Spectroscopic studies of nuclear spins polarized via spin exchange optical pumping and dynamic coupling in cryptophane host-guest complexes.

摘要

NMR is a powerful analytical spectroscopic tool used to perform detailed studies of structure and dynamics of molecules in solution. However, despite NMR's excellent spectral sensitivity, most NMR methods suffer from low detection sensitivity. This low detection sensitivity results largely from extremely small (Boltzmann) nuclear spin polarization at thermal equilibrium---in even the strongest of magnets. This dissertation focuses on selected research areas that maybe used to combat the limitations presented by NMR and measure weak spectral responses with atomic-scale precision. In particular, these methods involve the use of laser-polarized xenon, liquid crystals, and polarization transfer (cross-polarization) techniques to enhance NMR sensitivity and/or measure weak interactions. The potential use of these tools to study host-guest interactions is of particular interest.;In certain systems the sensitivity problem of conventional NMR/MRI can be overcome by applying optical pumping (OP) methods to enhance nuclear spin polarization. For instance, OP of noble gases (such as xenon) is employed to dramatically increase their nuclear spin polarization by transferring angular momentum of laser light to electronic and then nuclear spins.1;Next, cryptophane complexes are ideal choices for fundamental studies of prototypical host-guest interactions. Of general interest when studying host-guest interactions is how (1) physical confinement at the nanoscale and (2) interactions between guest and host may affect the properties, dynamics, interactions, and/or reactivity of a trapped molecule and the host/guest complex as a whole. As a more specific example, we are interested in probing host-guest dynamic coupling,2 which refers to the relative motion of the guest within the host, determined by the relative sizes and geometries---as well as the interactions involved. With the development of new NMR methods and techniques, we hope to gain insight into mechanisms that underlie complex formation by probing the structures, dynamics and energetic contributions involved in ligand binding, where molecular contributions such as: orientational and motional freedom of the guest; and structure, dynamics, and ordering of the host can influence the behavior of inclusion complexes.;Chapter one deals with the basic fundamentals of NMR spectroscopy that are needed in order to understand the concepts and phenomena involved in this dissertation. Chapter two concerns the basic fundamentals of liquid crystals such as PBLG and Cromolyn. Furthermore, this chapter also provides a basic understanding of liquid crystal NMR. Chapter three provides background regarding optical pumping: Firstly, the physical and chemical properties of xenon are discussed in detail. Secondly, basic fundamentals of how laser diode arrays and volume holographic gratings operate. Lastly the theoretical and experimental aspects of alkali metal spin exchange optical pumping are discussed. Chapter four discusses some fundamental properties of cryptophanes that are studied in this dissertation. The last of the background chapters, chapter five discusses the aspects of Density Functional theory used to perform quantum chemical calculations on cryptophane complexes.;Chapter six presents the improved generation of laser-polarized xenon using a fiber-coupled LDA narrowed with an integrated volume holographic grating (VHG) used to study the dependence of PXe and PRb on the VHG-LDA excitation profile. Chapter seven presents the uses of a VHG narrowed LDA to study the effects of laser power, linewidth, and gas densities on the production of laser polarized xenon. Chapter eight discusses the study of cryptophane and chloroform as host-guest complexes in PBLG liquid crystals to study dynamic coupling with the aid of adiabatic Hartmann-Hahn cross-polarization. Chapter nine pertains to the uses water-soluble cryptophanes and xenon aligned in cromolyn liquid crystals in preliminary studies to achieve intermolecular adiabatic Hartmann-Hahn cross-polarization. Lastly, Chapter ten involves the preliminary studies to obtain thermodynamic and kinetic information from chloroform cryptophane complexes using both computational and experimental methods.