ti resonances were characterized by a distribution of isotropic chemical shifts, whereas the Si-O-Si resonance was characterized by very small distributions of both EFG tensor and isotropic chemical shift, which means that the disorder in the glass is closely related to Ti. In addition to the order/disorder issue, the most striking feature about the 17O off-MAS experiments on KTS2 is the lack of signals from Ti-O-Ti, which contradicts linking between corner sharing TiO5 units. Therefore, the structure must consist of linkages between TiO5 units and SiO4 tetrahedra and linkages between SiO4 tetrahedra.

-J. Phys. Chem. B, 111 (30), 8691 -8694, 2007. 10.1021/jp0734979 S1520-6106(07)03497-9

Microscopic Access to Long-Range Diffusion Parameters of the Fast Lithium Ion Conductor Li7BiO6 by Solid State 7Li Stimulated Echo NMR

Martin Wilkening,* Claus Mühle, Martin Jansen, and Paul Heitjans
Institute of Physical Chemistry and Electrochemistry, and Center for Solid State Chemistry and New Materials, Leibniz University Hannover, Callinstr. 3a, 30167 Hannover, Germany, and Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

Abstract: Li self-diffusion in rhombohedral Li7BiO6, being a promising basic material for cathodes of rechargeable ion batteries, is studied by means of 7Li stimulated echo NMR. Using the pulse sequence introduced by Jeener and Broekaert, a spin-alignment echo is created whose amplitude decay is recorded as a function of mixing time. The so-obtained two-time correlation functions follow stretched exponential behavior and lead to decay rates which can be identified directly with microscopic Li motional correlation rates ( -1). Using a jump distance of about 0.2 nm, this results in a diffusion coefficient (D) of about 0.5 × 10-16 m2 s-1 at 294 K. The activation energy turned out to be 0.53(3) eV which is in very good agreement with recently obtained results by means of dc-conductivity measurements probing long-range diffusion parameters. This shows that stimulated echo NMR, due to its inherent time scale, gives microscopic access to long-range transport. The prefactor 0-1 of the corresponding Arrhenius law lies in the typical range of phonon frequencies, 0-1 = 3 × 1012 s-1.

-J. Phys. Chem. B, 111 (30), 8860 -8867, 2007. 10.1021/jp070782j S1520-6106(07)00782-1

1H NMR and Viscometric Studies on Cationic Gemini Surfactants in Presence of Aromatic Acids and Salts

Kabir-ud-Din,* Waseefa Fatma, Ziya Ahmad Khan, and Aijaz Ahmad Dar
Department of Chemistry, Aligarh Muslim University, Aligarh - 202 002, India, and Department of Chemistry, University of Kashmir, Srinagar - 190 006, Jammu and Kashmir, India

Abstract: In this paper, we are reporting the influence of addition of aromatic acids (anthranilic and benzoic acid) and their sodium salts on the micellar morphological changes in three cationic gemini surfactant solutions, viz. 5 mM tetramethylene-1,4-bis(N-hexadecyl-N,N-dimethylammonium bromide), 10 mM pentamethylene-1,5-bis(N-hexadecyl-N,N-dimethylammonium bromide), and 10 mM hexamethylene-1,6-bis(N,-hexadecyl-N,N-dimethylammonium bromide). The solubilization site of the counterions (obtained from the additives) near the micellar surface are inferred by 1H NMR. The behavior is explained in the light of binding of counterions to the micelle as well as the nature of the functional group attached to the additive.

-J. Phys. Chem. B, 111 (30), 9172 -9178, 2007. 10.1021/jp072755z S1520-6106(07)02755-1

High-Field 1H MAS and 15N CP-MAS NMR Studies of Alanine Tripeptides and Oligomers: Distinction of Antiparallel and Parallel -Sheet Structures and Two Crystallographically Independent Molecules

Yu Suzuki, Michi Okonogi, Kazuo Yamauchi, Hiromichi Kurosu, Masataka Tansho, Tadashi Shimizu, Hazime Saitô, and Tetsuo Asakura*
Department of Biotechnology, Tokyo University of Agriculture and Technology, 2-24-16, Nakacho, Koganei, Tokyo 184-8588, Japan, Graduate School of Humanities and Sciences, Nara Women's University, Kitauoya-Nishimachi, Nara 630-8506, Japan, and National Institute for Material Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan

Abstract: -Strand peptides are known to assemble into either antiparallel (AP) or parallel (P) -sheet forms which are very important motifs for protein folding and fibril formations occurring in silk fibroin or amyloid proteins. Well-resolved 1H NMR signals including NH protons were observed for alanine tripeptides (Ala)3 with the AP and P structures as well as (Ala)n (n = 4-6) by high-field/fast magic-angle spinning NMR. Amide NH and amino NH3+ 1H signals of (Ala)3 with the P structure were well resonated at 7.5 and 8.9 ppm, respectively, whereas they were not resolved for the AP structure. Notably, NH 1H signals of (Ala)3 and (Ala)4 taking the P structure are resonated at higher field than those of the AP structure by 1.0 and 1.1 ppm, respectively. Further, NH 15N signals of (Ala)3 with the AP structure were resonated at lower field by 2 to 5 ppm than those of (Ala)3 with the P structure. These relative 1H and 15N hydrogen bond shifts of the P structure with respect to those of the AP structure are consistent with the relative hydrogen bond lengths of the interstrand N-H···O=C bonds. Distinction between the two crystallographically independent chains present in the AP and P structures was feasible by 15N chemical shifts but not by 1H chemical shifts because of insufficient spectral resolution in the latter. Calculated 1H and 15N shielding constants by density functional theory are generally consistent with the experimental data, although some discrepancies remain depending upon the models used.

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