Thursday, May 21, 2009

J. Chem. Phys. 130, 194506 (2009)

Molecular dynamics in supercooled glycerol: Results from 13C NMR spectroscopy
J. Chem. Phys. 130, 194506 (2009); DOI:10.1063/1.3138179
P. Jain, A. Levchenko, P. Yu, and S. Sen
13C NMR spectra of glycerol are collected over the entire temperature range of supercooling: Tg(185 K)TTm(293 K). The temperature dependent evolution of the 13C NMR line shape indicates dynamical averaging of the chemical shift anisotropy at the carbon sites with increasing temperature, resulting from isotropic tumbling of the constituent molecules. This isotropic reorientation dynamics involves random molecular jumps over all possible angles, and its time scale is in excellent agreement with the -relaxation time scale of the supercooled liquid. The increasing activation energy of such molecular jumps with decreasing temperature and hence the fragility of supercooled glycerol are likely to be related to the corresponding temperature dependence of the average number of hydrogen bonds per molecule. The absence of any peak in the dielectric relaxation spectra of supercooled glycerol is possibly related to a strong coupling between intramolecular dynamics and the tumbling of the entire molecule as a whole.

Chem. Mater., Article ASAP

A One-Step Mechanochemical Route to Core−Shell Ca2SnO4 Nanoparticles Followed by 119Sn MAS NMR and 119Sn Mssbauer Spectroscopy

Vladimir epelk* and Klaus Dieter Becker, Ingo Bergmann‡ and Shigeru Suzuki, Sylvio Indris, Armin Feldhoff and Paul Heitjans, Clare P. Grey

Calcium stannate (Ca2SnO4) nanoparticles with an average size of about 15 nm were synthesized via single-step mechanochemical processing of binary oxide precursors at room temperature. High-resolution TEM studies revealed a nonuniform structure of mechanosynthesized Ca2SnO4 nanoparticles consisting of an ordered core surrounded by a disordered surface shell region. The inner core of a Ca2SnO4 nanoparticle possesses a fully ordered orthorhombic structure, and the surface shell exhibits the thickness of about 1.5 nm. The volume fraction of surface shell regions in the nanostructured mechanosynthesized stannate is estimated to be about 50%. Because of the ability of both solid-state 119Sn MAS NMR and 119Sn Mssbauer spectroscopies to probe the local environment of Sn nuclei, valuable complementary insight into the local structural disorder in mechanosynthesized Ca2SnO4 was obtained. It was concluded that the near-surface layers of stannate nanoparticles are disordered because of broadly distorted geometry of SnO6 octahedra. The octahedra are deformed in such a way that they become more regular.

Tuesday, May 19, 2009

Inorg. Chem., 2009, 48 (10), pp 4342–4353

Polymorphism of Potassium Ferrocyanide Trihydrate as Studied by Solid-State Multinuclear NMR Spectroscopy and X-ray Diffraction
Inorg. Chem., 2009, 48 (10), pp 4342–4353 DOI: 10.1021/ic802134j
Mathew J. Willans, Roderick E. Wasylishen and Robert McDonald
The polymorphism of bulk powder samples of potassium ferrocyanide trihydrate (K4Fe(CN)6·3H2O, KFCT) has been studied using 1H, 13C, and 15N NMR spectroscopy in combination with X-ray diffraction. At room temperature, KFCT typically crystallizes in a monoclinic C2/c form, which converts irreversibly to a monoclinic Cc form upon cooling below −25 °C. The structure of both of these forms has been determined using single-crystal X-ray diffraction. A less common metastable tetragonal I41/a form is also known to exist at room-temperature. This tetragonal form also converts to the monoclinic Cc form upon cooling, although this phase transition is irreversible and occurs at −60 °C. Initial room-temperature 15N MAS NMR spectra and powder X-ray diffraction patterns of ground powder samples of KFCT prepared using a variety of crystallization methods suggested that only the C2/c form was obtained from a bulk crystallization. The 13C MAS NMR spectra consisted of six peaks with equal integrated areas, a result that is inconsistent with the 15N NMR spectra and known crystal structures. When the samples were not ground, the relative areas of the 13C NMR peaks were altered, indicating that the bulk samples in fact consisted of the two known forms of KFCT. Using the known temperature dependence of these two polymorphs, the 13C peaks corresponding to each of the C2/c and I41/a forms were assigned. The 13C NMR spectra and powder X-ray diffraction results demonstrate that upon grinding, a near 50−50 mixture of the two forms is always produced, rather than a new form entirely. The insensitivity of the 15N NMR spectra to the polymorphism of KFCT is surprising, and likely arises from a fortuitous overlap of the 15N NMR peaks of the two forms.

Tuesday, May 12, 2009

Macromolecules, Volume 42, Issues 7-9

Local Deformation in Carbon Black-Filled Polyisoprene Rubbers Studied by NMR and X-ray Diffraction

Stphane Dupres, Didier R. Long, Pierre-Antoine Albouy and Paul Sotta*
Macromolecules, 2009, 42 (7), pp 2634–2644
Abstract: We study polyisoprene elastomers reinforced with carbon blacks (CBs) of various grades. In addition to mechanical characterization at medium/large elongation ratios, we use deuterium NMR experiments on stretched samples to measure the local strain within the elastomer matrix and X-ray scattering to measure the onset of strain-induced crystallization in the reinforced systems. We show that NMR experiments and measurements of the onset of crystallization are indeed sensitive to the different degrees of reinforcement observed according to the various CB grades and volume fractions. The measurements show a good correlation between macroscopic (mechanical) and microscopic (NMR, crystallization) measurements. This indicates that the techniques used here are valuable techniques for characterizing reinforced systems in which fillers have complex morphologies and dispersion states. The discrepancies between the results of both macroscopic and microscopic measurements are analyzed. The local strain as measured by NMR is lower than the macroscopic strain. This indicates the presence of some degree of local strain (or stress) inhomogeneity within the elastomer matrix in the reinforced systems. This inhomogeneity is more pronounced in the presence of more reinforcing carbon black grades

Monday, May 11, 2009

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.

Friday, May 08, 2009

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

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
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.

Thursday, May 07, 2009

J. Am. Chem. Soc., 2009, 131 (18), pp 6340–6341

Side Chain Resonances in Static Oriented Proton-Decoupled 15N Solid-State NMR Spectra of Membrane Proteins

Christopher Aisenbrey†‡, Lydia Prongidi-Fix†, Alexandre Chenal§, Daniel Gillet and Burkhard Bechinger†‡

Proton-decoupled 15N solid-state NMR spectra are used to analyze the structure, dynamics, and membrane topology of proteins uniformly labeled with 15N. Preparation of the proteins by bacterial overexpression results in the labeling not only of the backbone amides but also of nitrogens localized within the side chains of arginine, glutamine, tryptophan, asparagines, lysines, and histidines. Most of these side chain resonances appear in the spectral region of the anisotropic backbone amides, and residual intensities have been observed also in cross-polarization spectra. In the past this issue has received little attention although it can cause ambiguities during assignment. Here we show that by combining cross-polarization and Hahn echo solid-state NMR experiments, it is possible to differentiate between side chain and backbone resonances. This is demonstrated using experimental and simulated 15N spectra of oriented purple membranes, diphtheria toxin T domain and Bcl-xL.

Inorg. Chem. Vol. 48, Iss. 9

Cadmium(II) Cysteine Complexes in the Solid State: A Multispectroscopic Study
Inorg. Chem., 2009, 48 (9), pp 4219–4230 DOI: 10.1021/ic900145n
Farideh Jalilehvand, Vicky Mah, Bonnie O. Leung, Janos Mink, Guy M. Bernard
and Laszlo Hajba
Cadmium(II) cysteinate compounds have recently been recognized to provide an environmentally friendly route for the production of CdS nanoparticles, used in semiconductors. In this article, we have studied the coordination for two cadmium(II) cysteinates, Cd(HCys)2·H2O (1) and {Cd(HCys)2·H2O}2·H3O+ClO4− (2), by means of vibrational (Raman and IR absorption), solid-state NMR (113Cd and 13C), and Cd K- and L3-edge X-ray absorption spectroscopy. Indistinguishable Cd K-edge extended X-ray absorption fine structure (EXAFS) and Cd L3-edge X-ray absorption near edge structure (XANES) spectra were obtained for the two compounds, showing similar local structure around the cadmium(II) ions. The vibrational spectra show that the cysteine amine group is protonated (NH3+) and not involved in bonding. The 113Cd solid-state cross-polarization magic angle spinning NMR spectra showed a broad signal in the 500−700 ppm range, with the peak maximum at about 650 ppm, indicating three to four coordinated thiolate groups. Careful analyses of low-frequency Raman and far-IR spectra revealed bridging and terminal Cd−S vibrational bands. The average Cd−S distance of 2.52 ± 0.02 Å that constantly emerged from least-squares curve-fitting of the EXAFS spectra is consistent with CdS4 and CdS3O coordination. Both structural models yielded reasonable values for the refined parameters, with a slightly better fit for the CdS3O configuration, for which the Cd−O distance of 2.27 ± 0.04 Å was obtained. The Cd L3-edge XANES spectra of 1 and 2 resembled that of the CdS3O model compound and showed that the coordination around Cd(II) ions in 1 and 2 cannot be exclusively CdS4. The small separation of 176 cm−1 between the infrared symmetric and antisymmetric COO− stretching modes indicates monodentate or strongly asymmetrical bidentate coordination of a cysteine carboxylate group in the CdS3O units. The combined results are consistent with a “cyclic/cage” type of structure for both the amorphous solids 1 and 2, composed of CdS4 and CdS3O units with single thiolate (Cd−S−Cd) bridges, although a minor amount of cadmium(II) sites with CdS3O2−3 and CdS4O coordination geometries cannot be ruled out.

Monday, May 04, 2009

J. Am. Chem. Soc., 2009, 131 (19), pp 6658–6659

Fast and Simple Acquisition of Solid-State 14N NMR Spectra with Signal Enhancement via Population Transfer
Luke A. O’Dell and Robert W. Schurko*

A new approach for the acquisition of static, wideline 14N NMR powder patterns is outlined. The method involves the use of frequency-swept pulses which serve two simultaneous functions: (1) broad-band excitation of magnetization and (2) signal enhancement via population transfer. The signal enhancement mechanism is described using numerical simulations and confirmed experimentally. This approach, which we call DEISM (Direct Enhancement of Integer Spin Magnetization), allows high-quality 14N spectra to be acquired at intermediate field strengths in an uncomplicated way and in a fraction of the time required for previously reported methods.

Marcel's Update

Can. J. Chem. 87(1): 348–360 (2009)

Probing solid iminobis(diorganophosphine chalcogenide) systems with multinuclear magnetic resonance

Bryan A. Demko and Roderick E. Wasylishen

Abstract: A 31P and 77Se solid-state NMR investigation of the iminobis(diorganophosphine chalcogenide) HN(R2PE)2 (R = Ph,iPr; E = O, S, Se) systems is presented. The NMR results are discussed in terms of the known HN(R2PE)2 structures available from X-ray crystallography. The phosphorus chemical shift tensors are found to be sensitive to the nature of the alkyl and chalcogen substituents. The nature of the R group also influences the selenium chemical shift tensors of HN(R2PSe)2 (R = Ph, iPr), which are shown to be sensitive to hydrogen bonding in the dimer structure of HN(Ph2PSe)2 and to the presence of disorder in the case of HN(iPr2PSe)2. Scalar relativistic ZORA DFT nuclear magnetic shielding tensor calculations were performed yielding the orientations of the corresponding chemical shift tensors. A theoretical investigation into the effect of the E-P···P-E “torsion” angle on the phosphorus and selenium chemical shift tensors of a truncated HN(Me2PSe)2 system indicates that the electronic effect of the alkyl group on the respective nuclear magnetic shielding tensors are more important than the steric effect of the E-P···P-E torsion angle.
Key words: iminobis(diorganophosphine chalcogenide), solid-state NMR, 31P NMR, 77Se NMR, ZORA DFT.

Can. J. Chem. 87(4): 563–570 (2009)

Measurement and calculation of 13C and 15N NMR chemical-shift tensors of a push–pull ethylene

Saeed K. Amini, Mohsen Tafazzoli, Hilary A. Jenkins, Gillian R. Goward, and Alex D. Bain

Abstract: Methyl 3-dimethylamino-2-cyanocrotonate (MDACC) has a remarkably weak carbon–carbon double bond. It has strong electron-withdrawing groups on one end and electron-donating groups on the other: a so-called push–pull ethylene. To investigate this unusual electronic structure, we have determined the crystal structure and measured both the 13C and 15N NMR chemical-shift tensors. These measurements are supplemented by shielding-tensor calculations done with density functional methods. The large difference (approximately 100 ppm) between isotropic chemical shifts of the two alkenyl carbons reflects a large charge release from the electron-donating side of C=C double bond to the electron-withdrawing groups. Comparison of the calculated orientations of the principal components of the alkenyl carbons obtained from ab initio calculations shows that the primary changes in charge density occur in the molecular plane. On the other hand, smaller charge density changes above and below the plane of the C=C double bond establish the conjugation of donor and acceptor groups with π* and π molecular orbitals of the central double bond, respectively, which lowers the barrier to rotation about this bond.
Key words: density functional methods, NMR, shielding anisotropy, CSA, push–pull ethylenes, crystal structure.

J. Mater. Chem., 2009, 19, 1151 – 1159

Sol-gel synthesis of sodium-modified AlPO4–SiO2 glasses and structural characterization by solid state NMR

Rashmi R. Deshpande, Long Zhang and Hellmut Eckert

Abstract: Sodium aluminophosphosilicate gels and glasses along the composition line (Na2O)x–[(AlPO4)0.5(SiO2)0.5]1-x were synthesized via the sol-gel process using sodium acetate, aluminium lactate, phosphoric acid and tetraethyl orthosilicate (TEOS) as precursors. The structural evolution from solution to gel and to the final glass was monitored by 27Al, 31P, 29Si and 23Na magic-angle spinning (MAS) NMR. In the solution state aluminium forms a mixture of octahedral coordinated complexes coordinated to lactate, water and phosphate units with varying degrees of polymerization. With increasing temperature up to about 150 °C the lactate ligands are successively replaced by phosphate, resulting in an increased extent of Al–O–P linking. Annealing the xerogel at 400 °C results in a glassy network containing mostly four-coordinated Al species (Al(IV)), in addition to small concentrations of Al(V) and Al(VI). While the distribution of Al coordination numbers remains constant, the compositional trend of the 27Al chemical shift indicates a gradual replacement of Al–O–P linkages by Al–O–Si linkages with increasing sodium content. This conclusion is confirmed by 27Al{31P} Rotational Echo Double Resonance (REDOR) experiments, which reveal a reduction in the number of Al–O–P linkages. Concomitantly, the interaction of Na with the phosphate species is successively increased as indicated by 31P chemical shift trends and 31P{23Na} REDOR results. The results illustrate that the network segregation found in pure AlPO4–SiO2 sol-gel glasses can be overcome, at least partially, by introduction of the network modifier Na2O.

J. Mater. Chem., 2009, 19, 2683 – 2694

Solid state NMR investigation of photoresist molecular glasses including blend behavior with a photoacid generator

David L. VanderHart, Vivek M. Prabhu, Anuja De Silva, Nelson M. Felix and Christopher K. Ober

Abstract: We have examined four molecular glass (MG) materials that show promise as photoresists for extreme-ultraviolet (EUV) lithography. These glass-forming materials were investigated by proton and 13C solid state nuclear magnetic resonance (NMR) techniques in the bulk state as pure materials and as mixtures with (5 or 10) % by mass of the photoacid generator (PAG), triphenylsulfonium perfluorobutanesulfonate. The 13C techniques gave information about crystallinity, purity, and the qualitative existence of multiple phases. Proton studies focused on using spin diffusion to characterize the intimacy of mixing of the PAG and MG blends. The four MGs were largely aromatic materials containing several hydroxyl groups that were partially protected by t-butoxycarbonyl (t-BOC) groups. In two cases, this fraction was varied and the impact on mixing noted. Phase separation of the PAG into PAG-rich larger domains was never seen; the PAG was always finely distributed and the maximum size for any PAG clustering was estimated; however, in some cases, the average local concentration of PAG appeared to vary. Crystallinity was only seen associated with the underivatized materials implying that the mixing of the PAG with any derivatized MG was not restricted by crystallization. It was also noted that some very strong hydrogen bonds exist in three of the four underivatized materials and were eliminated or weakened upon partial derivatization with t-BOC.

Friday, May 01, 2009

J. Am. Chem. Soc., 2008, 130 (31), pp 10233–10239

NMR-Assisted Prediction of RNA Secondary Structure: Identification of a Probable Pseudoknot in the Coding Region of an R2 Retrotransposon

James M. Hart†, Scott D. Kennedy‡, David H. Mathews‡ and Douglas H. Turner†

As the rate of functional RNA sequence discovery escalates, high-throughput techniques for reliable structural determination are becoming crucial for revealing the essential features of these RNAs in a timely fashion. Computational predictions of RNA secondary structure quickly generate reasonable models but suffer from several approximations, including overly simplified models and incomplete knowledge of significant interactions. Similar problems limit the accuracy of predictions for other self-folding polymers, including DNA and peptide nucleic acid (PNA). The work presented here demonstrates that incorporating unassigned data from simple nuclear magnetic resonance (NMR) experiments into a dynamic folding algorithm greatly reduces the potential folding space of a given RNA and therefore increases the confidence and accuracy of modeling. This procedure has been packaged into an NMR-assisted prediction of secondary structure (NAPSS) algorithm that can produce pseudoknotted as well as non-pseudoknotted secondary structures. The method reveals a probable pseudoknot in the part of the coding region of the R2 retrotransposon from Bombyx mori that orchestrates second-strand DNA cleavage during insertion into the genome.

J. Am. Chem. Soc., 2008, 130 (51), pp 17230–17231

19F NMR Spectroscopy for the Analysis of RNA Secondary Structure Populations

Dagmar Graber, Holger Moroder and Ronald Micura*

Labeling of RNA with site-specific fluorine atoms has become straightforward in recent years and in particular has become an integrated part of engineered functional RNA with therapeutic potential, e.g. for siRNA technologies. Here, we demonstrate that temperature-dependent one-dimensional 19F NMR spectroscopy of oligoribonucleotides is a powerful tool to obtain direct information on the conformational behavior. The approach is particularly useful for the quantification of monomolecular vs bimolecular secondary structure equilibria.

J. Am. Chem. Soc., 2009, 131 (17), pp 6102–6104

A Direct and Nondestructive Approach To Determine the Folding Structure of the I-Motif DNA Secondary Structure by NMR

Jixun Dai†, Attila Ambrus†, Laurence H. Hurley†‡§ and Danzhou Yang

I-motifs are four-stranded DNA secondary structures formed in C-rich DNA sequences and consist of parallel-stranded DNA duplexes zipped together in an antiparallel orientation by intercalated, hemiprotonated cytosine+−cytosine base pairs. I-motif structures have been indicated to form in various regions of the human genome as well as in nanotechnological applications. While NMR is a major tool for structural studies of I-motifs, the determination of the folding topologies of unimolecular I-motifs has been a challenging and arduous task using conventional NMR spectral assignment strategies, due to the inherent sequence redundancy of the C-rich strands in the formation of unimolecular I-motif structures. We report here a direct and nondestructive method that can be utilized to unambiguously determine the hemiprotonated C+−C base pairs and thus the folding topology of unimolecular I-motif structures formed from native C-rich DNA sequences. The reported approach uses affordable low-enrichment site-specific labeling. More significantly, the reported method can directly and unambiguously determine the equilibrating multiple conformations coexisting in a single DNA sequence, which would be a very difficult task using conventional assignment strategies. Additionally, this method can be applied to the direct detection of the base-paired thymines that are involved in the capping structures.

J. Am. Chem. Soc., 2009, 131 (17), pp 6048–6049

Accurate Sampling of High-Frequency Motions in Proteins by Steady-State 15N−{1H} Nuclear Overhauser Effect Measurements in the Presence of Cross-Correlated Relaxation

Fabien Ferrage†‡, David Cowburn† and Ranajeet Ghose§

The steady-state {1H}−15N NOE experiment is used in most common NMR analyses of backbone dynamics to accurately ascertain the effects of the fast dynamic modes. We demonstrate here that, in its most common implementation, this experiment generates an incorrect steady state in the presence of CSA/dipole cross-correlated relaxation leading to large errors in the characterization of these high-frequency modes. This affects both the quantitative and qualitative interpretation of 15N backbone relaxation in dynamic terms. We demonstrate further that minor changes in the experimental implementation effectively remove these errors and allow a more accurate interpretation of protein backbone dynamics.

J. Am. Chem. Soc., Article ASAP

Mssbauer, NMR, Geometric, and Electronic Properties in S = 3/2 Iron Porphyrins

Yan Ling and Yong Zhang*

Iron porphyrins with the intermediate spin S = 3/2 or admixed with S = 5/2 or 1/2 are models for a number of heme protein systems, including cytochromes c′. The 57Fe Mssbauer quadrupole splittings and 1H and 13C NMR chemical shifts have been found to be useful probes of their electronic states. We present the results of the first successful quantum chemical calculations of the Mssbauer and NMR properties in various S = 3/2 iron porphyrin complexes, covering four-, five-, and six-coordinate states and three commonly seen porphyrin conformations: planar, ruffled, and saddled. Several interesting correlations among these useful experimental spectroscopic probes and geometric and electronic properties were discovered. These results should facilitate future investigations of related heme proteins and model systems.

J. Am. Chem. Soc., Article ASAP

Probing the Structure and Charge State of Glutathione-Capped Au25(SG)18 Clusters by NMR and Mass Spectrometry

Zhikun Wu, Chakicherla Gayathri, Roberto R. Gil and Rongchao Jin*

Despite the recent crystallographic determination of the crystal structure of Au25(SCH2CH2Ph)18 clusters, the question−whether all thiolate-capped, 25-atom gold clusters adopt the same structure, regardless of the types of thiols (e.g., long-chain alkylthiols, aromatic thiols, or other functionalized ones)−still remains unanswered. To crystallize long-chain or bulky ligand (e.g., glutathione)-capped Au25(SR)18 clusters has proven to be difficult due to the major amorphousness caused by such ligands; therefore, one needs to seek other strategies to probe the structural information of such gold clusters. Herein, we report a strategy to probe the Au25 core structure and surface thiolate ligand distribution by means of NMR in combination with mass spectrometry. We use glutathione-capped Au25(SG)18 clusters as an example to demonstrate the utility of this strategy. One-dimensional (1D) and two-dimensional (2D) correlation NMR spectroscopic investigation of Au25(SG)18 reveals fine spectral features that explicitly indicate two types of surface binding modes of thiolates, which is consistent with the ligand distribution in the Au25(SCH2CH2Ph)18 cluster. Laser desorption ionization (LDI) mass spectrometry analysis shows that Au25(SG)18 exhibits an identical ionization and core fragmentation pattern with phenylethylthiolate-capped Au25 clusters. The charge state of the native Au25(SG)18 clusters was determined to be −1 by comparing their optical spectrum with those of [Au25(SCH2CH2Ph)18]q of different charge states (q = −1, 0). Taken together, our results led to the conclusion that glutathione-capped Au25(SG)18 clusters indeed adopt the same structure as that of Au25(SCH2CH2Ph)18. This conclusion is also valid for other types of thiolate-capped Au25 clusters, including hexyl- and dodecylthiolates. Interestingly, the chiral optical responses (e.g., circular dichroism (CD) signals in the visible wavelength region) from the Au25(SG)18 clusters seem to be imparted by the chiral glutathione ligands because no similar CD signals were observed in Au25(SCH2CH2Ph)18.