J. Phys. Chem. B and C, vol. 114, Issues 37-42

Accurate Determination of Interstrand Distances and Alignment in Amyloid Fibrils by Magic Angle Spinning NMR

Marc A. Caporini†§, Vikram S. Bajaj†, Mikhail Veshtort†, Anthony Fitzpatrick‡, Cait E. MacPhee‡, Michele Vendruscolo‡, Christopher M. Dobson‡, and Robert G. Griffin*†

J. Phys. Chem. B, 2010, 114 (42), pp 13555–13561
DOI: 10.1021/jp106675h
Publication Date (Web): October 6, 2010
Copyright © 2010 American Chemical Society

Abstract: Amyloid fibrils are structurally ordered aggregates of proteins whose formation is associated with many neurodegenerative and other diseases. For that reason, their high-resolution structures are of considerable interest and have been studied using a wide range of techniques, notably electron microscopy, X-ray diffraction, and magic angle spinning (MAS) NMR. Because of the excellent resolution in the spectra, MAS NMR is uniquely capable of delivering site-specific, atomic resolution information about all levels of amyloid structure: (1) the monomer, which packs into several (2) protofilaments that in turn associate to form a (3) fibril. Building upon our high-resolution structure of the monomer of an amyloid-forming peptide from transthyretin (TTR105−115), we introduce single 1-13C labeled amino acids at seven different sites in the peptide and measure intermolecular carbonyl−carbonyl distances with an accuracy of 0.11 A. Our results conclusively establish a parallel, in register, topology for the packing of this peptide into a β-sheet and provide constraints essential for the determination of an atomic resolution structure of the fibril. Furthermore, the approach we employ, based on a combination of a double-quantum filtered variant of the DRAWS recoupling sequence and multispin numerical simulations in SPINEVOLUTION, is general and should be applicable to a wide range of systems.




Solid-State NMR Study of Cysteine on Gold Nanoparticles

Anuji Abraham, Eugene Mihaliuk, Bharath Kumar, Justin Legleiter, and Terry Gullion*
Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States
J. Phys. Chem. C, 2010, 114 (42), pp 18109–18114
DOI: 10.1021/jp107112b
Publication Date (Web): September 30, 2010
Copyright © 2010 American Chemical Society

Abstract: Solid-state NMR spectroscopy is used to characterize the interaction of l-cysteine with gold nanoparticles. The experiments show that there are two types of cysteine in the gold−cysteine complex, with nearly equal populations. We postulate that cysteine forms a two-layer boundary around the gold nanoparticles. The first layer is made of cysteine molecules forming a thiolate bond with the gold surface and having its charged amino and carboxyl groups oriented away from the gold surface. The second layer has its amino and carboxyl groups oriented toward the first layer and its sulfur group oriented away from the gold particles.




Pore Size Distribution Analysis of Mesoporous TiO2 Spheres by 1H Nuclear Magnetic Resonance (NMR) Cryoporometry

Su-Yeol Ryu, Dong Suk Kim, Jae-Deok Jeon, and Seung-Yeop Kwak*

J. Phys. Chem. C, 2010, 114 (41), pp 17440–17445
DOI: 10.1021/jp105496h
Publication Date (Web): September 21, 2010
Copyright © 2010 American Chemical Society

Abstract: Mesoporous TiO2 spheres with various pore sizes were prepared by varying the calcination temperature in the range of 300−700 °C. Increasing calcination temperature was found to increase the crystal size, decrease the surface area, and increase the pore size. The morphologies of mesoporous TiO2 spheres consist of well-defined spherical shapes of monodisperse sizes near 0.8 μm. To determine the pore size distributions (PSDs) of these mesoporous TiO2 spheres, 1H nuclear magnetic resonance (NMR) cryoporometry and Barrett−Joyner−Halenda (BJH) analysis were conducted. NMR cryoporometry is based on the theory of the melting point depression (MPD) of a probe molecule confined within a pore, which is dependent on the pore diameter. MPD was determined by analyzing the variation of the NMR spin−echo intensity with temperature. From the resulting spin−echo intensity versus temperature (I−T) curves, it was found that the maximum MPD of a probe molecule confined within the pores of mesoporous TiO2 decreases with increasing calcination temperature; that is, the pore size increases with increasing calcination temperature. Because mesoporous TiO2 spheres consist of aggregates of nanocrystallite TiO2 and mesopores located at intercrystallites, an increase in the calcination temperature induces an increase in the crystallite size and, thus, in the pore size because the small pores collapse and the large pores increase in size. We also confirmed by BJH analysis that the pore size of mesoporous TiO2 increases with increasing calcination temperature. This trend is in agreement with our 1H NMR cryoporometry results. Overall, these findings indicate that NMR cryoporometry is a very effective method for determining the PSDs of mesoporous TiO2 spheres.



Defect Functionalization of Hexagonal Boron Nitride Nanosheets

Yi Lin*†, Tiffany V. Williams‡, Wei Cao§, Hani E. Elsayed-Ali§, and John W. Connell‡

J. Phys. Chem. C, 2010, 114 (41), pp 17434–17439
DOI: 10.1021/jp105454w
Publication Date (Web): September 21, 2010
Copyright © 2010 American Chemical Society

Abstract:A pristine hexagonal boron nitride (h-BN) powder sample with layered crystalline sheetlike particles of 1−10 μm in lateral sizes and a few hundred nanometers in thicknesses was mechanically treated using a ball-mill to intentionally introduce defect sites. The h-BN was ball-milled for various times and subsequently was functionalized with a long alkyl chain amine via Lewis acid−base interactions between the amino groups and the boron atoms of h-BN to obtain soluble amine-attached nanosheet samples as the products. The functionalized h-BN nanosheet samples were characterized via various microscopic and spectroscopic techniques. The results strongly support a direct correlation between increasing defect site concentrations of the h-BN nanosheet samples and improved reaction efficiency with the amine. This suggests the enhanced reactivity of defect boron atoms in comparison to conjugated ones on an unperturbed h-BN plane. NMR investigations provided the strongest evidence supporting the hypothesis that the amino groups reacted with the h-BN at specific defect sites induced by ball-milling. The mechanistic implications are discussed.



Access to Well-Defined Ruthenium Mononuclear Species Grafted via a Si−Ru Bond on Silane Functionalized Silica†

Fernando Rascn‡, Romain Berthoud‡, Raphal Wischert‡, Wayne Lukens§, and Christophe Copret*‡

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp1064962
Publication Date (Web): September 20, 2010
Copyright © 2010 American Chemical Society

Abstract: A functionalized silica with T3 silane surface groups (i.e., (≡SiO)3Si−H) was prepared and interacted with Ru(cod)(cot), resulting in the formation of monometallic surface species attached to the surface via a Si−Ru bond, according to EXAFS spectroscopy, infrared spectroscopy, and solid-state NMR.



Time-Resolved and Site-Specific Insights into Migration Pathways of Li+ in α-Li3VF6 by 6Li 2D Exchange MAS NMR

M. Wilkening*†, E. E. Romanova†‡, S. Nakhal§, D. Weber§, M. Lerch§, and P. Heitjans†

J. Phys. Chem. C, 2010, 114 (44), pp 19083–19088
DOI: 10.1021/jp103433h
Publication Date (Web): September 14, 2010
Copyright © 2010 American Chemical Society

Abstract: Two-dimensional (2D) exchange nuclear magnetic resonance (NMR) spectroscopy carried out under magic angle spinning (MAS) conditions is ideally suited to study site-specific Li diffusion parameters of cathode materials required for the target-oriented development of so-called high-energy density 4 V-lithium-ion batteries. In the present study, we took advantage of Li NMR hyperfine shifts to record temperature-variable 1D and mixing-time dependent 2D exchange MAS 6Li NMR spectra on α-Li3VF6 serving as both a potential cathode material as well as an application-oriented model substance with three magnetically inequivalent Li sites. By comparing the NMR results with structural details of the material we were able to obtain detailed insights into the migration pathways and Li exchange rates which are of the order of some hundreds of Li jumps per second at approximately 340 K. Site-specific Li jump rates τ−1 reveal the electrochemically active sites and provide information how to modify the material in order to increase its relatively low Li diffusivity found at room temperature.



Adsorbate Effect on AlO4(OH)2 Centers in the Metal−Organic Framework MIL-53 Investigated by Solid-State NMR Spectroscopy

Christian Lieder, Sabine Opelt, Michael Dyballa, Harald Henning, Elias Klemm, and Michael Hunger*

J. Phys. Chem. C, 2010, 114 (39), pp 16596–16602
DOI: 10.1021/jp105700b
Publication Date (Web): September 10, 2010
Copyright © 2010 American Chemical Society

Abstract:1H and 27Al MAS NMR spectroscopies have been applied for studying the effect of water molecules, nitrogen bases, and o-xylene on the hydroxyl protons of bridging AlOH groups and framework aluminum atoms in the metal−organic framework (MOF) MIL-53. For water molecules adsorbed on the low-temperature form MIL-53lt, two 1H MAS NMR signals were found indicating the formation of different O−H···O hydrogen bonds to neighboring oxygen atoms, such as to carboxylic oxygens. Upon adsorption of the nitrogen bases acetonitrile, ammonia, and pyridine, a linear increase of the quadrupole coupling constant, CQ, of the framework aluminum atoms in dehydrated MIL-53 from CQ = 8.5 MHz (unloaded material) to maximum 10.8 MHz (pyridine-loaded material) as a function of the proton affinity of the adsorbates was observed. Adsorption of o-xylene led to three stepwise changes of the quadrupole coupling constants, CQ, of framework aluminum atoms in dehydrated MIL-53. While the first two stepwise changes of the CQ values (CQ = 8.0 and 8.7 MHz) occur for o-xylene loadings of lower than 4 molecules per unit cell and for all AlO4(OH)2 centers, the third change of the CQ value to 9.4 MHz was observed for o-xylene loadings higher than 4 o-xylene molecules per unit cell and for maximum 50% of the framework aluminum atoms. This third adsorbate-induced change of the CQ value of framework aluminum atoms in MIL-53 is accompanied by a significant decrease of the adsorbate mobility



Impact of Controlling the Site Distribution of Al Atoms on Catalytic Properties in Ferrierite-Type Zeolites†

Yuriy Romn-Leshkov, Manuel Moliner, and Mark E. Davis*
Chemical Engineering, California Institute of Technology, Pasadena, California 91125
J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp106247g
Publication Date (Web): September 9, 2010
Copyright © 2010 American Chemical Society

Abstract: Zeolites with the ferrierite (FER) topology are synthesized using a combination of tetramethylammonium (TMA) cations with differently sized cyclic amines (pyrrolidine (Pyr), hexamethyleneimine (HMI), and 1,4-diazabicyclo[2.2.2]octane (DAB)). Using these organic structure-directing agents (SDAs), low Si/Al ratios and concentrated synthesis mixtures favor the crystallization of FER materials. Increasing the size of the cyclic amine or decreasing the aluminum content leads to the crystallization of other phases or the creation of excessive amounts of connectivity defects. TMA cations play a decisive role in the synthesis of the FER materials, and their presence allows the use of HMI to synthesize FER. Proton MAS NMR is used to quantify the accessibility of pyridine to acid sites in these FER samples, where it is found that the FER+HMI+TMA sample contains only 27% acid sites in the 8-MR channels, whereas FER+Pyr and FER+Pyr+TMA contain 89% and 84%, respectively. The constraint index (CI) test and the carbonylation of dimethyl ether (DME) with carbon monoxide are used as probe reactions to evaluate how changes in the aluminum distribution in these FER samples affect their catalytic behavior. Results show that the use of Pyr as an SDA results in the selective population of acid sites in the 8-MR channels, whereas the use of HMI generates FER zeolites with an increased concentration of acid sites in the 10-MR channels.

J. Phys. Chem. B and C, volume 114, Issues 43 - 45 + November ASAPs

Quantum Oscillations and Polarization of Nuclear Spins in Photoexcited Triplet States†

Gerd Kothe*‡, Tomoaki Yago‡, Jrg-Ulrich Weidner‡, Gerhard Link‡, Michail Lukaschek‡, and Tien-Sung Lin§

J. Phys. Chem. B, 2010, 114 (45), pp 14755–14762
DOI: 10.1021/jp103508t
Publication Date (Web): July 28, 2010
Copyright © 2010 American Chemical Society

Abstract: The unique physical properties of photoexcited triplet states have been explored in numerous spectroscopic studies employing electron paramagnetic resonance (EPR). So far, however, no quantum interference effects were found in these systems in the presence of a magnetic field. In this study, we report the successful EPR detection of nuclear quantum oscillations in an organic triplet state subject to an external magnetic field. The observed quantum coherences can be rationalized using an analytical theory. Analysis suggests that the nuclear spins are actively involved in the intersystem crossing process. The novel mechanism also acts as a source of oscillatory nuclear spin polarization that gives rise to large signal enhancement in nuclear magnetic resonance (NMR). This opens new perspectives for the analysis of chemically induced dynamic nuclear polarization in mechanistic studies of photoactive proteins.



W/Mo-Oxide Nanomaterials: Structure−Property Relationships and Ammonia-Sensing Studies†

Ying Zhou‡, Kaibo Zheng§, Jan-Dierk Grunwaldt, Thomas Fox‡, Leilei Gu§, Xiaoliang Mo§, Guorong Chen§, and Greta R. Patzke*‡

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp106439n
Publication Date (Web): November 30, 2010
Copyright © 2010 American Chemical Society

Abstract:W/Mo-oxides of the hexagonal tungsten bronze (HTB) type have been investigated by X-ray absorption spectroscopy to obtain detailed insight into the substitution process of W by Mo that leads to mixed HTB frameworks. Both the morphology of the nanostructured W/Mo-HTBs as well as the oxidation state of Mo are significantly influenced through the incorporation of different alkali cations into the hexagonal channels of this open structure. A variety of complementary analytical methods, including TG, in situ and ex situ XRD, SEM, and solid-state NMR analyses, were applied to determine the thermal stability of the obtained W/Mo-HTB materials with respect to their alkali cation and NH4+ contents. A strong correlation between composition and stability was found with the Rb-W/Mo-HTBs exhibiting the highest structural and morphological resistance among the series (up to 580 °C). The NH3-sensing properties of selected W/Mo-oxides in test atmospheres furthermore point to promising features of the Rb-stabilized hexagonal framework materials




Hydrogen Physisorption in a Cu(II) Metallacycle

Tanja Pietraβ*†, Itza Cruz-Campa‡, Justine Kombarakkaran†, Suman Sirimulla§, Atta M. Arif§, and Juan C. Noveron*‡
J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp104544r
Publication Date (Web): November 19, 2010
Copyright © 2010 American Chemical Society

Abstract: The interaction of molecular hydrogen with a novel microporous dinuclear Cu(II) complex, [bis-μ-di(4-pyridyl)methanol-1,4,7-triazacyclononane copper(II)] triflate (1), and its derivatives formed from oxidation and solvent removal was studied with 2H NMR and density functional theory (DFT). The Cu-complex 1 was characterized with X-ray diffraction methods and consists of a dinuclear macrocycle that forms one-dimensional channels of 9.55 Å in diameter. 2H NMR studies of deuterium gas adsorption by 1 suggest that physisorption condensation of D2 occurs within two distinct microenvironments: in the interior and in-between the microtubular structures. The assignment of NMR resonances to specific adsorption sites is supported by spectral decomposition and analysis of the line widths and integrated signal intensities of the components. The dynamics of the system are probed by spin−lattice relaxation time measurements and spectral hole-burn experiments as a function of temperature and pressure. NMR and DFT calculations suggest that hydrogen uptake is mediated through interactions with the Cu(II) centers via dipole−ion interactions.




Influence of Structure on the Spectroscopic Properties of the Polymorphs of Piroxicam

Wei Liu†, Wei David Wang†, Wei Wang†, Shi Bai*†‡, and Cecil Dybowski‡

J. Phys. Chem. B, ASAP
DOI: 10.1021/jp1084444
Publication Date (Web): November 18, 2010
Copyright © 2010 American Chemical Society

Abstract: The complete 13C NMR chemical-shift tensors for the carbon sites of the two polymorphic forms (PI and PII) and the monohydrate form (PM) of the analgesic drug, piroxicam, are reported. The NMR parameters (isotropic chemical shifts, chemical-shielding anisotropies and asymmetries, and dipolar couplings), X-ray powder diffraction, and density functional calculations of piroxicam are analyzed in terms of hydrogen bonding and structure. The integration of all the data gives an improved model of the local solid-state structures of the polymorphs. In particular, the solid-state NMR spectra demonstrate that the asymmetric unit of the monohydrate, PM, contains two zwitterionic piroxicam molecules.


Heterogeneities in Gelatin Film Formation Using Single-Sided NMR

Sushanta Ghoshal*, Carlos Mattea, Paul Denner, and Siegfried Stapf

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp1068363
Publication Date (Web): November 18, 2010
Copyright © 2010 American Chemical Society

Abstract: Gelatin solutions were prepared in D2O. The drying process of cast solutions was followed with a single-sided nuclear magnetic resonance (NMR) scanner until complete solidification occurred. Spin−spin relaxation times (T2) were measured at different layers with microscopic resolution and were correlated with the drying process during film formation. Additionally, the evaporation of the gelatin solution was observed optically from the reduction of the sample thickness, revealing that at the macroscopic level, the rate of evaporation is not uniform throughout the experiment. A crossover in the spatial evolution of the drying process is observed from the NMR results. At the early stages, the gel appears to be drier in the upper layers near the evaporation front, while this tendency is inverted at the later stages, when drying is faster from the bottom. XRD (X-ray diffraction) data showed that a structural heterogeneity persists in the final film.



Understanding the Properties of the Coagel and Gel Phases: A 2H and 13C NMR Study of Amphiphilic Ascorbic Acid Derivatives

Silvia Borsacchi†, Moira Ambrosi‡, Pierandrea Lo Nostro‡, and Marco Geppi*†

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp107324e
Publication Date (Web): November 15, 2010
Copyright © 2010 American Chemical Society

Abstract: The coagel and gel phases formed by the d and l diastereoisomers of ascorbyl-dodecanoate (ASC12) in deuterated water were studied through solid-state NMR techniques. In particular, the dynamic properties of water and surfactant chains were investigated by 2H and 13C NMR static spectra, respectively. Two fractions of water with very different dynamics were found in the coagel phases, one solidlike and one liquidlike, assigned to water strongly bound to the surfactant polar heads and bulk water, respectively. Only one kind of “intermediate” water was instead detected in the gel phase suggesting that the merging of the two types of water in the interlayers between the surfactant lamellae occurs at the coagel-to-gel transition. Moreover, the surfactant chains, very rigid in the coagel phase, give rise to fast trans−gauche interconformational jumps in the gel phase, where almost isotropic reorientations of the whole aggregates also occur. A different dynamic behavior was found for the two diastereoisomers in particular for what concerns the surfactant molecules in the gel phase and the water molecules in the coagel presumably ascribable to different inter- and intramolecular interactions that involve the polar heads



The “Alkyl” and “Carbenium” Pathways of Methane Activation on Ga-Modified Zeolite BEA: 13C Solid-State NMR and GC-MS Study of Methane Aromatization in the Presence of Higher Alkane

Mikhail V. Luzgin, Anton A. Gabrienko, Vladimir A. Rogov, Alexander V. Toktarev, Valentin N. Parmon, and Alexander G. Stepanov*
J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp1078899
Publication Date (Web): November 11, 2010
Copyright © 2010 American Chemical Society

Abstract:By using 13C solid-state NMR spectroscopy and GC-MS analysis, the activation of methane and coaromatization of methane and propane have been monitored on gallium-modified zeolite BEA at 573−823 K. A noticeable degree involvement of the 13C-label from methane-13C into the aromatic reaction products (benzene, toluene) has been demonstrated. The major intermediate of the methane activation represents gallium-methyl species, which are formed by methane dissociative adsorption on Ga2O3 species of the zeolite. The minor species of methane activation, Ga-methoxy groups, provide the involvement of methane into aromatics by the methylation of aromatic molecules, which are generated exclusively from propane, by the mechanism of electrophilic substitution. Ga-methyl species can serve as methylating nucleophilic agent for the reaction of nucleophilic substitution with participation of aromatic molecules, which contain the electron-withdrawing substitutes.



Slow Exchange Model of Nonrigid Rotational Motion in RNA for Combined Solid-State and Solution NMR Studies

Prashant S. Emani†, Gregory L. Olsen‡, Dorothy C. Echodu‡, Gabriele Varani‡§, and Gary P. Drobny*‡

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp107193z
Publication Date (Web): November 10, 2010
Copyright © 2010 American Chemical Society

Abstract: Functional RNA molecules are conformationally dynamic and sample a multitude of dynamic modes over a wide range of frequencies. Thus, a comprehensive description of RNA dynamics requires the inclusion of a broad range of motions across multiple dynamic rates which must be derived from multiple spectroscopies. Here we describe a slow conformational exchange theoretical approach to combining the description of local motions in RNA that occur in the nanosecond to microsecond window and are detected by solid-state NMR with nonrigid rotational motion of the HIV-1 transactivation response element (TAR) RNA in solution as observed by solution NMR. This theoretical model unifies the experimental results generated by solution and solid-state NMR and provides a comprehensive view of the dynamics of HIV-1 TAR RNA, a well-known paradigm of an RNA where function requires extensive conformational rearrangements. This methodology provides a quantitative atomic level view of the amplitudes and rates of the local and collective displacements of the TAR RNA molecule and provides directly motional parameters for the conformational capture hypothesis of this classical RNA−ligand interaction.



Molecular Dynamics of Amorphous Gentiobiose Studied by Solid-State NMR

Teresa G. Nunes*†, Hermnio P. Diogo†, Susana S. Pinto†, and Joaquim J. Moura Ramos‡

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp106371w
Publication Date (Web): November 10, 2010
Copyright © 2010 American Chemical Society

Abstract: A solid-state NMR (SSNMR) study is reported on the effect of temperature on the molecular mobility of amorphous gentiobiose, which is complemented with data obtained from crystalline samples. 13C cross-polarization/magic-angle-spinning (CPMAS) spectra and 1H MAS spectra were obtained for gentiobiose at natural abundance, in the amorphous state, from 293 K up to the glass transformation region (Tg = 359 K). Two well-defined molecular mobility regimes were observed, corresponding to different motional modes. NMR results on molecular dynamics are discussed and compared with those obtained by thermally stimulated depolarization currents (TSDC) and dielectric relaxation spectroscopy (DRS). SSNMR spectra presented evidence for a new polymorphic form of gentiobiose, not yet reported in the literature, which is obtained by slow heating of the amorphous solid up to 364 K inside the NMR zirconia rotor.



73Ge Solid-State NMR of Germanium Oxide Materials: Experimental and Theoretical Studies

Vladimir K. Michaelis and Scott Kroeker*

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp1071082
Publication Date (Web): November 10, 2010
Copyright © 2010 American Chemical Society

Abstract: A comprehensive series of crystalline germanates has been studied by ultrahigh-field 73Ge NMR and quantum chemical calculations. Despite its low gyromagnetic ratio, low natural abundance and large quadrupole moment, interpretable spectra were obtained in almost all cases, demonstrating that 73Ge is an accessible NMR nucleus. The spectra yield a wide range of quadrupole coupling constants (CQ = 9 to 35 MHz), with calculations indicating a range twice that, which are rationalized principally in terms of the variation in Ge−O bond lengths. The isotropic chemical shifts appear to fall into distinct regions for four-, five-, and six-coordinate Ge, with increasing coordination number corresponding to lower frequencies. Both CASTEP and WIEN2k consistently underestimate the CQs, suggesting that the exchange-correlation functional is poorly optimized for these systems. 73Ge NMR spectra of alkali germanate glasses are broad and featureless, rendering them difficult to interpret in terms of specific structural elements, even with the well understood NMR parameters from the crystalline systems. This study represents the first systematic 73Ge NMR investigation of solids, and shows that valuable structural information can be obtained in favorable cases.



NMR Study of LiBH4 with C60

David T. Shane*†, Robert L. Corey‡, Laura H. Rayhel†, Matthew Wellons§, Joseph A. Teprovich, Jr.§, Ragaiy Zidan§, Son-Jong Hwang, Robert C. Bowman, Jr., and Mark S. Conradi†

J. Phys. Chem. C, 2010, 114 (46), pp 19862–19866
DOI: 10.1021/jp107911u
Publication Date (Web): November 3, 2010
Copyright © 2010 American Chemical Society

Abstract:LiBH4 doped with 1.6 mol % well-dispersed C60 is studied with solid-state nuclear magnetic resonance (NMR). Variable-temperature hydrogen NMR shows large changes between the data upon first heating and after exposure to 300 °C. After heating, a large fraction on the order of 50% of the hydrogen signal appears in a motionally narrowed peak, similar to a previous report of LiBH4 in a porous carbon aerogel nanoscaffold. Magic-angle spinning (MAS) NMR of 13C in a 13C-enriched sample finds the C60 has reacted already in the as-mixed (unheated) material. Dehydriding and rehydriding result in further 13C spectral changes, with nearly all intensity being found in a broad peak corresponding to aromatic carbons. It thus appears that the previously reported improved dehydriding and rehydriding kinetics of this material at least partially result from in situ formation of a carbon framework. The method may offer a new route to dispersal of hydrides in carbon support structures.



Investigation of Si Atom Migration in the Framework of MSE-Type Zeolite YNU-2

Takuji Ikeda*†, Satoshi Inagaki‡, Taka-aki Hanaoka†, and Yoshihiro Kubota‡

J. Phys. Chem. C, 2010, 114 (46), pp 19641–19648
DOI: 10.1021/jp1079586
Publication Date (Web): November 2, 2010
Copyright © 2010 American Chemical Society

Abstract: The change in distribution of Si atom defects in the framework of zeolite YNU-2 by steam treatment was investigated using powder X-ray diffraction and solid-state NMR spectroscopy. The precursor of zeolite YNU-2 (abbreviated to YNU-2P) with a three-dimensional pore system has a large number of Si atom defects (more than 10% of all T sites in a unit cell) around the supercage. These defect sites were confirmed by observation of a Q3 ((−SiO)3Si−OH) resonance peak by 29Si magic angle spinning NMR spectroscopy. We have shown that steam treatment of YNU-2P at 523 K for 24 h significantly decreases the relative intensity ratio of the observed Q3 resonance peak for the Q4((−SiO)4Si) peak. The Rietveld analysis of steam-treated YNU-2P (YNU-2PST) shows a marked increase in the site occupancies of the defective Si sites. Furthermore, the Si atom defects in YNU-2PST almost disappeared after calcination, yielding siliceous zeolite YNU-2. These results indicate that the defective framework structure was almost restored by steam treatment. Experimental results suggest that Si atom migration in the framework of YNU-2P takes place during steam treatment. The migrated Si atom fragment fills defect sites and is connected with adjacent silanol groups. In addition, the quantity of the half amount of structure-directing agent molecules was removed from the micropores in YNU-2PST by steam treatment.



Ryan M. Ravenelle†, Florian Schüβler‡, Andrew D’Amico†, Nadiya Danilina§, Jeroen A. van Bokhoven§, Johannes A. Lercher‡, Christopher W. Jones†, and Carsten Sievers*†

J. Phys. Chem. C, 2010, 114 (46), pp 19582–19595
DOI: 10.1021/jp104639e
Publication Date (Web): November 2, 2010
Copyright © 2010 American Chemical Society

Abstract: Zeolites Y and ZSM-5 with varying Si/Al ratios are treated in liquid water at 150 and 200 °C under autogenic pressure to assess their hydrothermal stability. The changes in the structure are characterized by atomic absorption spectroscopy, X-ray diffraction, scanning electron microscopy, argon physisorption, 27Al and 29Si MAS NMR spectroscopy, temperature-programmed desorption of ammonia, and pyridine adsorption followed by IR spectroscopy. During treatment in hot water, zeolite Y with a Si/Al ratio of 14 or higher is transformed into an amorphous material, and the rate of this degradation increases with increasing Si/Al ratio. In contrast, ZSM-5 is not modified under the same conditions. The main degradation mechanism is suggested to be hydrolysis of the siloxane bonds (Si−O−Si) as opposed to dealumination, which dominates under steaming conditions. In the resulting amorphous phase, Al remains tetrahedrally coordinated, but the micropore volume and concentration of accessible acid sites is reduced dramatically. The results demonstrate that potential structural changes of zeolites have to be considered when these materials are used as catalysts for aqueous phase conversion of biomass.



Structure and Characterization of KSc(BH4)4

Radovan ern*†, Dorthe B. Ravnsbæk‡, Godwin Severa§, Yaroslav Filinchuk, Vincenza D’ Anna, Hans Hagemann, Drthe Haase#, Jørgen Skibsted‡, Craig M. Jensen*§, and Torben R. Jensen*‡

J. Phys. Chem. C, 2010, 114 (45), pp 19540–19549
DOI: 10.1021/jp106280v
Publication Date (Web): October 25, 2010
Copyright © 2010 American Chemical Society

Abstract: A new potassium scandium borohydride, KSc(BH4)4, is presented and characterized by a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. The title compound, KSc(BH4)4, forms at ambient conditions in ball milled mixtures of potassium borohydride and ScCl3 together with a new ternary chloride K3ScCl6, which is also structurally characterized. This indicates that the formation of KSc(BH4)4 differs from a simple metathesis reaction, and the highest scandium borohydride yield (31 mol %) can be obtained with a reactant ratio KBH4:ScCl3 of 2:1. KSc(BH4)4 crystallizes in the orthorhombic crystal system, a = 11.856(5), b = 7.800(3), c = 10.126(6) Å, V = 936.4(8) Å3 at RT, with the space group symmetry Pnma. KSc(BH4)4 has a BaSO4 type structure where the BH4 tetrahedra take the oxygen positions. Regarding the packing of cations, K+, and complex anions, [Sc(BH4)4]−, the structure of KSc(BH4)4 can be seen as a distorted variant of orthorhombic neptunium, Np, metal. Thermal expansion of KSc(BH4)4 in the temperature range RT to 405 K is anisotropic, and the lattice parameter b shows strong nonlinearity upon approaching the melting temperature. The vibrational and NMR spectra are consistent with the structural model, and previous investigations of the related compounds ASc(BH4)4 with A = Li, Na. KSc(BH4)4 is stable from RT up to 405 K, where the compound melts and then releases hydrogen in two rapid steps approximately at 460−500 K and 510−590 K. The hydrogen release involves the formation of KBH4, which reacts with K3ScCl6 and forms a solid solution, K(BH4)1−xClx. The ternary potassium scandium chloride K3ScCl6 observed in all samples has a monoclinic structure at room temperature, P21/a, a = 12.729(3), b = 7.367(2), c = 12.825(3) Å, β = 109.22(2)°, V = 1135.6(4) Å3, which is isostructural to K3MoCl6. The monoclinic polymorph transforms to cubic at 635 K, a = 10.694 Å (based on diffraction data measured at 769 K), which is isostructural to the high temperature phase of K3YCl6.



Phase Behavior and 13C NMR Spectroscopic Analysis of the Mixed Methane + Ethane + Propane Hydrates in Mesoporous Silica Gels

Seungmin Lee, Inuk Cha, and Yongwon Seo*

J. Phys. Chem. B, 2010, 114 (46), pp 15079–15084
DOI: 10.1021/jp108037m
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society

Abstract: In this study, the phase behavior and quantitative determination of hydrate composition and cage occupancy for the mixed CH4 + C2H6 + C3H8 hydrates were closely investigated through the experimental measurement of three-phase hydrate (H)−water-rich liquid (LW)−vapor (V) equilibria and 13C NMR spectra. To examine the effect of pore size and salinity, we measured hydrate phase equilibria for the quaternary CH4 (90%) + C2H6 (7%) + C3H8 (3%) + water mixtures in silica gel pores of nominal diameters of 6.0, 15.0, and 30.0 nm and for the quinary CH4 (90%) + C2H6 (7%) + C3H8 (3%) + NaCl + water mixtures of two different NaCl concentrations (3 and 10 wt %) in silica gel pores of a nominal 30.0 nm diameter. The value of hydrate−water interfacial tension for the CH4 (90%) + C2H6 (7%) + C3H8 (3%) hydrate was found to be 47 ± 4 mJ/m2 from the relation of the dissociation temperature depression with the pore size of silica gels at a given pressure. At a specified temperature, three-phase H−LW−V equilibrium curves of pore hydrates were shifted to higher pressure regions depending on pore sizes and NaCl concentrations. From the cage-dependent 13C NMR chemical shifts of enclathrated guest molecules, the mixed CH4 (90%) + C2H6 (7%) + C3H8 (3%) gas hydrate was confirmed to be structure II. The cage occupancies of each guest molecule and the hydration number of the mixed gas hydrates were also estimated from the 13C NMR spectra.




Biomimetic Apatite Mineralization Mechanisms of Mesoporous Bioactive Glasses as Probed by Multinuclear 31P, 29Si, 23Na and 13C Solid-State NMR

Philips N. Gunawidjaja†, Andy Y. H. Lo†, Isabel Izquierdo-Barba‡§, Ana Garca‡§, Daniel Arcos‡§, Baltzar Stevensson†, Jekabs Grins, Mara Vallet-Reg‡§, and Mattias Edn*†

J. Phys. Chem. C, 2010, 114 (45), pp 19345–19356
DOI: 10.1021/jp105408c
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society

Abstract: An array of magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy experiments is applied to explore the surface reactions of a mesoporous bioactive glass (MBG) of composition Ca0.10Si0.85P0.04O1.90 when subjected to a simulated body fluid (SBF) for variable intervals. Powder X-ray diffraction and 31P NMR techniques are employed to quantitatively monitor the formation of an initially amorphous calcium phosphate surface layer and its subsequent crystallization into hydroxycarbonate apatite (HCA). Prior to the onset of HCA formation, 1H → 29Si cross-polarization (CP) NMR evidence dissolution of calcium ions; a slightly increased connectivity of the speciation of silicate ions is observed at the MBG surface over 1 week of SBF exposure. The incorporation of carbonate and sodium ions into the bioactive orthophosphate surface layer is explored by 1H → 13C CPMAS and 23Na NMR, respectively. We discuss similarities and distinctions in composition−bioactivity relationships established for traditional melt-prepared bioglasses compared to MBGs. The high bioactivity of phosphorus-bearing MBGs is rationalized to stem from an acceleration of their surface reactions due to presence of amorphous calcium orthophosphate clusters of the MBG pore wall.




Analysis of the 7Li NMR signals in the Monoclinic Li3Fe2(PO4)3 and Li3V2(PO4)3 Phases
A. Castets, D. Carlier*, K. Trad, C. Delmas, and M. Mntrier

J. Phys. Chem. C, 2010, 114 (44), pp 19141–19150
DOI: 10.1021/jp106871z
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society

Abstract: The monoclinic Li3Fe2(PO4)3 and Li3V2(PO4)3 phosphates are materials for positive electrodes in Li-ion batteries. They also have interesting structures to test and improve the understanding of Li NMR signals in paramagnetic compounds. The position of such signals is governed by the transfer of electron spin density from the transition metal ion to the Li nucleus. These mechanisms are based on delocalization and polarization effects which induce positive and negative Fermi contact shifts, respectively. We have characterized Li3Fe2(PO4)3 by Li NMR. To understand the signals observed, we have analyzed the electron spin density transfer mechanisms (i) by considering the different Li environments, (ii) by using DFT calculations. We compare our analysis to the one very recently reported by Davis et al. These analyses have been extended to Li3V2(PO4)3 studied by NMR by Cahill et al.



Mechanically, Magnetically, and “Rotationally Aligned” Membrane Proteins in Phospholipid Bilayers Give Equivalent Angular Constraints for NMR Structure Determination

Sang Ho Park, Bibhuti B. Das, Anna A. De Angelis, Mario Scrima, and Stanley J. Opella*

J. Phys. Chem. B, 2010, 114 (44), pp 13995–14003
DOI: 10.1021/jp106043w
Publication Date (Web): October 20, 2010
Copyright © 2010 American Chemical Society

Abstract: The native environment for membrane proteins is the highly asymmetric phospholipid bilayer, and this has a large effect on both their structure and dynamics. Reproducing this environment in samples suitable for spectroscopic and diffraction experiments is a key issue, and flexibility in sample preparation is essential to accommodate the diverse size, shape, and other physical properties of membrane proteins. In most cases, to ensure that the biological activities are maintained, this means reconstituting the proteins in fully hydrated planar phospholipid bilayers. The asymmetric character of protein-containing bilayers means that it is possible to prepare either oriented or unoriented (powder) samples. Here we demonstrate the equivalence of mechanical, magnetic, and what we refer to as “rotational alignment” of membrane proteins in phospholipid bilayer samples for solid-state NMR spectroscopy. The trans-membrane domain of virus protein “u” (Vpu) from human immunodeficiency virus (HIV-1) and the full-length membrane-bound form of fd bacteriophage coat protein in phospholipid bilayers are used as examples. The equivalence of structural constraints from oriented and unoriented (powder) samples of membrane proteins is based on two concepts: (1) their alignment is defined by the direction of the bilayer normal relative to the magnetic field and (2) they undergo rapid rotational diffusion about the same bilayer normal in liquid crystalline membranes. The measurement of angular constraints relative to a common external axis system defined by the bilayer normal for all sites in the protein is an essential element of oriented sample (OS) solid-state NMR.



Controlled Interactions between Anhydrous Keggin-Type Heteropolyacids and Silica Support: Preparation and Characterization of Well-Defined Silica-Supported Polyoxometalate Species

Eva Grinenval†, Xavier Rozanska§, Anne Baudouin†, Elise Berrier‡, Franoise Delbecq§, Philippe Sautet§, Jean-Marie Basset†, and Frdric Lefebvre*†

J. Phys. Chem. C, 2010, 114 (44), pp 19024–19034
DOI: 10.1021/jp107317s
Publication Date (Web): October 20, 2010
Copyright © 2010 American Chemical Society

Abstract:Anhydrous Keggin-type phosphorus heteropolyacids were deposited on partially dehydroxylated silica by using the surface organometallic chemistry (SOMC) strategy. The resulting solids were characterized by a combination of physicochemical methods including IR, Raman, 1D and 2D 1H, and 31P MAS NMR, electron microscopy experiments and density functional theory (DFT) calculations. It is shown that the main surface species is [≡Si(OH...H+)]2[H+]1[PM12O403−] where the polyoxometalate is linked to the support by proton interaction with two silanols. Two other minor species (10% each) are formed by coordination of the polyoxometalate to the surface via the interaction between all three protons with three silanol groups or via three covalent bonds formed by dehydroxylation of the above species. Comparison of the reactivity of these solids and of compounds prepared by a classical way shows that the samples prepared by the SOMC approach contain ca. 7 times more acid sites




Thermal Spreading As an Alternative for the Wet Impregnation Method: Advantages and Downsides in the Preparation of MoO3/SiO2−Al2O3 Metathesis Catalysts

Damien P. Debecker*†, Mariana Stoyanova‡, Uwe Rodemerck‡, Pierre Eloy†, Alexandre Lonard§, Bao-Lian Su§, and Eric M. Gaigneaux*†

J. Phys. Chem. C, 2010, 114 (43), pp 18664–18673
DOI: 10.1021/jp1074994
Publication Date (Web): October 14, 2010
Copyright © 2010 American Chemical Society

Abstract:Silica−alumina-supported MoO3 catalysts are classically prepared via impregnation of the support with a molybdenum salt solution, usually ammonium heptamolybdate, and subsequent drying and calcination (three steps). The downsides of such a route for the synthesis of heterogeneous metathesis catalysts are linked to the limited control on the nature of the MoOx stabilized at the surface, to the uneven distribution of the deposit in the pores of the support, and to the build up of inactive species that find their origin in the wet step of the preparation. In opposition, the direct thermal spreading of molybdenum oxide onto the support is a straightforward (one step) method involving no wet stage. It allows the conversion of bulk MoO3 crystals to amorphous molybdate species dispersed at the surface of the silica−alumina support. This contribution shows that the catalysts obtained via both methods exhibit similar performances in the self-metathesis of propene to butene and ethene. However, based on XRD, XPS, Raman spectroscopy, ICP-AES, N2 physisorption, TEM, and MAS-NMR spectroscopy, it is shown that the origin of active and inactive species in the two systems is different. Whereas the activity of wet-made catalysts is limited by the formation of bulky MoO3 crystals and of aluminum molybdate, the performances of dry-made catalysts are limited by the incomplete spreading of MoO3 nanocrystallites.




On the Performance of Spin Diffusion NMR Techniques in Oriented Solids: Prospects for Resonance Assignments and Distance Measurements from Separated Local Field Experiments

Nathaniel J. Traaseth†, T. Gopinath†, and Gianluigi Veglia*†‡

J. Phys. Chem. B, 2010, 114 (43), pp 13872–13880
DOI: 10.1021/jp105718r
Publication Date (Web): October 11, 2010
Copyright © 2010 American Chemical Society

Abstract: NMR spin diffusion experiments have the potential to provide both resonance assignment and internuclear distances for protein structure determination in oriented solid-state NMR. In this paper, we compared the efficiencies of three spin diffusion experiments: proton-driven spin diffusion (PDSD), cross-relaxation-driven spin diffusion (CRDSD), and proton-mediated proton transfer (PMPT). As model systems for oriented proteins, we used single crystals of N-acetyl-L-15N-leucine (NAL) and N-acetyl-L-15N-valyl-L-15N-leucine (NAVL) to probe long and short distances, respectively. We demonstrate that, for short 15N/15N distances such as those found in NAVL (3.3 Å), the PDSD mechanism gives the most intense cross-peaks, while, for longer distances (>6.5 Å), the CRDSD and PMPT experiments are more efficient. The PDSD was highly inefficient for transferring magnetization across distances greater than 6.5 Å (NAL crystal sample), due to small 15N/15N dipolar couplings (<4.5>

Journal of Physical Chemistry B and C, v114, Issues 23

Crystalline Aluminum Hydroxide Fluorides AlFx(OH)3−x·H2O: Structural Insights from 1H and 2H Solid State NMR and Vibrational Spectroscopy

G. Scholz*, S. Brehme, R. Knig, D. Heidemann and E. Kemnitz*
J. Phys. Chem. C, 2010, 114 (23), pp 10535–10543
DOI: 10.1021/jp1023857

AbstractFor the first time, 1H/2H MAS NMR signals of crystalline hydroxide fluorides AlFx(OH)3−x·H2O, as well as of the dehydrated samples, both with pyrochlore structure, were resolved, identified, and assigned in direct correlation with vibrational bands of respective FT IR spectra. The use of magnetically diluted samples in combination with 1H spin−echo experiments, 2H MAS, and 19F−2H CP and 1H−2H CP MAS NMR experiments gave information on different 2H (1H) sites in relation to present structural motifs known from the crystal structure.

J. Phys. Chem B and C., vol. 114, Issues 22

Multireference Ab Initio Calculations of g tensors for Trinuclear Copper Clusters in Multicopper Oxidases

Steven Vancoillie‡, Jakub Chalupsk§, Ulf Ryde, Edward I. Solomon, Kristine Pierloot‡, Frank Neese¶* and Lubomr Rulek§*

J. Phys. Chem. B, 2010, 114 (22), pp 7692–7702
DOI: 10.1021/jp103098r

Abstract: EPR spectroscopy has proven to be an indispensable tool in elucidating the structure of metal sites in proteins. In recent years, experimental EPR data have been complemented by theoretical calculations, which have become a standard tool of many quantum chemical packages. However, there have only been a few attempts to calculate EPR g tensors for exchange-coupled systems with more than two spins. In this work, we present a quantum chemical study of structural, electronic, and magnetic properties of intermediates in the reaction cycle of multicopper oxidases and of their inorganic models. All these systems contain three copper(II) ions bridged by hydroxide or O2− anions and their ground states are antiferromagnetically coupled doublets. We demonstrate that only multireference methods, such as CASSCF/CASPT2 or MRCI can yield qualitatively correct results (compared to the experimental values) and consider the accuracy of the calculated EPR g tensors as the current benchmark of quantum chemical methods. By decomposing the calculated g tensors into terms arising from interactions of the ground state with the various excited states, the origin of the zero-field splitting is explained. The results of the study demonstrate that a truly quantitative prediction of the g tensors of exchange-coupled systems is a great challenge to contemporary theory. The predictions strongly depend on small energy differences that are difficult to predict with sufficient accuracy by any quantum chemical method that is applicable to systems of the size of our target systems.

Journal of Physical Chemistry B, vol. 114, Issues 18

Molecular Level Characterization of the Inorganic−Bioorganic Interface by Solid State NMR: Alanine on a Silica Surface, a Case Study

Ira Ben Shir†, Shifi Kababya†, Tal Amitay-Rosen‡, Yael S. Balazs† and Asher Schmidt*†

J. Phys. Chem. B, 2010, 114 (18), pp 5989–5996
DOI: 10.1021/jp100114v
Publication Date (Web): April 16, 2010

Abstract: The molecular interface between bioorganics and inorganics plays a key role in diverse scientific and technological research areas including nanoelectronics, biomimetics, biomineralization, and medical applications such as drug delivery systems and implant coatings. However, the physical/chemical basis of recognition of inorganic surfaces by biomolecules remains unclear. The molecular level elucidation of specific interfacial interactions and the structural and dynamical state of the surface bound molecules is of prime scientific importance. In this study, we demonstrate the ability of solid state NMR methods to accomplish these goals. l-[1-13C,15N]Alanine loaded onto SBA-15 mesoporous silica with a high surface area served as a model system. The interacting alanine moiety was identified as the −NH3+ functional group by 15N{1H}SLF NMR. 29Si{15N} and 15N{29Si}REDOR NMR revealed intermolecular interactions between the alanine −NH3+ and three to four surface Si species, predominantly Q3, with similar internuclear N···Si distances of 4.0−4.2 Å. Distinct dynamic states of the adsorbed biomolecules were identified by 15N{13C}REDOR NMR, indicating both bound and free alanine populations, depending on hydration level and temperature. In the bound populations, the −NH3+ group is surface anchored while the free carboxylate end undergoes librations, implying the carboxylate has small or no contributions to surface binding. When surface water clusters grow bigger with increased hydration, the libration amplitude of the carboxyl end amplifies, until onset of dissolution occurs. Our measurements provide the first direct, comprehensive, molecular-level identification of the bioorganic−inorganic interface, showing binding functional groups, geometric constraints, stoichiometry, and dynamics, both for the adsorbed amino acid and the silica surface.



Selective Chemical Shift Assignment of Bacteriochlorophyll a in Uniformly [13C−15N]-Labeled Light-Harvesting 1 Complexes by Solid-State NMR in Ultrahigh Magnetic Field

Anjali Pandit*, Francesco Buda, Adriaan J. van Gammeren†, Swapna Ganapathy and Huub J. M. de Groot
Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
J. Phys. Chem. B, 2010, 114 (18), pp 6207–6215
DOI: 10.1021/jp100688u

Abstract: Magic-angle spinning (MAS) 13C−13C correlation NMR spectroscopy was used to resolve the electronic ground state characteristics of the bacteriochlorophyll a (BChl a) cofactors in light-harvesting 1 (LH1) complexes of Rhodopseudomonas acidophila (strain 10050). The BChl a 13C isotropic chemical shifts of the LH1 complexes are compared to the 13C chemical shifts for BChl a dissolved in acetone-d6 and to 13C NMR data that has been obtained for the B800 and B850 BChl molecules in Rps. acidophila peripheral light-harvesting complexes (LH2). Since both complexes contain BChl a cofactors, we can address the chemical shift variability for specific carbon responses between the two types of antennae. The global shift pattern of the LH1 BChl's resembles the shift patterns of the LH2 α- and β-B850 BChl's, while some carbon responses, in particular the C3 and C31, show significant deviations. A comparison with density functional theory (DFT) shift calculations provides insight into the BChl concomitant structural and electronic interactions in the ground state. The differences in the LH1 BChl observed chemical shifts relative to the 13C responses of BChl a in solution cannot be explained by local side chain interactions, such as hydrogen bonding or nonplanarity of the C3 acetyl, but appear to be dominated by protein-induced macrocycle distortion. Such shaping of the macrocycle will contribute significantly to the red shift of the BChl Qy absorbance band in purple bacterial light-harvesting complexes.




Solid-State 137Ba NMR Spectroscopy: An Experimental and Theoretical Investigation of

Hiyam Hamaed†, Eric Ye‡, Konstantin Udachin§ and Robert W. Schurko*†

J. Phys. Chem. B, 2010, 114 (18), pp 6014–6022
DOI: 10.1021/jp102026m

Abstract: Ultrawideline 137Ba SSNMR spectra of several barium-containing systems (barium nitrate, barium carbonate, barium chlorate monohydrate, barium chloride dihydrate, anhydrous barium chloride, and barium hydrogen phosphate) were acquired at two different magnetic field strengths (9.4 and 21.1 T) using frequency-stepped techniques. The recently reported WURST−QCPMG pulse sequence (O’Dell et al. Chem. Phys. Lett. 2008, 464, 97−102) is shown to be very useful for rapidly acquiring high signal-to-noise 137Ba SSNMR spectra. The breadths of the second-order quadrupolar-dominated spectra and experimental times are notably reduced for experiments conducted at 21.1 T. Analytical simulations of the 137Ba SSNMR spectra at both fields yield the quadrupolar parameters, and in select cases the barium chemical shift anisotropies (CSAs). Quadrupolar interactions dominate the 137Ba powder patterns, with quadrupolar coupling constants, CQ(137Ba), ranging from 7.0 to 28.8 MHz. The 137Ba electric field gradient (EFG) parameters extracted from these spectra are correlated to the local environments at the barium sites, via consideration of molecular symmetry and structure, and first principles calculations of 137Ba EFG tensors performed using CASTEP software. The rapidity with which 137Ba SSNMR spectra can be acquired using the WURST pulse sequence and/or at ultrahigh magnetic fields and the sensitivity of the 137Ba EFG tensor parameters to the changes in the barium environment suggest that 137Ba SSNMR has great potential for structural characterization of a variety of barium-containing materials.

Journal of Physical Chemistry B and C, Vol. 114, Issues 14 and 15

7Li NMR Knight Shifts in Li−Sn Compounds: MAS NMR Measurements and Correlation with DFT Calculations

Emilie Bekaert†, Florent Robert‡, Pierre Emmanuel Lippens‡ and Michel Mntrier*†

J. Phys. Chem. C, 2010, 114 (14), pp 6749–6754
DOI: 10.1021/jp100365u

Abstract: Several Li−Sn crystalline phases, LiSn, Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5, were prepared by ball-milling and studied by 7Li MAS NMR spectroscopy with silica as a diluting agent to avoid field penetration limitations. All phases except for LiSn exhibit exchanged NMR signals at room temperature for the various types of Li present in the unit cells, in the 10 to 100 ppm range. Electronic structure calculations based on first-principles method led to a rather good correlation between the participation of the Li 2s orbital to the density of states (DOS) at the Fermi level and the corresponding NMR Knight shift for the two Li crystallographic types in the case of LiSn, and for the weighted average of the different crystallographic types in the case of the NMR-exchanged signals for the other compounds.



A Novel Phase Transformation Phenomenon in Mesostructured Aluminophosphate

Wanling Shen†, Shenhui Li†, Jun Xu†, Hailu Zhang†, Wei Hu†, Dan Zhou‡, Jianan Zhang‡, Jihong Yu‡, Wujun Xu§, Yao Xu§ and Feng Deng*†

J. Phys. Chem. C, 2010, 114 (15), pp 7076–7084
DOI: 10.1021/jp911959u

Abstract: A novel phase transformation phenomenon that involves two successive phase transformation events was found for the first time in the synthesis of mesostructured aluminophosphate and studied by XRD, TEM, and multinuclear solid-state NMR techniques. The results showed that a hexagonal phase Hex and two lamellar phases, L1 and L2, were formed after hydrothermal treatment for 1, 3, and 50 h, respectively. The status of the surfactant was found to be arrayed interdigitated in a bilayer with a tilt angle in L1 phase but upright in L2 phase. A mechanism that the exciting of the alkane tail of the surfactant together with the condensation of aluminophosphate cooperatively promoted the phase transformation was proposed for the observed phenomenon. Additionally, a ZON microporous structure was found for the first time existing in the framework of the mesostructured aluminophosphate.

J. Phys. Chem. B and C, v114, Issues 13

Distinguishing Polymorphs of the Semiconducting Pigment Copper Phthalocyanine by Solid-State NMR and Raman Spectroscopy

Medhat A. Shaibat†, Leah B. Casabianca†, Diana Y. Siberio-Prez§‡, Adam J. Matzger*‡ and Yoshitaka Ishii*†

Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, and Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109
J. Phys. Chem. B, 2010, 114 (13), pp 4400–4406

Abstract: Cu(II)(phthalocyanine) (CuPc) is broadly utilized as an archetypal molecular semiconductor and is the most widely used blue printing pigment. CuPc crystallizes in six different forms; the chemical and physical properties are substantially modulated by its molecular packing among these polymorphs. Despite the growing importance of this system, spectroscopic identification of different polymorphs for CuPc has posed difficulties. This study presents the first example of spectroscopic distinction of α- and β-forms of CuPc, the most widely used polymorphs, by solid-state NMR (SSNMR) and Raman spectroscopy. 13C high-resolution SSNMR spectra of α- and β-CuPc using very-fast magic angle spinning (VFMAS) at 20 kHz show that hyperfine shifts sensitively reflect polymorphs of CuPc. The experimental results were confirmed by ab initio chemical shift calculations. 13C and 1H SSNMR relaxation times of α- and β-CuPc under VFMAS also showed marked differences, presumably because of the difference in electronic spin correlation times in the two forms. Raman spectroscopy also provided another reliable method of differentiation between the two polymorphs.

Journal of Physical Chemistry B, Vol. 113, Issues 25 to 30

Glassy Dynamics in Nanoconfinement as Revealed by 31P NMR
S. Gradmann, P. Medick and E. A. Rssler*
Experimentalphysik II, Universitt Bayreuth, 95440 Bayreuth, Germany
J. Phys. Chem. B, 2009, 113 (25), pp 8443–8445

Abstract: We investigated the glass former m-tricresyl-phosphate confined in different nanoporous silica matrices with defined pore radii from 2−150 nm. While applying different 31P NMR techniques, we were able to detect the extremely stretched correlation functions extending over 7−8 decades in time and reflecting strong dynamic heterogeneities. The experimental results were explained by a topological model for which the broad distribution of correlation times G(ln τ) becomes inhomogeneous in space; that is, the “local” dynamics given by a correlation time τ(r) depend on the distance from the pore center. As τ(r) changes with temperature, we were able to reintroduce the idea of a dynamic correlation length.



Hierarchical Dynamics of As2P2S8 Quasi-Molecular Units in a Supercooled Liquid in the As−P−S System: A 31P NMR Spectroscopic Study

E. L. Gjersing and S. Sen*
J. Phys. Chem. B, 2009, 113 (25), pp 8514–8519
DOI: 10.1021/jp901388j
Abstract: The dynamics of As2P2S8 quasi-molecular units caged in an As−S network in the supercooled chalcogenide liquid of composition (As2S3)90(P2S5)10 have been studied near the glass transition region (Tg = 468 ≤ T ≤ 628 K) using 31P NMR line shape analysis and spin−lattice relaxation techniques. 31P NMR line shape analysis indicates the presence of isotropic rotational reorientation of As2P2S8 quasi-molecular units at frequencies on the order of tens of kilohertz at T <>


Proton Assisted Recoupling at High Spinning Frequencies†
Jzef R. Lewandowski‡§, Gal De Pape‡, Matthew T. Eddy‡, Jochem Struppe, Werner Maas and Robert G. Griffin*‡
J. Phys. Chem. B, 2009, 113 (27), pp 9062–9069
DOI: 10.1021/jp810280t

2008 marked the Centennial of the American Chemical Society’s Division of Physical Chemistry. To celebrate and to highlight the field of physical chemistry from both historical and future perspectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem. Senior Editors.,

Abstract: We demonstrate the successful application of 13C−13C proton assisted recoupling (PAR) on [U−13C,15N] N-f-MLF-OH and [U−13C,15N] protein GB1 at high magic angle spinning (MAS) frequencies (ωr/2π = 65 kHz). Specifically, by combining PAR mixing with low power heteronuclear decoupling (ω1H/2π 16 kHz) and high spinning frequencies, we obtain high resolution 2D spectra displaying long-range 13C−13C contacts from which distance estimates can be extracted. These experiments therefore demonstrate the possibility of performing high resolution structural studies in the limit of high spinning frequency and low power 1H decoupling, a regime which optimizes the resolution of protein samples and preserves their integrity.



Self-Diffusion and Mutual Diffusion of Small Molecules in High-Set Curdlan Hydrogels Studied by 31P NMR
Marc-Andr Gagnon and Michel Lafleur*
J. Phys. Chem. B, 2009, 113 (27), pp 9084–9091
DOI: 10.1021/jp811105p

Abstract: Self-diffusion and mutual diffusion are two different transport mechanisms experimentally characterized on different length and time scales. NMR spectroscopy is a highly suitable technique to characterize these two phenomena as both mechanisms can be studied on the same system and in the same experimental conditions. Pulsed field gradient (PFG) NMR was used to measure the self-diffusion whereas 31P NMR profiling provided an approach to determine the mutual diffusion coefficients. We have characterized the diffusion of phosphate, trimetaphosphate, alendronate, and d-glucose-6-phosphate in hydrogels prepared with 10% (w/v) curdlan, a bacterial polysaccharide built of linear (1→3)-β-d-glucose repeating units. These solutes are small compared to the average pore size of the hydrogel, as inferred from environmental scanning electron microscopy (eSEM). Our results show that the self- and mutual-diffusion coefficients of small molecules in curdlan hydrogels are similar and are reduced by 30% compared to those measured in aqueous solutions. These observations are validated for the complete series of investigated analytes. It is therefore concluded that, for this system, the analyte diffusion in the gel is essentially reduced because of interactions at the molecular level and that the open structure of this gel has a very limited influence at the mesoscopic length scale. A literature survey indicates that these conditions prevail for the large majority of the systems that have been investigated up to now.


Validating a Strategy for Molecular Dynamics Simulations of Cyclodextrin Inclusion Complexes through Single-Crystal X-ray and NMR Experimental Data: A Case Study
Giuseppina Raffaini*†, Fabio Ganazzoli†, Luciana Malpezzi†, Claudio Fuganti†, Giovanni Fronza‡, Walter Panzeri‡ and Andrea Mele*†
J. Phys. Chem. B, 2009, 113 (27), pp 9110–9122
DOI: 10.1021/jp901581e

Abstract: A theoretical and experimental study about the formation and structure of the inclusion complex (−)-menthyl-O-β-D-glucopyranoside 1 with β-cyclodextrin (β-CD) 2 is presented as paradigmatic case study to test the results of molecular dynamics (MD) simulations. The customary methodological approach—the use of experimental geometrical parameters as restraints for MD runs—is logically reversed and the calculated structures are a posteriori compared with those obtained from NMR spectroscopy in D2O solution and single crystal X-ray diffraction so as to validate the simulation procedure. The guest molecule 1 allows for a broad repertoire of intermolecular interactions (dipolar, hydrophobic, hydrogen bonds) concurring to stabilize the host−guest complex, thus providing the general applicability of the simulation procedure to cyclodextrin physical chemistry. Many starting geometries of the host−guest association were chosen, not assuming any a priori inclusion. The simulation protocol, involving energy minimization and MD runs in explicit water, yielded four possible inclusion geometries, ruling out higher-energy outer adducts. By analysis of the average energy at room temperature, the most stable geometry in solution was eventually obtained, while the kinetics of formation showed that it is also kinetically favored. The reliability of such geometry was thoroughly checked against the NOE distances via the pair distribution functions, that is, the statistical distribution of intermolecular distances among selected diagnostic atoms calculated from the MD trajectories at room temperature. An analogous procedure was adopted both with implicit solvent and in vacuo. The most stable geometry matched that found with explicit solvent but major differences were observed in the relative stability of the metastable complexes as a consequence of the lack of hydration on the polar moiety of the guest. Finally, a control set of geometrical parameters of the thermodynamically favored complex matched the corresponding one obtained from the X-ray structure, while local conformational differences were indicative of packing effects.


1H Solid-State NMR Investigation of Structure and Dynamics of Anhydrous Proton Conducting Triazole-Functionalized Siloxane Polymers
mit Akbey†, Sergio Granados-Focil‡, E. Bryan Coughlin§, Robert Graf† and Hans Wolfgang Spiess*†
J. Phys. Chem. B, 2009, 113 (27), pp 9151–9160
DOI: 10.1021/jp9030909

Abstract:1H MAS solid-state NMR methods are applied to elucidate the conduction mechanism of an anhydrous proton conducting triazole-functionalized polysiloxane. At temperatures below T = 260 K, hydrogen bonding between neighboring heterocycles is observed and a dimer formation can be excluded. From the temperature dependence of 1H MAS NMR spectra, different dynamic processes of the triazole ring contributing to the proton conduction process are qualitatively and quantitatively analyzed and detailed insight into the conduction mechanism and temperature-dependent structural changes is obtained. Although the dynamics processes on the molecular level are qualitatively in good agreement with the findings from macroscopic conductivity measurements, temperature-dependent factors on mesoscopic scales beyond the local molecular mobility influence the macroscopic conductivity and hamper quantitative interpretation.


Intra- and Intermolecular Effects on 1H Chemical Shifts in a Silk Model Peptide Determined by High-Field Solid State 1H NMR and Empirical Calculations
Yu Suzuki†, Rui Takahashi†, Tadashi Shimizu‡, Masataka Tansho‡, Kazuo Yamauchi†, Mike P. Williamson§ and Tetsuo Asakura*†
J. Phys. Chem. B, 2009, 113 (29), pp 9756–9761
DOI: 10.1021/jp903020p

Abstract: A combination of solid state 1H NMR chemical shift measurements and empirical chemical shift calculations has been used to interpret 1H solid state chemical shifts of a model peptide (Ala-Gly)15 for the crystalline domain of Bombyx mori silk fibroin in silk I and silk II structures, including a treatment of both intra- and intermolecular arrangements. Silk I and silk II are the structures of silk fibroin before and after spinning, respectively. Two peaks with equal intensity were observed for the amide protons of (AG)15 in silk I, whereas only one broad peak was observed for silk II, reflecting a difference of 1.1 ppm in Ala HN shift between silk I and silk II, but a difference of only 0.2 ppm in Gly HN shift. Chemical shift calculations predicted chemical shifts that are in good agreement with the experimental observations and showed that the origin of these chemical shift differences was predominantly the magnetic anisotropy effect from the C═O bond that hydrogen bonds with HN, which has a more favorable geometry for Ala HN in silk II than for the other HN. This result shows that we could distinguish between proton chemical shift effects arising from intermolecular interactions and those from intramolecular interactions by combining observation of the solid state 1H NMR chemical shift and empirical chemical shift calculation.


1H Photo-CIDNP Enhancements in Heteronuclear Correlation NMR Spectroscopy
Ashok Sekhar and Silvia Cavagnero*
J. Phys. Chem. B, 2009, 113 (30), p 10548
DOI: 10.1021/jp905605u
Publication Date (Web): July 9, 2009
Copyright © 2009 American Chemical Society

J. Phys Chem B and C, vol. v113, i24

1H Photo-CIDNP Enhancements in Heteronuclear Correlation NMR Spectroscopy

Ashok Sekhar and Silvia Cavagnero*
J. Phys. Chem. B, 2009, 113 (24), pp 8310–8318

Abstract:Photochemically induced dynamic nuclear polarization (photo-CIDNP) is usually employed as a probe of solvent exposure in biomolecular NMR. The potential of the photo-CIDNP effect for sensitivity enhancement, however, remains poorly explored. Here, we introduce 1H-photo-CIDNP in heteronuclear correlation spectroscopy at low laser irradiation power (1 W), and compare the sensitivity of various 1H-photo-CIDNP-enhanced- (HPE) 1H−15N heteronuclear correlation pulse sequences, including HSQC, HMQC, and SOFAST-HMQC, in terms of their ability to detect the Trp indole Hε1 resonance. Both Trp and the Trp-containing protein apoHmpH were analyzed using flavin mononucleotide as photosensitizer in aqueous solutions either containing or lacking urea. We find that 1H−15N photo-CIDNP-SOFAST-HMQC, denoted here as HPE-SOFAST-HMQC, yields a 2-fold higher signal-to-noise per unit time than the parent SOFAST-HMQC, for the solvent-exposed Trp of urea-unfolded apoHmpH. Thus, HPE-SOFAST-HMQC is the most sensitive heteronuclear correlation pulse sequence for the detection of solvent-exposed Trp.



Solid-State NMR Study of Nanodiamonds Produced by the Detonation Technique
Marc Dubois*†, Katia Gurin†, Elodie Petit†, Nicolas Batisse†, Andr Hamwi†, Naoki Komatsu‡, Jrme Giraudet§, Pascal Pirotte§ and Francis Masin*§
J. Phys. Chem. C, 2009, 113 (24), pp 10371–10378

Abstract:Nanodiamonds obtained by the detonation method have been investigated by means of solid-state magnetic nuclear resonance (NMR) and electron paramagnetic resonance. 13C and 1H magic angle spinning (MAS) NMR and 13C MAS NMR with 1H to 13C cross-polarization allow the determination of surface-hydrogenated groups (CH, CH2, and COH) and the quasi-absence of an sp2 carbon fullerene-like shell on the diamond surface to be underlined. The 1H and 13C spin−lattice relaxation time (T1) and second moment measurements are presented as a function of the temperature. Relaxation is shown to be mainly caused by paramagnetic centers in the case of 13C nuclei, whereas the presence of a molecular motion with an activation energy of 11.15 kJ·mol−1 is involved for 1H nuclei.



Identification of Mixed Valence Vanadium in ETS-10 Using Electron Paramagnetic Resonance, 51V Solid-State Nuclear Magnetic Resonance, and Density Functional Theory Studies

Kristopher Ooms† and Tatyana Polenova
Michael J. Nash‡ and Raul F. Lobo*
J. Phys. Chem. C, 2009, 113 (24), pp 10477–10484

Abstract: Microporous vanadium-substituted titanosilicate ETS-10 solids are promising photocatalysts for decomposition of organic molecules. The dopant vanadium metal modulates the electronic environment of the titanosilicate matrix and plays a major role in the enhancement of the photocatalytic activity. However, the local electronic and geometric structure of the vanadium sites in these materials is a subject of controversy. Using vanadium electron paramagnetic resonance (EPR) and 51V nuclear magnetic resonance (NMR) spectroscopy, we have characterized the local environments of the vanadium sites in vanadium-substituted ETS-10 samples with different vanadium loadings. The measurements reveal clearly the presence of V(IV) and V(V) oxidation states. The EPR results suggest that V(IV) is in octahedral sites and, therefore, must substitute for Ti in the framework. 51V NMR studies indicate that the V(V) species are adjacent to the V(IV) species in most cases on the basis of significant electron−nuclear dipolar interaction between the V(V) nuclei and the unpaired electron on V(IV). The NMR chemical shift and electric field gradient parameters estimated from the NMR spectra are used in conjunction with density functional theory calculations to propose a model where the V(V) species preferentially occupy sites at the ends of the octahedral chains.



Long-Time Scale Ionic Dynamics in Dense Clay Sediments Measured by the Frequency Variation of the 7Li Multiple-Quantum NMR Relaxation Rates in Relation with a Multiscale Modeling
Patrice Porion*, Anne Marie Faugre and Alfred Delville*
J. Phys. Chem. C, 2009, 113 (24), pp 10580–10597

Abstract: 7Li NMR relaxation measurements under spin-locking conditions are used to probe the dynamical properties of the lithium counterions within dense dispersions of charged anisotropic nanoplatelets. By simultaneously measuring the T1ρ and T2ρ relaxation times in addition to triple-quantum filtered relaxation times under the same spin-locking conditions, it is possible to separately quantify the contributions from the quadrupolar and the heterogeneous dipolar relaxation mechanisms. Thanks to the contribution from the residual quadrupolar coupling felt by the condensed lithium counterions, that procedure allows a broad dynamical range to be probed by performing spin-locking relaxation measurements using a limited number of irradiation powers. As illustrated by a multiscale modeling of the lithium diffusion and relaxation within such heterogeneous system, the frequency variation of the spectral densities characterizing the decorrelation of the quadrupolar coupling is a sensitive probe of the ionic mobility and the structure of the colloidal dispersion.



Proton Dynamics in Layered Double Hydroxides: A 1H T1 Relaxation and Line Width Investigation

Marc X. Reinholdt*†, Panakkattu K. Babu†‡§ and R. James Kirkpatrick†
J. Phys. Chem. C, 2009, 113 (24), pp 10623–10631

Abstract:The investigation of the dynamics of water and organic species confined in minerals or adsorbed at their surface is of significant geochemical, environmental, catalytic, biomedicine, and life’s growth interests but is poorly understood on the molecular scale. This work explores the behavior of water molecules and glutamate species adsorbed on and between the double hydroxide layers of hydrotalcite [HT; (Mg2Al)(OH)6A−·nH2O, where A− is a counteranion which may bear different charges] and compares the results to those for HT containing small inorganic anions. The relative humidity (RH) dependence of the 1H T1 relaxation rates for all samples reveals the existence of two separate spin systems with 1/T1 relaxation rates differing by a factor of approximately 2 × 103. The static 1H spectral line widths allow assigning the fast relaxing protons to the fixed “static” interlayer and adsorbed species—i.e. bound water, bound organic species, and most of the structural hydroxyl groups (-OH)—and the slow ones to the “mobile” species—i.e. free water and solvated organic molecules and some of the structural -OH groups.




Transformation of AlPO-53 to JDF-2: Reversible Dehydration of a Templated Aluminophosphate Studied by MAS NMR and Diffraction

Sharon E. Ashbrook*†, Marica Cutajar‡, John M. Griffin†, Zoe A. D. Lethbridge§, Richard I. Walton*§ and Stephen Wimperis*‡
J. Phys. Chem. C, 2009, 113 (24), pp 10780–10789

Abstract:We describe a detailed study of the aluminum phosphate AlPO-53 in both its as-made and calcined forms. In its as-made state, AlPO-53(A), the material is templated by methylammonium cations and contains occluded water molecules and also hydroxide ions that bridge pairs of aluminum atoms, increasing their coordination number to 5. Solid-state NMR experiments confirm the local environment of the aluminum and phosphorus atoms proposed in a previous structural model from powder X-ray diffraction. 31P NMR shows the presence of four distinct resonances with an intensity ratio of 1:1:2:2, consistent with the expected six crystallographic P sites. 27Al triple-quantum MAS NMR resolves five aluminum peaks, two with NMR parameters characteristic of four-coordinate Al and three of five-coordinate Al. One of these latter signals has a greater intensity than that of the others, consistent with the presence of two overlapping signals from two distinct crystallographic Al sites. First-principles calculations of NMR parameters provide a complete spectral assignment and confirm our interpretation of unresolved spectra. AlPO-53(A) is found to convert easily into a second crystalline phase on moderate heating (upon spinning in the NMR rotor for an extended period, for example), and variable-temperature powder X-ray experiments, together with TGA, suggest that this is a dehydration process yielding a second aluminophosphate, JDF-2. This is confirmed using both 31P and 27Al NMR, with the spectral assignment of JDF-2 supported by first-principles calculations. Calcination of AlPO-53(A) or of the dehydrated material, JDF-2, at 300 °C yields the microporous open-framework material AlPO-53(B), a tetrahedral network with three Al and three P sites, as confirmed by NMR and first-principles calculations. In addition to demonstrating the power of the combined use of NMR, first-principles calculations, and diffraction for detailed structural investigations, we show that the possibility of a reversible dehydration in as-made AlPO-53 and similar systems is an important consideration in structural studies and provides evidence that the published structural model for AlPO-53(A) may be incomplete.

J. Phys. Chem. B. , Vol. 113, Issues 16-19

Solid State NMR Study and Density Functional Theory (DFT) Calculations of Structure and Dynamics of Poly(p-xylylenes)

A. Sroka-Bartnicka†, S. Olejniczak†, W. Ciesielski†, A. Nosal‡, H. Szymanowski‡, M. Gazicki-Lipman‡ and M. J. Potrzebowski*†

J. Phys. Chem. B, 2009, 113 (16), pp 5464–5472
DOI: 10.1021/jp900788m

Abstract:High resolution solid state 13C nuclear magnetic resonance (SS NMR) measurements were carried out on poly(p-xylylene) (PPX). The samples comprised vapor-deposited specimens as well as pure α and β polymorphs of this polymer. The measurements were performed using cross-polarization and magic angle spinning (CP/MAS) techniques. Density functional theory gauge-including-atomic-orbital (DFT GIAO) calculations of NMR shielding parameters 13C σii were performed for the optimized geometry and structure of a xylylene trimer, acquired from the X-ray data, including intermolecular interactions. Two-dimensional phase adjusted spinning sideband (2D PASS) correlation was employed for the assignment of the values of the principal elements 13C δii of the chemical shift tensor (CST). A comparative analysis of shielding (σii) versus chemical shift (δii) parameters showed substantial differences between the molecular dynamics of α and β polymorphs. This observation was further supported by the measurements of 13C T1 relaxation times and the analysis of cross-polarization kinetics. Frequency switched Lee−Goldburg heteronuclear correlation (FSLG HETCOR) for the 1H−13C system was used in order to analyze molecular packing in both polymorphs. As a result of all of the above measurements, new insight into the mechanism of thermal phase transition from the α to the β polymorph of poly(p-xylylene) is presented.



Approximate Reconstruction of Continuous Spatially Complex Domain Motions by Multialignment NMR Residual Dipolar Couplings

Charles K. Fisher and Hashim M. Al-Hashimi*
J. Phys. Chem. B, 2009, 113 (18), pp 6173–6176
DOI: 10.1021/jp900411z

Abstract:NMR spectroscopy is one of the most powerful techniques for studying the internal dynamics of biomolecules. Current formalisms approximate the dynamics using simple continuous motional models or models involving discrete jumps between a small number of states. However, no approach currently exists for interpreting NMR data in terms of continuous spatially complex motional paths that may feature more than one distinct maneuver. Here, we present an approach for approximately reconstructing spatially complex continuous motions of chiral domains using NMR anisotropic interactions. The key is to express Wigner matrix elements, which can be determined experimentally using residual dipolar couplings, as a line integral over a curve in configuration space containing an ensemble of conformations and to approximate the curve using a series of geodesic segments. Using this approach and five sets of synthetic residual dipolar couplings computed for five linearly independent alignment conditions, we show that it is theoretically possible to reconstruct salient features of a multisegment interhelical motional trajectory obtained from a 65 ns molecular dynamics simulation of a stem−loop RNA. Our study shows that the 3-D atomic reconstruction of complex motions in biomolecules is within experimental reach.





A Dynamic Magic Angle Spinning NMR Study of the Local Mobility of Alanine in an Aqueous Environment at the Inner Surface of Mesoporous Materials

Tal Amitay-Rosen†, Shifi Kababya‡ and Shimon Vega*†
J. Phys. Chem. B, 2009, 113 (18), pp 6267–6282
DOI: 10.1021/jp810572r
Abstract: Dynamic deuterium magic angle spinning NMR has been applied to study the slow motion of small molecules close to a silica surface. In particular, alanine-d3 molecules dissolved in an aqueous solution were loaded into the pores of the mesoporous materials SBA-15 and MCM-41. Deuterium spectra were measured as a function of the water content of these materials and the temperature. From the analysis of these spectra and the corresponding proton spectra, using a simple molecular exchange model, relatively slow desorption rates of the binding of alanine to the inner pore surface were obtained and were correlated with the low proton concentrations at the pore surfaces.




Proton Mobilities in Phosphonic Acid-Based Proton Exchange Membranes Probed by 1H and 2H Solid-State NMR Spectroscopy

Gunther Brunklaus*, Siri Schauff, Dilyana Markova, Markus Klapper, Klaus Mllen and Hans-Wolfgang Spiess
Max-Planck-Institut fr Polymerforschung, Postfach 3148, D-55021 Mainz, Germany
J. Phys. Chem. B, 2009, 113 (19), pp 6674–6681
Abstract:Two novel phosphonic acid-based “dry” proton exchange membrane materials that may allow for fuel cell operation above 100 °C have been prepared and characterized via solid-state 1H and 2H MAS NMR spectroscopy. We obtained information on both the nature of hydrogen bonding and local proton mobilities among phosphonic acid moieties. In particular, 2H MAS NMR line shape analysis yielded apparent activation energies of the underlying motional processes. Using this approach, we have investigated both a model compound and a novel PEM system. It was found that the relation of estimated hydrogen-bond strength and local proton mobility accessible by solid-state NMR and bulk proton conductivity is complex. Improvements through admixture of a second component with protogenic groups are suggested.