Use of atomic and molecular probes in NMR studies of materials and construction of a xenon-129 hyperpolarizer

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Xenon atoms and sulfur hexafluoride (SF₆) molecules can be dissolved in liquids and liquid crystals or adsorbed in porous materials. Nuclear magnetic resonance (NMR) spectra of ¹²⁹Xe or ¹⁹F nuclei reveal information about their surroundings. This means that xenon atoms and SF₆ molecules can be used as probes to indirectly study materials by NMR spectroscopy. The change in the spectra arises from a NMR interaction called shielding. Especially in the case of xenon, shielding reveals even the slightest changes, for example, in the density of a liquid it is dissolved in. Because a change in temperature leads to a change in the density of the liquid as well, temperature change is observed as a shift of the resonance line in the ¹²⁹Xe NMR spectrum. This property can be utilized in the accurate determination of the sample temperature. Self-diffusion measurements of the gases provide additional information on a larger scale rather than just the immediate surroundings of atoms or molecules. Various liquid crystals were studied using xenon and SF₆ as probes proving their applicability.</p><p>It is often considered that the signal observed in NMR experiments is very weak and limits the full potential of the method. This is true especially with the samples in gaseous form. The Spin-Exchange Optical Pumping (SEOP) hyperpolarization method solves this problem in the case of xenon. A ¹²⁹Xe NMR signal can be enhanced by a factor of 10⁴–10⁵ by SEOP and this opens access to techniques that are not otherwise possible. The remote detection technique, which separates the encoding and detection steps of the typical NMR experiment both temporally and spatially, is one of these techniques. The potential of the combination of SEOP and remote detection techniques was shown in studies of thermally modified Pinus Sylvestris.</p></div>