Ein- und zweidimensionale "7Li-Festkoerper-NMR-Spektroskopie anorganischer und metallorganischer Lithium-Verbindungen

SIGLEAvailable from TIB Hannover: DW 7541 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

(129)Xe NMR of Mesoporous Silicas

The porosities of three mesoporous silica materials were characterized with {sup 129}Xe NMR spectroscopy. The materials were synthesized by a sol-gel process with r = 0, 25, and 70% methanol by weight in an aqueous cetyltrimethylammonium bromide solution. Temperature dependent chemical shifts and spin lattice relaxation times reveal that xenon does not penetrate the pores of the largely disordered (r= 70%) silica. For both r = 0 and 25%, temperature dependent resonances corresponding to physisorbed xenon were observed. An additional resonance for the r = 25% sample was attributed to xenon between the disordered cylindrical pores. 2D NMR exchange experiments corroborate the spin lattice relaxation data which show that xenon is in rapid exchange between the adsorbed and the gas phase

Probing zeolite internal structures using very low temperature {sup 129}Xe NMR

In recent years, probing pore structure with {sup 129}Xe NMR has received a bad reputation. This is due to the fact that the method is more complex than was originally suggested so the data is somewhat difficult to interpret. The authors find that the use of a wide temperature range (40--350 K) allows them to interpret {sup 129}Xe chemical shifts in terms of van der Waals attraction between the xenon atom and oxygen in zeolite walls. Using rather simple models from the literature, they can extract useful pore size information as well as the van der Waals potential energy

High Field Cross Polarization NMR from Laser Polarized Xenon to a Polymer Surface

Surface-selective characterization of materials with NMR has been quite useful in the few cases where sufficient sensitivity and selectivity have been achieved.1 In this communication we report the use of laser-polarized xenon as the source of magnetization for a high-field cross polarization experiment, obtaining surfaceselective magnetization transfer. Gas-phase xenon with nuclear spin polarization several orders of magnitude higher than thermal Boltzmann levels in a high magnetic field can be produced using optically pumped rubidium vapor according to the pioneering work of Happer and co-workers.1 2 The angular momentum of circularly polarized laser light is transferred, via the rubidium electron spins, to the (slowly relaxing) xenon nuclear spin system. We have previously used xenon, with a large polarization enhancement («10 000), as a probe of low surface area materials.3 More recently, using thermal mixing in low field,4 polarization has been transferred between xenon isotopes5 and from xenon to 13C02 occluded in solid xenon.6 The approach of low-field mixing has the disadvantages of nonselective magnetization transfer, typically very short spin-lattice relaxation times in low field, and the necessity of rapid field switching or mechanical transport of the sample to high field for detection. High-field cross polarization methods should be advantageous in allowing nuclear spin selectivity in the transfer step as well as circumventing relaxation and transport problems. In the present work, contact between laser-polarized xenon and surface spins was achieved in high field by Hartmann-Hahn matching of the energy levels in the rotating frame with direct NMR detection of the polarized species.7-8 Proton spins are observed due to their abundance at the surface and the dominant dipolar interactions with adsorbed xenon