J. Am. Chem. Soc., 129 (5), 1386 -1392, 2007.
Structures of Alkali Metals in Silica Gel Nanopores: New Materials for Chemical Reductions and Hydrogen Production
Mouath Shatnawi, Gianluca Paglia, James L. Dye,* Kevin C. Cram, Michael Lefenfeld,* and Simon J. L. Billinge*
Alkali metals and their alloys can be protected from spontaneous reaction with dry air by intercalation (with subsequent heating) into the pores of silica gel (SG) at loadings up to 40 wt %. The resulting loose, black powders are convenient materials for chemical reduction of organic compounds and the production of clean hydrogen. The problem addressed in this paper is the nature of the reducing species present in these amorphous materials. The atomic pair distribution function (PDF), which considers both Bragg and diffuse scattering components, was used to examine their structures. Liquid Na-K alloys added to silica gel at room temperature (stage 0) or heated to 150 C (stage I) as well as stage I Na-SG, retain the overall pattern of pure silica gel. Broad oscillations in the PDF show that added alkali metals remain in the pores as nanoscale metal clusters. 23Na MAS NMR studies confirm the presence of Na0 and demonstrate that Na+ ions are formed as well. The relative amounts of Na0 and Na+ depend on both the overall metal loading and the average pore size. The results suggest that ionization occurs near or in the SiO2 walls, with neutral metal present in the larger cavities. The fate of the electrons released by ionization is uncertain, but they may add to the silica gel lattice, or form an "electride-like plasma" near the silica gel walls. A remaining mystery is why the stage I material does not show a melting endotherm of the encapsulated metal and does not react with dry oxygen. Na-SG when heated to 400 C (stage II) yields a dual-phase reaction product that consists of Na4Si4 and Na2SiO3.
J. Am. Chem. Soc., 129 (5), 1293 -1303, 2007.
Structure of Molecular Tweezer Complexes in the Solid State: NMR Experiments, X-ray Investigations, and Quantum Chemical Calculations
Torsten Schaller,* Uta P. Büchele, Frank-Gerrit Klärner, Dieter Bläser, Roland Boese, Steven P. Brown, Hans Wolfgang Spiess, Felix Koziol, Jörg Kussmann, and Christian Ochsenfeld
The structure of supramolecular complexes formed by a naphthalene-spaced tweezer molecule as host and 1,4-dicyanobenzene (DCNB), 1,2,4,5-tetracyanobenzene (TCNB), and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) as aromatic, electron-deficient guests is investigated by solid-state NMR and X-ray diffraction measurements. Quantum chemical calculations using linear scaling methods are applied to predict and to assign the 1H NMR chemical shifts of the complexes. By combining experiment and theory, insights into intra- and intermolecular effects influencing the proton chemical shifts of the host-guest system are provided in the solid state.
J. Am. Chem. Soc., 129 (4), 839 -845, 2007.
Rotational Dynamics in a Crystalline Molecular Gyroscope by Variable-Temperature 13C NMR, 2H NMR, X-Ray Diffraction, and Force Field Calculations
Tinh-Alfredo V. Khuong, Hung Dang, Peter D. Jarowski, Emily F. Maverick, and Miguel A. Garcia-Garibay*
A combination of solid-state 13C CPMAS NMR, 2H NMR, X-ray-determined anisotropic displacement parameters (ADPs), and molecular mechanics calculations were used to analyze the rotational dynamics of 1,4-bis[3,3,3-tris(m-methoxyphenyl)propynyl]benzene (3A), a structure that emulates a gyroscope with a p-phenylene group acting as a rotator and two m-methoxy-substituted trityl groups acting as a stator. The line shape analysis of VT 13C CPMAS and broad-band 2H NMR data were in remarkable agreement with each other, with rotational barriers of 11.3 and 11.5 kcal/mol, respectively. The barriers obtained by analysis of ADPs obtained by single-crystal X-ray diffraction at 100 and 200 K, assuming a sinusoidal potential, were 10.3 and 10.1 kcal, respectively. A similar analysis of an X-ray structure solved from data acquired at 300 K suggested a barrier of only 8.0 kcal/mol. Finally, a rotational potential calculated with a finite cluster model using molecular mechanics revealed a symmetric but nonsinusoidal potential that accounts relatively well for the X-ray-derived values and the NMR experimental results. It is speculated that the discrepancy between the barriers derived from low and high-temperature X-ray data may be due to an increase in anharmonicity, or to disorder, at the higher temperature values.
J. Am. Chem. Soc., 129 (4), 728 -729, 2007.
Proton Assisted Insensitive Nuclei Cross Polarization
Józef R. Lewandowski, Gaël De Paëpe, and Robert G. Griffin*
This Communication presents a solid-state NMR 15N-13C polarization transfer scheme applicable at high B0 and high MAS frequencies, requiring moderate rf powers and mixing time (1-6 ms). The sequence, PAIN-CP, involves the abundant nearby protons in the heteronuclear recoupling dynamics and provides a new tool for obtaining long distance 15N-13C contacts. It should be of major interest for biomolecular structural studies.
J. Am. Chem. Soc., 129 (3), 470 -471, 2007.
Simultaneous Supralinear Line-Narrowing and Sensitivity Enhancement at High Fields in Magic Angle Spinning NMR of Spin-1/2 Nuclei in Solids
Riqiang Fu,* Ozge Gunaydin-Sen, and Naresh S. Dalal*
We report that, in the 15N CP/MAS NMR measurements on NH4H2AsO4 (ADA), the line widths decrease nonlinearly on going from 7.0 to 21.1 T (0.260 to 0.071 ppm, respectively), which can be attributed to an increase in the spin-spin relaxation time, T2. Additionally, besides the expected (B0)3/2 effect, the sensitivity is further enhanced by a factor of 1.6 from 7.0 to 14.1 T, and 2.7 on going to 21.1 T. The enhanced resolution and sensitivity at 21.1 T have enabled us to probe the nature of the antiferroelectric transition at 215 K in ADA to a depth not possible at lower fields.
J. Am. Chem. Soc., 129 (3), 462 -463, 2007.
NMR Detection of Protein 15N Spins near Paramagnetic Lanthanide Ions
Michael John, Ah Young Park, Nicholas E. Dixon, and Gottfried Otting*
Pronounced paramagnetic relaxation enhancement (PRE) due to paramagnetic metal ions prevents the observation of NMR signals from 1H spins near the metal. While 15N spins are less prone to PRE, the intrinsic sensitivity of 15N NMR spectroscopy is low. This Communication presents a 1H detected out-and-back Nz-exchange experiment which allows the measurement of pseudocontact shifts of 15N spins located as close as 6 Å from a Dy3+ ion in a 30 kDa protein complex. The experiment relies on the chemical exchange between paramagnetic and diamagnetic metal ions during two mixing times during which the 15N magnetization is stored as PRE-insensitive longitudinal magnetization. It is demonstrated with the complex between the subunit and the N-terminal domain of the subunit of E. coli DNA polymerase III, prepared with a mixture of Dy3+ and La3+. Pseudocontact shifts were measured for 61 15N spins which were not observable in a conventional 15N-HSQC spectrum.
J. Am. Chem. Soc., 129 (2), 335 -346, 2007.
17O Magic Angle Spinning NMR Studies of Brnsted Acid Sites in Zeolites HY and HZSM-5
Luming Peng, Hua Huo, Yun Liu, and Clare P. Grey*
High-resolution 17O/1H double resonance NMR spectra were obtained for two zeolites, one with a low Si/Al ratio (zeolite HY) and one with a high Si/Al ratio (HZSM-5), to investigate their local structure and Brnsted acidity. Two different oxygen signals, corresponding to Brnsted acid sites in supercages and sodalite cages of zeolite HY were readily resolved in the two-dimensional (2-D) 1H-17O heteronuclear correlation (HETCOR) NMR spectra allowing the 17O isotropic chemical shift (CS) and quadrupolar coupling parameters (quadrupolar coupling constant, QCC, and asymmetry parameter, ) for the two oxygen atoms to be extracted. Similar experiments for HZSM-5 showed that the sites in this system are associated with a much larger distribution in NMR parameters than found in HY. 17O-1H rotational echo double resonance (REDOR) NMR was applied to probe the O-H distances in zeolites HY and HZSM-5. Weaker 17O-1H dephasing was observed for zeolite HZSM-5 in comparison to that of HY, consistent with longer O-H bonds and/or increased proton mobility.