https://home.uni-leipzig.de/energy/publist/pfeifer1985.pdf

Nuclear magnetic resonance studies on the acidity of zeolites and related catalysts

Confinement of a nematogen in submicrometer cavities can produce substantial effects on the nematic-isotropic phase transition and on order and orientation of the nematic phase. We combine the methods of 13C- and 1H-NMR and broadband dielectric spectroscopy to probe liquid crystalline order and molecular mobility. We find that the dynamics of mesogenic molecules (5CB) in the free volume of nanoporous sol-gel glass (5 nm pore diameter) is as fast as in the free bulk phase. In addition, a boundary layer of molecules with reduced mobility covers the pore walls. Isotropic-nematic transition as well as crystallization of the confined nematogen are considerably suppressed. In order to extract the temperature dependence of the nematic order parameter from NMR data, a mathematical model is applied which describes the combined influences of orientational order and diffusional averaging in randomly oriented pores on NMR spectra of the nematogens.

Measurement of orientational order and mobility of a nematic liquid crystal in random nanometer confinement

Application of the NMR broad line technique to study phase transitions in microporous materials

Measurements of self-diffusion in the cholesteric liquid-crystalline phase by means of 13C NMR

13C-NMR measurements of the chiral mesogen cholesteryl-undecylcarbonate (CHUC) adsorbed in Anopore membranes with cylindrical pores of 0.2μm diameter are presented. The pitch of CHUC in the cholesteric phase is of order of the pore diameter, and in view of the planar axial anchoring a strong influence of the walls on the director field can be expected. Simulation of the angular dependence of 13C-NMR line shape allows to determine a mean tilt angle of 60μ of the director with respect to the helical axes which are oriented along the channels. Furthermore, at T=341K a correlation time of 130μs for reorientational motions could be obtained which is connected with molecular diffusion along the pores. Optical and NMR-measurements are compatible with a tilted helical director field for the cholesteric LC material within the pores.

Induction of Tilted Cholesteric Structures in Microconfined Liquid Crystals

Es wird ein statistisches Modell zum Einflus der thermischen Bewegung auf die NMR-Linienform vorgestellt, das die Verschmalerung von Festkorper-Spektren bei wachsender Temperatur beschreibt. Das Modell geht von der Annahme aus, das nach einer Ortsveranderung eines Kerns infolge thermischer Bewegung jede beliebige Kernresonanzfrequenz mit der durch das Festkorperspektrum vorgegebenen Wahrscheinlichkeit angenommen werden kann. Am Beispiel der Festkorper-Gauslinie wird der Unterschied zu dem bekannten Modell von ANDERSON und WEISS verdeutlicht.



A Statistical Model for the Influence of Thermal Motion on N. M. R. Spectra in Solids



A theory is proposed which allows to describe the narrowing of n. m. r.-line width in the presence of thermal motions of the spins. The model is based on the assumption, that the local resonance frequency of a given spin immediately after the jump is distributed according to the n. m. r.-line shape of the rigid lattice. The difference to the well-known ANDERSON-WEISS-model of spectral narrowing is demonstrated for a gaussian line shape.

D. Freude

Papers

Ein statistisches Modell zum Einfluß der thermischen Bewegung auf NMR-Festkörperspektren