Wednesday, June 23, 2010

Hiyam's Journal Update

J. Am. Chem. Soc., 2010, 132 (21), pp 7321–7337

Molecular Silicate and Aluminate Species in Anhydrous and Hydrated Cements
Aditya Rawal, Benjamin J. Smith†, George L. Athens, Christopher L. Edwards, Lawrence Roberts, Vijay Gupta and Bradley F. Chmelka

The compositions and molecular structures of anhydrous and hydrated cements are established by using advanced solid-state nuclear magnetic resonance (NMR) spectroscopy methods to distinguish among different molecular species and changes that occur as a result of cement hydration and setting. One- and two-dimensional (2D) solid-state 29Si and 27Al magic-angle spinning NMR methodologies, including T1-relaxation-time- and chemical-shift-anisotropy-filtered measurements and the use of very high magnetic fields (19 T), allow resonances from different silicate and aluminate moieties to be resolved and assigned in complicated spectra. Single-pulse 29Si and 27Al NMR spectra are correlated with X-ray fluorescence results to quantify the different crystalline and disordered silicate and aluminate species in anhydrous and hydrated cements. 2D 29Si{1H} and 27Al{1H}heteronuclear correlation NMR spectra of hydrated cements establish interactions between water and hydroxyl moieties with distinct 27Al and 29Si species. The use of a 29Si T1-filter allows anhydrous and hydrated silicate species associated with iron-containing components in the cements to be distinguished, showing that they segregate from calcium silicate and aluminate components during hydration. The different compositions of white Portland and gray oilwell cements are shown to have distinct molecular characteristics that are correlated with their hydration behaviors.

A Resonance Assignment Method for Oriented-Sample Solid-State NMR of Proteins
Robert W. Knox†, George J. Lu‡, Stanley J. Opella‡ and Alexander A. Nevzorov

J. Am. Chem. Soc., 2010, 132 (24), pp 8255–8257

A general sequential assignment strategy for uniformly 15N-labeled uniaxially aligned membrane proteins is proposed. Mismatched Hartmann−Hahn magnetization transfer is employed to establish proton-mediated correlations among the neighboring 15N backbone spins. Magnetically aligned Pf1 phage coat protein was used to illustrate the method. Exchanged and nonexchanged separated local field spectra were acquired and overlaid to distinguish the cross-peaks from the main peaks. Most of the original assignments from the literature were confirmed without selectively labeled samples. This method is applicable to proteins with arbitrary topology and will find use in assigning solid-state NMR spectra of oriented membrane proteins for their subsequent structure determination.

The Polar Phase of NaNbO3: A Combined Study by Powder Diffraction, Solid-State NMR, and First-Principles Calculations
Karen E. Johnston†, Chiu C. Tang‡, Julia E. Parker‡, Kevin S. Knight§, Philip Lightfoot*† and Sharon E. Ashbrook*†

J. Am. Chem. Soc., 2010, 132 (25), pp 8732–8746

A polar phase of NaNbO3 has been successfully synthesized using sol-gel techniques. Detailed characterization of this phase has been undertaken using high-resolution powder diffraction (X-ray and neutron) and 23Na multiple-quantum (MQ) MAS NMR, supported by second harmonic generation measurements and density functional theory calculations. Samples of NaNbO3 were also synthesized using conventional solid-state methods and were observed to routinely comprise of a mixture of two different polymorphs of NaNbO3, namely, the well-known orthorhombic phase (space group Pbcm) and the current polar phase, the relative quantities of which vary considerably depending upon precise reaction conditions. Our studies show that each of these two polymorphs of NaNbO3 contains two crystallographically distinct Na sites. This is consistent with assignment of the polar phase to the orthorhombic space group P21ma, although peak broadenings in the diffraction data suggest a subtle monoclinic distortion. Using carefully monitored molten salt techniques, it was possible to eradicate the polar polymorph and synthesize the pure Pbcm phase.

NMR Methods for Characterizing the Pore Structures and Hydrogen Storage Properties of Microporous Carbons
Robert J. Anderson†, Thomas P. McNicholas‡, Alfred Kleinhammes*†, Anmiao Wang‡, Jie Liu‡ and Yue Wu†

J. Am. Chem. Soc., 2010, 132 (25), pp 8618–8626

1H NMR spectroscopy is used to investigate a series of microporous activated carbons derived from a poly(ether ether ketone) (PEEK) precursor with varying amounts of burnoff (BO). In particular, properties relevant to hydrogen storage are evaluated such as pore structure, average pore size, uptake, and binding energy. High-pressure NMR with in situ H2 loading is employed with H2 pressure ranging from 100 Pa to 10 MPa. An N2-cooled cryostat allows for NMR isotherm measurements at both room temperature (290 K) and 100 K. Two distinct 1H NMR peaks appear in the spectra which represent the gaseous H2 in intergranular pores and the H2 residing in micropores. The chemical shift of the micropore peak is observed to evolve with changing pressure, the magnitude of this effect being correlated to the amount of BO and therefore the structure. This is attributed to the different pressure dependence of the amount of adsorbed and non-adsorbed molecules within micropores, which experience significantly different chemical shifts due to the strong distance dependence of the ring current effect. In pores with a critical diameter of 1.2 nm or less, no pressure dependence is observed because they are not wide enough to host
non-adsorbed molecules; this is the case for samples with less than 35% BO. The largest estimated pore size that can contribute to the micropore peak is estimated to be around 2.4 nm. The total H2 uptake associated with pores of this size or smaller is evaluated via a calibration of the isotherms, with the highest amount being observed at 59% BO. Two binding energies are present in the micropores, with the lower, more dominant one being on the order of 5 kJ mol−1 and the higher one ranging from 7 to 9 kJ mol−1.

Characterization of RNA Invasion by 19F NMR Spectroscopy
Anu Kiviniemi and Pasi Virta*

J. Am. Chem. Soc., 2010, 132 (25), pp 8560–8562

19F NMR spectroscopy offers an efficient tool for monitoring RNA invasion. The invasion of 2′-O-methyl oligoribonucleotides into a 19F-labeled HIV-1 TAR RNA model and the temperature-dependent behavior of the complexes obtained have been examined.

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