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>

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

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

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

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

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

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.

J. Phys. Chem. C., vol. 114, Issue 20

1H, 29Si, and 27Al MAS NMR as a Tool to Characterize Volcanic Tuffs and Assess Their Suitability for Industrial Applications

Piero Ciccioli†, Paolo Plescia‡ and Donatella Capitani*§

J. Phys. Chem. C, 2010, 114 (20), pp 9328–9343
DOI: 10.1021/jp103082h
Publication Date (Web): April 29, 2010

Abstract: The type and quality of the information provided by the direct analysis of volcanic tuffs by 1H, 29Si, and 27Al NMR were investigated. At this aim, five tuffs, characterized by different origin, bonding mechanism, and clast composition, were used as test materials. Results consistent with the different nature of the tuff matrix and mineral composition were obtained. While the relative content of Al in the crystal and amorphous phase was determined by 27Al MAS and 3Q MAS NMR, the prevalent glassy or zeolitic nature of the matrix was assessed by 29Si and 1H MAS NMR. Zeolites present at levels as low as 15% w/w were detected by 29Si MAS NMR, and in some tuffs, identification of their framework type was performed together with the determination of the Si/Al ratio and, for the first time, of their configurational entropy. Data obtained were coherent with those provided by X-ray fluorescence (XRF), X-ray powder diffraction (XPRD), thermogravimetric analysis (TGA), differential thermal gravimetry (DTG), cation exchange capacity (CEC) determinations, and scanning electron microscopy, used in both backscattering imaging mode (SEM) and for elemental analysis (SEM-EDS). Results show that, under favorable conditions, solid state NMR techniques can provide a comprehensive view of the chemical and physicochemical behavior of a tuff. A combined use of these techniques is suitable for characterization of tuffs on a routine basis, and can be particularly useful to decide if a material is suitable for industrial applications.




The Comparison in Dehydrogenation Properties and Mechanism between MgCl2(NH3)/LiBH4 and MgCl2(NH3)/NaBH4 Systems

L. Gao†, Y. H. Guo†, Q. Li‡ and X. B. Yu*†

J. Phys. Chem. C, 2010, 114 (20), pp 9534–9540
DOI: 10.1021/jp103012t
Publication Date (Web): May 5, 2010

Abstract: The dehydrogenation properties and mechanism of MgCl2(NH3)/MBH4 (here, M is Li or Na) were investigated by thermogravimetric analysis and mass spectrometry, X-ray diffraction (XRD), solid-state 11B NMR, Fourier transform infrared, and differential scanning calorimetry (DSC). As for the MgCl2(NH3)/LiBH4 system, it was found that a new phase, namely, MgCl2(NH3)·LiBH4, to which the following dehydrogenation relates, is formed after ball milling. Judging from the reaction products, it is confirmed that MgCl2 is inclined to work as an ammonia carrier, and the ligand NH3, transferring from MgCl2, is able to combine with the LiBH4 to release H2 with a trace of ammonia at ca. 240 °C. With the increase of LiBH4 content in the mixture, the emission of ammonia was totally suppressed, and Mg(BH4)2 was produced by the decomposition reaction of MgCl2 with the excessive LiBH4 after the ligand NH3 was exhausted, resulting in an improved dehydrogenation in the whole system. As for the MgCl2(NH3)/NaBH4 system, no new phases are detected by XRD after ball milling. The MgCl2 works as a BH4− acceptor, and the ligand NH3 stays with Mg2+ to combine with the BH4−, which transfers from NaBH4 to Mg2+, resulting in a totally different decomposition route and thermal effects as compared with the MgCl2(NH3)/LiBH4 system. DSC results revealed that the decomposition of MgCl2(NH3)/LiBH4 presented an exothermic reaction with an enthalpy of −3.8 kJ mol−1 H2, while the MgCl2(NH3)/NaBH4 showed two apparent endothermic peaks associated with its two-step dehydrogenation with enthalpies of 8.6 and 2.2 kJ mol−1 H2, respectively. Moreover, the MS profiles of the MgCl2(NH3)/2NaBH4, with excessive BH4−, still released a trace of NH3, indicating that the NaBH4 is not so effective in suppressing the emission of NH3 as LiBH4 did.

J. Phys. Chem. C., vol. 114, Issue 17

Solid-State 65Cu and 31P NMR Spectroscopy of Bis(triphenylphosphine) Copper Species

Bryan E. G. Lucier†, Joel A. Tang†, Robert W. Schurko*†, Graham A. Bowmaker‡, Peter C. Healy§ and John V. Hanna*

J. Phys. Chem. C, 2010, 114 (17), pp 7949–7962
DOI: 10.1021/jp907477m

Abstract: Frequency-stepped ultrawideline (UW) 65Cu solid-state NMR (SSNMR) experiments have been performed on a series of nine bis(triphenylphosphine) copper(I) species, with eight of these having an oxyanion-based ligand and one a borohydride ligand. These copper atoms reside in spherically asymmetric environments featuring two covalent Cu−P bonds and coordination from single bidentate ligands. The QCPMG pulse sequence was utilized in NMR experiments on all of the samples, along with the WURST-QCPMG sequence on select samples, to acquire UWNMR spectra of high quality. In all cases, large 65Cu quadrupolar coupling constants (CQ) between 40.8 and 51.7 MHz are observed, and are confirmed by NQR measurements. The immense quadrupolar interactions and their correspondingly large contributions to the central-transition powder patterns make accurate quantification of copper chemical shift anisotropy (CSA) difficult, though CSA effects are observed. 1H−31P CP/MAS NMR spectra reveal one-bond J-couplings, 1J(65/63Cu, 31P), for all complexes, as well as the presence of residual dipolar coupling, which enables determinations of both the sign of CQ and the orientation of the EFG tensor with respect to the Cu−P dipolar vector (both of which are unavailable from standard 65Cu SSNMR experiments). The 65Cu EFG parameters and 1J(65/63Cu, 31P) coupling constants are sensitive to the local geometry and bond lengths about the Cu center. Ab initio calculations are used to confirm experimentally predicted orientations of the Cu EFG tensors, to predict experimental CQ, ηQ, and CS tensor values, and to aid in identifying relationships between the copper NMR parameters and molecular structures. This combination of experimental and theoretical NMR data enables the correlation of symmetry and local structure with copper NMR parameters, further extending the applicability of copper SSNMR spectroscopy to a wide variety of copper-containing systems.

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.

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. C, vol. 113, Issues 20 to 23

MgCl2·4(CH3)2CHOH: A New Molecular Adduct and Super Active Polymerization Catalyst Support

K. S. Thushara†, Renny Mathew‡, T. G. Ajithkumar‡, P. R. Rajamohanan‡, Sumit Bhaduri*§ and Chinnakonda S. Gopinath*†

J. Phys. Chem. C, 2009, 113 (20), pp 8556–8559
DOI: 10.1021/jp9026546
Abstract: A new molecular adduct, MgCl2·4(CH3)2CHOH, has been synthesized and characterized for structural aspects and demonstrated for super active ethylene polymerization activity with TiCl4 to ultrahigh molecular weight polyethylene in high yield.



Structural Characterization of Hydrothermal Carbon Spheres by Advanced Solid-State MAS 13C NMR Investigations

Niki Baccile*†‡, Guillaume Laurent‡, Florence Babonneau‡, Franck Fayon§, Maria-Magdalena Titirici† and Markus Antonietti†

J. Phys. Chem. C, 2009, 113 (22), pp 9644–9654
DOI: 10.1021/jp901582x
Abstract: The local structure of carbon spheres obtained via the hydrothermal carbonization process is characterized by using a combination of advanced solid-state 13C NMR techniques. Glucose was chosen as the starting product because it offers the possibility of 13C isotopic enrichment and is regarded as a model compound for more complex polysaccharides and biomass, as reported in recent studies. A number of 13C solid-state MAS NMR techniques (single-pulse, cross-polarization, inversion recovery cross-polarization, INEPT, 13C−13C proton-driven magnetization exchange, and 13C−13C double-quantum−single-quantum correlation experiments) were combined to retrieve information about binding motifs and C−C closest neighbor relations. We found that the core of the carbonaceous scaffold is composed of furan rings cross-linked by domains containing short keto-aliphatic chains instead of otherwise expected graphene-type sheets, as mainly reported either for hydrothermal carbon spheres or for biomass-related carbons obtained by low-temperature (<350>




Optimized Synthesis and Structural Characterization of the Borosilicate MCM-70

Dan Xie†, Lynne B. McCusker*†, Christian Baerlocher†, Lisa Gibson#‡, Allen W. Burton‡ and Son-Jong Hwang§
J. Phys. Chem. C, 2009, 113 (22), pp 9845–9850
DOI: 10.1021/jp903500q

Abstract:A structure analysis of the borosilicate zeolite MCM-70, whose synthesis had been patented in 2003, was reported in 2005. Unfortunately, that structure analysis was somewhat ambiguous. Anisotropic line broadening made it difficult to model the peak shape, some peaks in the electron density map could not be interpreted satisfactorily, the framework geometry was distorted, and MAS NMR results were partially contradictory. In an attempt to resolve some of these points, an optimization of the synthesis was undertaken, and the structure was reinvestigated. The structure was solved from synchrotron powder diffraction data collected on an as-synthesized sample (Pmn21, a = 13.3167(1) Å, b = 4.6604(1) Å, c = 8.7000(1) Å) using a powder charge-flipping algorithm. The framework topology, with a 1-dimensional, 10-ring channel system, is identical to the one previously reported. However, the B in this new sample was found to be ordered in the framework, fully occupying one of the four tetrahedral sites. Two extra-framework K+ ion positions, each coordinated to five framework O atoms and one water molecule, were also found. The solid state 29Si, 11B and 1H NMR results are fully consistent with this ordered structure.



LiSc(BH4)4 as a Hydrogen Storage Material: Multinuclear High-Resolution Solid-State NMR and First-Principles Density Functional Theory Studies

Chul Kim†, Son-Jong Hwang*†, Robert C. Bowman, Jr.‡, Joseph W. Reiter‡, Jason A. Zan‡, James G. Kulleck‡, Houria Kabbour§#, E. H. Majzoub and V. Ozolins

J. Phys. Chem. C, 2009, 113 (22), pp 9956–9968
DOI: 10.1021/jp9011685
Abstract: A lithium salt of anionic scandium tetraborohydride complex, LiSc(BH4)4, was studied both experimentally and theoretically as a potential hydrogen storage medium. Ball milling mixtures of LiBH4 and ScCl3 produced LiCl and a unique crystalline hydride, which has been unequivocally identified via multinuclear solid-state nuclear magnetic resonance (NMR) to be LiSc(BH4)4. Under the present reaction conditions, there was no evidence for the formation of binary Sc(BH4)3. These observations are in agreement with our first-principles calculations of the relative stabilities of these phases. A tetragonal structure in space group I (#82) is predicted to be the lowest energy state for LiSc(BH4)4, which does not correspond to structures obtained to date on the crystalline ternary borohydride phases made by ball milling. Perhaps reaction conditions are resulting in formation of other polymorphs, which should be investigated in future studies via neutron scattering on deuterides. Hydrogen desorption while heating these Li−Sc−B−H materials up to 400 °C yielded only amorphous phases (besides the virtually unchanged LiCl) that were determined by NMR to be primarily ScB2 and [B12H12]−2 anion containing (e.g., Li2B12H12) along with residual LiBH4. Reaction of a desorbed LiSc(BH4)4 + 4LiCl mixture (from 4LiBH4/ScCl3 sample) with hydrogen gas at 70 bar resulted only in an increase in the contents of Li2B12H12 and LiBH4. Full reversibility to reform the LiSc(BH4)4 was not found. Overall, the Li−Sc−B−H system is not a favorable candidate for hydrogen storage applications.



A 47/49Ti Solid-State NMR Study of Layered Titanium Phosphates at Ultrahigh Magnetic Field

Jianfeng Zhu†, Nick Trefiak‡, Tom K. Woo‡ and Yining Huang*†

J. Phys. Chem. C, 2009, 113 (23), pp 10029–10037
DOI: 10.1021/jp901235w
Abstract: Layered titanium phosphates (TiPs) have many potentially important applications in ion exchange, catalysis, intercalation, and sorption. Characterization of metal local environments by solid-state 47/49Ti NMR has been difficult due to many unfavorable 47/49Ti NMR properties. In this work, we have directly characterized the local structures around Ti in several representative layered TiPs, including α-, β-, and γ-TiP, by examining the 47/49Ti static NMR spectra of these materials at an ultrahigh magnetic field of 21.1 T. The 47/49Ti chemical shielding and electric field gradient (EFG) tensors have been extracted from spectral analysis. The observed 47/49Ti spectra are mainly determined by the second-order quadrupolar interactions. The quadrupole coupling constants (CQ) are sensitive to the distortion of the TiO6 octahedron in this series of layered TiPs. Quantum mechanical calculations have been performed on several model clusters as well as periodic systems. The results indicate that, in addition to the oxygens in the first coordination sphere of Ti, the atoms in the second and third coordination spheres and beyond also have significant effects on the EFG at the metal center, and this long-range effect contributes substantially to the CQ. A relationship between observed CQ and the Ti−O bond length distortion parameter appears to exist, and this empirical correlation is also confirmed by theoretical calculations. Using sodium-exchanged α-TiP (α-Na-TiP) with an unknown structure as an example, we show that the 47/49Ti NMR spectra can provide partial information on the local environment of the metal center. For this material, the ion exchange does not affect the Ti local environment significantly. It appears that the layer in α-TiP is more robust compared to that of the zirconium analogue.



Electron−Nuclear Spin Dynamics in a Bacterial Photosynthetic Reaction Center

Eugenio Daviso†, A. Alia†, Shipra Prakash†, Anna Diller†, Peter Gast‡, Johan Lugtenburg†, Jrg Matysik*† and Gunnar Jeschke§

J. Phys. Chem. C, 2009, 113 (23), pp 10269–10278
DOI: 10.1021/jp900286q
Abstract: The solid-state photo-CIDNP effect is known to occur in natural photosynthetic reaction centers (RCs) where it can be observed by magic-angle spinning (MAS) NMR as strong modification of signal intensities under illumination compared to experiments performed in the dark. The origin of the effect has been debated. In this paper, we report time-resolved photo-CIDNP MAS NMR data of reaction centers of quinone depleted Rhodobacter sphaeroides. It is demonstrated that the build-up of nuclear polarization on the primary donor and the bacteriopheophytin acceptor depends on the presence and lifetimes of the molecular triplet states of the donor and carotenoid. Analysis of the data proves that up to three electron−nuclear spin-coupling mechanisms and two transient effects are working concomitantly in the spin-chemical machinery of the reaction center

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

Hyperpolarized 129Xe NMR Investigation of Ammonia Borane in Mesoporous Silica

Li-Qiong Wang*, Abhi Karkamkar, Tom Autrey and Gregory J. Exarhos
Fundamental & Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354
J. Phys. Chem. C, 2009, 113 (16), pp 6485–6490
DOI: 10.1021/jp810994p

Abstract: Hyperpolarized (HP) 129Xe NMR was used to probe the porosity of mesoporous silica (MCM) infused with ammonia borane (AB). Variable-temperature HP 129Xe NMR measurements have been systematically carried out on a series of MCM-41 materials with AB loading ranging from 33 to 75 wt % (1:2 to 3:1 AB:MCM). Three distinct types of pore environments are clearly evident: pristine mesopores, pores coated with AB inside the meso-channels, and interparticle spacing formed from AB aggregates outside the meso-channels. We found similarly uniform coating of AB on mesoporous silica channels with 1:2 and 1:1 AB:MCM loading (ratio of weight percent). When the loading of AB to MCM is greater than 1:1, AB starts to aggregate outside the meso-channels. Further increases in loading (≥3:1) result in the formation of partially blocked meso-channels as a result of excessive AB. The detailed information obtained from this study on how supported AB resides in nanoporous channels and how it evolves with the increase of AB loading is helpful for the rational design of novel materials with optimal hydrogen storage and release properties



High-Resolution 89Y and 45Sc NMR Spectroscopic Study of Short-Range Structural Order in Nanocrystalline Y- and Sc-doped CeO2 and ZrO2

Pragati Jain, Hugo J. Avila-Paredes, Christine Gapuz, Sabyasachi Sen* and Sangtae Kim

J. Phys. Chem. C, 2009, 113 (16), pp 6553–6560
Abstract: The effect of crystallite size on cation coordination environments and oxygen vacancy ordering has been investigated in micro- and nanocrystalline Y- and Sc-doped ZrO2 and CeO2 by using high-resolution 89Y and 45Sc magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Our results indicate that irrespective of crystallite size the vacancies are preferentially associated with the host cation (i.e., Zr) in Y-doped ZrO2 while they display a preference for the dopant cation (i.e., Sc) in Sc-doped ZrO2. On the other hand, vacancies prefer to be associated with the dopant cation in both Y- and Sc-doped CeO2. However, the reduction of crystallite size to a few nanometers shows an unexpected and remarkable effect of increasing randomness in the vacancy distribution in all materials. Such an effect is hypothesized to result from a higher degree of short-range structural disorder in the cation coordination environments in nanocrystals compared to that in their microcrystalline counterparts that controls the energetics of vacancy ordering via a complex balance between electrostatic and strain energy terms. Finally, a clear connection is established between vacancy ordering, oxygen ion transport, and electrical conductivity in microcrystalline Y-doped CeO2 and its possible implications on ionic transport in nanocrystalline materials are discussed.




Probing Porosity and Pore Interconnectivity in Crystalline Mesoporous TiO2 Using Hyperpolarized 129Xe NMR

Li-Qiong Wang*, Donghai Wang, Jun Liu and Gregory J. Exarhos, Shane Pawsey and Igor Moudrakovski
J. Phys. Chem. C, 2009, 113 (16), pp 6577–6583
DOI: 10.1021/jp809740e

Abstract: Hyperpolarized (HP) 129Xe NMR was used to probe the porosity and interconnectivity of pores in crystalline mesoporous TiO2. We have demonstrated that HP 129Xe NMR can be used to differentiate between similar sized pores within different crystalline phases. Pores of 4 nm size resident in mixed anatase and rutile mesoporous TiO2 phases were identified. Complementary to other pore characterization techniques, HP 129Xe NMR is able to probe the interconnectivity between pores present in these different phases. The cross peaks in 2D exchange (EXSY) NMR spectra between the signals of xenon in two types of pores are visible on millisecond timescale, indicating substantial pore interconnectivity. The obtained information on porosity and interconnectivity is important for the understanding of ion transport mechanisms in mesoporous TiO2 anode materials.




Structure, Activity, and Stability of Triphenyl Phosphine-Modified Rh/SBA-15 Catalyst for Hydroformylation of Propene: A High-Resolution Solid-State NMR Study

Xijie Lan, Weiping Zhang*, Li Yan, Yunjie Ding, Xiuwen Han, Liwu Lin and Xinhe Bao*
J. Phys. Chem. C, 2009, 113 (16), pp 6589–6595
DOI: 10.1021/jp810432p
Copyright © 2009 American Chemical Society

Abstract: A ligand (triphenyl phosphine, PPh3)-modified heterogeneous PPh3−Rh(CO)/SBA-15 catalyst and supported Wilkinson complex HRh(CO)(PPh3)3/SBA-15 catalyst were prepared and examined in the hydroformylation of propene. Heterogeneous PPh3−Rh(CO)/SBA-15 catalyst showed much higher activity and stability in this reaction. Multinuclear 1H, 29Si, 31P, and 17O MAS NMR and two-dimensional 17O MQ MAS NMR together with XRD and N2 adsorption were employed to study the local structures of these two catalysts. Quantitative 1H and 29Si MAS NMR and qualitative one- and two-dimensional 17O MAS and MQ MAS NMR indicate that in the presence of CO the silanols on the surface of SBA-15 can react with rhodium carbonyls to form the Si−O−Rh bonds at the interface between the catalyst and the support. 31P MAS NMR spectra demonstrate a similar Wilkinson complex structure is produced on the heterogeneous PPh3−Rh(CO)/SBA-15 catalyst. The formation of Si−O−Rh bonds at the interface may immobilize the Rh complex during the long reaction. These may be correlated to the higher performances of heterogeneous PPh3−Rh(CO)/SBA-15 catalyst in propene hydroformylation




Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-Dimensional NMR Relaxation Correlations

Daniel Weber, Jonathan Mitchell*, James McGregor and Lynn F. Gladden
J. Phys. Chem. C, 2009, 113 (16), pp 6610–6615
DOI: 10.1021/jp811246j
Copyright © 2009 American Chemical Society

Abstract
Two-dimensional nuclear magnetic resonance (NMR) relaxation time correlation measurements have been used to observe the behavior of liquids inside porous catalyst pellets; in particular, liquids of relevance to the hydrogenation of 2-butanone over a silica-supported ruthenium catalyst (Ru/SiO2). The behavior of 2-butanone is studied and compared to that of water and 2-propanol, which are used as solvents in this hydrogenation reaction. From the ratio of NMR relaxation times, T1/T2, for the liquids confined in the pores, it is possible to infer the relative strengths of the surface interaction for each liquid. Water is seen to have the strongest surface interaction, and 2-butanone has the weakest surface interaction. These results are supported by displacement experiments, in which one liquid replaces the other over time within the pore space of the catalyst. For comparison, the behavior of the same liquids in an alumina-supported palladium catalyst (Pd/Al2O3) was also studied. The variation in the strengths of surface interactions was more pronounced in the Pd/Al2O3 catalyst than in the Ru/SiO2 catalyst. This work demonstrates the applicability of NMR relaxation time correlation experiments to real catalytic systems containing metallic components. From these measurements, information on the access of reactants to surface adsorption sites can be inferred.




Characterization of HNbWO6 and HTaWO6 Metal Oxide Nanosheet Aggregates As Solid Acid Catalysts

Caio Tagusagawa†, Atsushi Takagaki‡, Shigenobu Hayashi§ and Kazunari Domen*†
J. Phys. Chem. C, 2009, 113 (18), pp 7831–7837

Abstract: Nanosheet aggregates prepared from protonated layered tungstates HMWO6 (M = Nb, Ta) are examined as potential solid acid catalysts. The nanosheet aggregates are formed by soft chemical processing of the layered compound with tetra(n-butylammonium) hydroxide, and the catalytic activity and acid strength of the aggregates are compared with those for HTiNbO5, HNb3O8, and a range of conventional solid acids. The catalytic activity for the Friedel−Crafts alkylation of anisole in the presence of benzyl alcohol increases in the order HTiNbO5 < m =" Nb," m =" Ti,">




First-Principles Nuclear Magnetic Resonance Structural Analysis of Vitreous Silica

Thibault Charpentier*†, Peter Kroll‡ and Francesco Mauri§
J. Phys. Chem. C, 2009, 113 (18), pp 7917–7929
DOI: 10.1021/jp900297r

Abstract:Gauge including projector augmented wave (GIPAW) NMR calculations combined with hybrid Monte Carlo/molecular dynamics simulations are carried out in order to investigate the relationships between the oxygen-17 and silicon-29 NMR spectra of vitreous silica and its local structure in terms of the Si−O−Si bond angle and Si−O distance distributions. Special attention is paid to the structure and NMR parameters of three- and four-membered rings, and the effect of their concentration on glass density is studied. It is shown that our simulations provide a new insight into the features of the 17O NMR parameters distribution. Accordingly, a new analytical model is presented and applied for the reconstruction of the Si−O−Si angle from the NMR spectrum. The reliability of the procedure is demonstrated conclusively through the excellent consistency of the analysis of the oxygen-17 and silicon-29 NMR experimental data of vitreous silica. Si−O−Si angle distribution mean values of 147.1° and 148.4°, respectively, and standard deviations of 11.2° and 10.8°, respectively, are obtained from the oxygen-17 and silicon-29 NMR experimental spectrum (Clark et al., ref 4) of the same sample.




SBE-Type Metal-Substituted Aluminophosphates: Detemplation and Coordination Chemistry

Daphne S. Bel n-Cordero†, Chul Kim‡§, Son-Jong Hwang‡ and Arturo J. Hern ndez-Maldonado*†
J. Phys. Chem. C, 2009, 113 (19), pp 8035–8049

Abstract: The detemplation process in Me-SBE (Me = Co2+, Mg2+, and Mn2+) aluminophosphates was studied to elucidate materials stability and framework characteristics. In addition, the hydrothermal synthesis conditions were optimized to obtain materials with minimal phase impurities. This was accomplished by means of decreasing reaction temperature and increasing aging periods. Scanning electron microscopy analysis of the Mg- and Mn-SBE as-synthesized samples revealed square plates with truncated corner morphologies grown in aggregated fashion and contrasting with the previously reported hexagonal platelike morphology of Co-SBE. Cautious detemplation in vacuum, using an evacuation rate of 10 mmHg/s and a temperature of 648 K, resulted in surface areas of about 700, 500, and 130 m2/g for Mg-, Co-, and Mn-SBE, respectively. Thermal gravimetric analysis and in situ high-temperature powder X-ray diffraction analyses indicate the frameworks for all of the SBE variants experienced collapse upon treatment with helium at temperatures above 700 K and subsequently formed an aluminophosphate trydimite dense phase. Detemplation in air at all times resulted in framework destruction during detemplation. In situ differential scanning calorimetry−powder X-ray diffraction data showed that the SBE frameworks experience breathing modes related to specific endothermic and exothermic scenarios during air treatment. Decomposition and elimination of the organic template during vacuum treatment was verified by Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy revealed that most of the Co atoms in vacuum-treated samples are in tetrahedral coordination, while the Mn atoms exhibit various coordination states. Ultraviolet-visible, electron paramagnetic resonance, and magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy corroborated the latter result in addition to providing evidence for the formation of Mn extra framework species. 27Al MAS NMR spectra for vacuum-detemplated Mg-SBE samples prior to and after dehydration confirmed the reversible formation of aluminum octahedral sites. This, however, did not affect the porous nature of detemplated Mg-SBE samples as these are capable of adsorbing 19 water molecules per super cage at 298 K.



Low Temperature 1H MAS NMR Spectroscopy Studies of Proton Motion in Zeolite HZSM-5

Hua Huo†, Luming Peng‡ and Clare P. Grey*†
J. Phys. Chem. C, 2009, 113 (19), pp 8211–8219
Abstract: Low temperature 1H MAS NMR spectroscopy is used to study protonic motion in zeolite HZSM-5 in both samplethat have been dried using procedures that are standard in the literature and samples that have been more carefully dehydrated. A significant enhancement of proton mobility is seen for the “standard” dehydrated HZSM-5 sample in comparison to that seen for the much drier sample. This is ascribed to a vehicle-hopping mechanism involving the residual water that is present in these zeolites. A gradual change of the framework structure is observed on cooling to approximately 213 K, as monitored via the change in 1H chemical shift values of the Brønsted acid resonances and by X-ray diffraction. A more sudden change in structure is seen by differential scanning calorimetry and NMR at approximately 220−230 K, which is associated with changes in both the mobility and the modes of binding of the residual water to the Brønsted acid sites and the zeolite framework.