19F Single-Quantum and 19F-33S Heteronuclear Multiple-Quantum Coherence NMR of SF6 in Thermotropic Nemtogens and in the Gas Phase.
H. Tervonen, J. Saunavaara, L.P. Ingman, and J. Jokisaari
19F single-quantum (SQC) and 19F-33S heteronuclear multiple-quantum coherence (HMQC) NMR spectroscopy of sulfur hexafluoride (SF6) dissolved in thermotropic liquid crystals (TLCs) were used to investigate the properties of TLCs. On one hand, environmental effects on the NMR parameters of SF6, 19F nuclear shielding, 19F-33S spin-spin coupling, secondary isotope effects of sulfur on 19F shielding, and the self-diffusion coefficient in the direction of the external magnetic field were studied as well. The temperature dependence of the 19F shielding of SF6 in TLCs was modeled with a function that takes into account the properties of both TLC and SF6. It appears that the TLC environment deforms the electronic system of SF6 so that the 19F shielding tensor becomes slightly anisotropic, with the anisotropy being from -0.5 to -1.4 ppm, depending upon the TLC solvent. On the contrary, no sign of residual dipolar coupling between 19F and 33S was found, meaning that the so-called deformational effects, which arise from the interaction between vibrational and reorientational motions of the molecule, on the geometry of the molecule are insignificant. Diffusion activation energies, Ea, were determined from the temperature dependence of the self-diffusion coefficients. In each TLC, Ea increases when moving from an isotropic phase to a nematic phase. The spin-spin coupling constant, J(19F,33S), increases by ca. 10 Hz when moving from the gas phase to TLC solutions. The secondary isotope shifts of 19F shielding are practically independent of TLC solvent and temperature. For the first time, 19F-33S heteronuclear multiple-quantum NMR spectra were recorded for SF6 in the gas phase and in a liquid-crystalline solution.
Atomistic Origin of Germanate Anomaly in GeO2 and Na-Germanate Glasses: Insights from Two-Dimensional; 17O NMR and Quantum Chemical Calculations.
S.K. Lee and B.H. Lee
J.Phys.Chem.B (2006)110, 16408.
The prominent problem in archetypal germanate glasses is the germanate anomaly where the density exhibits maxima at 15-20 mol % of the alkali oxide content. Here we report 17O two-dimensional NMR spectra for GeO2 and Na-germanate glasses where the presence of both bridging oxygen linking Ge and highly coordinated Ge ([5,6]Ge-O-Ge) and nonbridging oxygen, and an increase in topological disorder are demonstrated at the density maximum, manifesting atomic origins of the anomaly. These densification mechanisms in germanate glasses with Na content are remarkably similar to densification in v-B2O3 with pressure.
Thermally Stable Amorphous Mesoporous Alumniophosphates with Controllable P/Al Ratio: Synthesis, Characterization and Catalytic Performance for Selective O-Methylation of Catechol.
G. Liu, Z. Wang, M. Jia, X. Zou, X. Zhu, W. Zhang, and D Jiang
J.Phys.Chem.B (2006)110, 16953.
Amorphous mesoporous aluminophosphates (AlPO) with P/Al molar ratio in the range 0.8-1.15 are synthesized by using the citric acid (CA) route and are systematically characterized using N2-adsorption, XRD, SEM, solid-state CP-MAS NMR, FT-IR, TG-DTA, CO2-TPD, and NH3-TPD. The characterization studies show that the change in P/Al ratio could affect the structure, texture, thermal stability, and surface acid-base properties of AlPO. Samples with a relatively low P/Al ratio (1.0) exhibit uniform amorphous mesoporous character and high thermal stability (up to 1173 K). Partial crystallization of the AlPO framework easily occurred on the sample with higher P/Al ratio (1.1), thus leading to significant decrease of surface area and formation of particle pile mesopores. Both weak acid and weak base sites are observed over AlPO materials, and the amounts of acid-base sites can be effectively controlled by adjusting the P/Al ratio. The presence of suitable interaction between citric acid and AlPO framework is critical for the formation of mesoporous structures. Both CA and PO4 units are considered to be ligands to coordinate with aluminum ions, forming relative uniform complexes (such as CA-Al-PO4) in the as-synthesized AlPO materials. The mesoporous structure of AlPO materials is obtained after the rapid decomposition of citric acid. Vapor phase selective O-methylation of catechol with methanol reaction is carried out to investigate the catalytic performances of AlPO materials with different P/Al ratios. Among them, AlP1.1O shows the highest activity (88.4% conversion of catechol) and the highest yield of guaiacol (74.3%). The presence of suitable weak acid-base pairs may play an important role on the title reaction.
Selective NMR Measurements of Homonuclear Scalar Couplings in Isotropically Enriched Solids.
S. Cadars, A. Lesarge, N. Hedin, B.F. Chmelka, and L. Emsley
J.Phys.Chem.B (2006)110, 16982.
Scalar (J) couplings in solid-state NMR spectroscopy are sensitive to covalent through-bond interactions that make them informative structural probes for a wide range of complex materials. Until now, however, they have been generally unsuitable for use in isotopically enriched solids, such as proteins or many inorganic solids, because of the complications presented by multiple coupled but nonisolated spins. Such difficulties are overcome by incorporating a z-filter that results in a robust method for measuring pure J-coupling modulations between selected pairs of nuclei in an isotopically enriched spin system. The reliability of the new experimental approach is established by using numerical simulations and tested on fully 13C-labeled polycrystalline L-alanine. It is furthermore shown to be applicable to partially enriched systems, when used in combination with a selective double-quantum (DQ) filter, as demonstrated for the measurement of 2J(29Si-O-29Si) couplings in a 50% 29Si-enriched surfactant-templated layered silicate lacking long-range 3D crystallinity. J-coupling constants are obtained with sufficient accuracy to distinguish between different 29Si-O-29Si pairs, shedding insight on the local structure of the silicate framework. The new experiment is appropriate for fully or partially enriched liquid or solid samples.