J. Am. Chem. Soc., 130 (10), 3095 -3105, 2008.
Probing Local Structure in Zeolite Frameworks: Ultrahigh-Field NMR Measurements and Accurate First-Principles Calculations of Zeolite 29Si Magnetic Shielding Tensors
Darren H. Brouwer and Gary D. Enright
The principal components of zeolite 29Si magnetic shielding tensors have been accurately measured and calculated for the first time. The experiments were performed at an ultrahigh magnetic field of 21.1 T in order to observe the small anisotropies of the 29Si shielding interactions that arise for Si atoms in near-tetrahedral geometries. A robust two-dimensional (2D) chemical shift anisotropy (CSA) recoupling pulse sequence was employed that enables quasi-static powder patterns to be resolved according to the isotropic chemical shifts. For the zeolites Sigma-2 and ZSM-12, it is demonstrated that the 29Si chemical shift (CS) tensor components measured by the recoupling experiment are in excellent agreement with those determined from spinning sidebands in slow magic-angle spinning (MAS) experiments. For the zeolite ZSM-5, the principal components of the 29Si CS tensors of 15 of the 24 Si sites were measured using the 2D CSA recoupling experiment, a feat that would not be possible with a slow MAS experiment due to the complexity of the spectrum. A simple empirical relationship between the 29Si CS tensors and local structural parameters could not be established. However, the 29Si magnetic shielding tensors calculated using Hartree-Fock ab initio calculations on clusters derived from the crystal structures are in excellent agreement with the experimental results. The accuracy of the calculations is strongly dependent on the quality of the crystal structure used in the calculation, indicating that the 29Si magnetic shielding interaction is extremely sensitive to the local structure around each Si atom. It is anticipated that the measurement and calculation of 29Si shielding tensors could be incorporated into the "NMR crystallography" of zeolites and other related silicate materials, possibly being used for structure refinements that may lead to crystal structures with very accurate Si and O atomic coordinates.
J. Am. Chem. Soc., 130 (10), 2896 -2897, 2008.
Solid-State Deuterium NMR Studies Reveal s-ns Motions in the HIV-1 Transactivation Response RNA Recognition Site
Greg L. Olsen, Dorothy C. Echodu, Zahra Shajani, Michael F. Bardaro, Jr., Gabriele Varani, and Gary P. Drobny
Solution and solid-state NMR measurements were used together to examine motion in three sites in the HIV-1 TAR RNA. We wished to investigate the dynamics facilitating the conformational rearrangements the TAR RNA must undergo for Tat binding and in particular to characterize the full range of motional time scales accessible to this RNA. Our results demonstrate that the dynamics in TAR involving residues essential to Tat binding include not only the faster motions detected by solution relaxation measurements but also a significant component in the s-ns time scale.
Phys. Rev. B 77, 075421 (2008) (6 pages)
Highly rotational C60 dynamics inside single-walled carbon nanotubes: NMR observations
K. Matsuda, Y. Maniwa, and H. Kataura
Rotational dynamics of C60 molecules composing a one-dimensional (1D) linear array inside single-walled carbon nanotubes have been investigated by 13C NMR in a temperature range from 4.2 to 300 K. The temperature dependence of the NMR lineshape and spin-lattice relaxation time (T1) indicate that the encapsulated C60 molecules exhibit a quasi-free-rotation with correlation times of 5–10 ps at 300 K. With decreasing temperature, the large amplitude molecular rotation continues to 30 K with an activation energy of 467 K. It is suggested that the C60 linear array does not undergo an orientational phase transition, which is associated with its 1D nature. Furthermore, no evidence for the polymerization of C60 molecules is found from the 13C NMR line shape analysis.