J. Am. Chem. Soc., 130 (2), 414 -415, 2008.
Unexpected Fractionation in Site-Specific 13C Isotopic Distribution Detected by Quantitative 13C NMR at Natural Abundance
Eliot P. Botosoa, Elsa Caytan, Virginie Silvestre, Richard J. Robins, Serge Akoka, and Gérald S. Remaud
The recent development of 13C NMR to measure absolute 13C/12C ratios at natural abundance has made it possible to examine site-by-site variation at natural abundance. This technique has been applied to two common physical processes: distillation and column chromatography. It is shown that the target molecules, ethanol and vanillin, respectively, both show isotopic fractionation that differs in size and/or in sense at each carbon atom. The observation of this disparity in behavior is novel and unexpected. 13C SNIF-NMR appears as a new effective tool to study noncovalent isotope effects (NCIE) on target molecules.
J. Am. Chem. Soc., 130 (2), 481 -490, 2008.
31P Magic Angle Spinning NMR Spectroscopy of Paramagnetic Rare-Earth-Substituted Keggin and Wells-Dawson Solids
Wenlin Huang, Mark Schopfer, Cheng Zhang, Robertha C. Howell, Louis Todaro, Becky A. Gee, Lynn C. Francesconi,* and Tatyana Polenova*
Paramagnetic rare-earth elements have been examined as NMR structural probes in polyoxoanionic solids, which have a variety of applications as luminescent materials that are usually disordered and therefore intractable by traditional structural methods. Thirteen Keggin and Wells-Dawson polyoxotungstates containing substitutions with lanthanides of different effective magnetic moments have been examined by 31P magic angle spinning NMR spectroscopy. The electron-nuclear dipolar interaction dominating the spinning sideband envelopes is determined by the lanthanide's magnetic moment and was found to be a sensitive probe of the nature of the polyoxoanion, of the positional isomerism, and of the ion stoichiometry. Electron-nuclear dipolar anisotropies computed based on the point-dipole approximation are generally in good agreement with the experimental results. The choice of a specific lanthanide as a structural probe can be tailored to the desired distance range between the phosphorus atoms and the paramagnetic centers to be probed. This approach is expected to be particularly useful in the paramagnetic polyoxoanionic materials lacking long-range order.
Phys. Rev. B 76, 214424 (2007)
Magnetization and 63Cu NMR studies on granular FeCu alloys
B. Bandyopadhyay, B. Pahari, and K. Ghoshray
Nanostructured FeCu granular alloys (Fe ~1%–20%) have been prepared by borohydride reduction and characterized by energy dispersive spectroscopy, x-ray, and transmission electron microscopy studies. Study of zero-field-cooling–field-cooling magnetization yields blocking temperature distribution of magnetic fine particles system in all samples. At low Fe concentration, the magnetizations of the samples combine superparamagnetism and paramagnetism near room temperature and small hystereses at 5 K. High Fe content alloys are almost entirely ferromagnetic even at 300 K. However, in all these samples, the observation of the 63Cu nuclear magnetic resonance signal at all temperatures 4–300 K confirms the existence of a paramagnetic component having ~0.02–0.04 at. % Fe in Cu. The temperature dependence behavior of Fe contribution to 63Cu NMR linewidth and the Knight shift indicate that the paramagnetic component might exhibit a Kondo temperature of ~24 K that is significantly higher than that obtained in bulk dilute FeCu alloys.