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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2021)

Solid-state Nuclear Magnetic Resonance (ssNMR) combined with x-ray diffraction and first-principles computational chemistry, in a method called NMR-assisted crystallography, is used to determine the chemical structure of a variety of materials. 9-ter-butyl-anthracene ester (9TBAE) in molecular crystal nanorods can undergo a photodimerization that causes expansions of up to 15%. The expansion is a result of the formation of a metastable crystalline intermediate, the solid-state reacted dimer (SSRD). A molecular level understanding of 9TBAE using NMR-assisted crystallography is required to fully explain this expansion. ssNMR experiments on aligned nanorods and powder samples of 9TBAE allows for the observation that the nanorod expansion is a result of a rotation of the unit cells from monomer to SSRD. Additionally, the magnitude of expansion or contraction during the reaction can be predicted if the initial orientation of the unit cell with respect to the nanorod axis is known. These results are confirmed by diffraction and computational methods. ssNMR is used to reveal the structure of a few different materials in this work. Silica surface sites, where a lack of sensitivity in NMR makes observation of non-covalent bonding difficult, are observed using Dynamic Nuclear Polarization – Surface Enhanced NMR Spectroscopy (DNP-SENS). Thermally sensitive monomers can also be characterized and the negative thermal expansion they undergo can be described on a molecular level. Finally, a statistical method is described that can be used to discriminate between UV-Vis data of 1:1 and 1:2 host-guest complexes. Statistical methods are also used to compare different non-uniform sampling (NUS) methods in NMR.

    Cover page: Using Solid-State Nuclear Magnetic Resonance to Relate Chemical Structure With the Function of Materials
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2020)
    A spacious Fe(ii)-iminopyridine self-assembled cage complex can catalyze the oxidative dimerization of alkanethiols, with air as stoichiometric oxidant. The reaction is aided by selective molecular recognition of the reactants, and the active catalyst is derived from the Fe(ii) centers that provide the structural vertices of the host. The host is even capable of size-selective oxidation and can discriminate between alkanethiols of identical reactivity, based solely on size.
      Cover page: Selective, cofactor-mediated catalytic oxidation of alkanethiols in a self-assembled cage host
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2021)
      Crystals composed of photoreactive molecules represent a new class of photomechanical materials with the potential to generate large forces on fast timescales. An example is the photodimerization of 9-tert-butyl-anthracene ester (9TBAE) in molecular crystal nanorods that leads to an average elongation of 8%. Previous work showed that this expansion results from the formation of a metastable crystalline product. In this article, it is shown how a novel combination of ensemble oriented-crystal solid-state NMR, X-ray diffraction, and first principles computational modeling can be used to establish the absolute unit cell orientations relative to the shape change, revealing the atomic-resolution mechanism for the photomechanical response and enabling the construction of a model that predicts an elongation of 7.4%, in good agreement with the experimental value. According to this model, the nanorod expansion does not result from an overall change in the volume of the unit cell, but rather from an anisotropic rearrangement of the molecular contents. The ability to understand quantitatively how molecular-level photochemistry generates mechanical displacements allows us to predict that the expansion could be tuned from +9% to -9.5% by controlling the initial orientation of the unit cell with respect to the nanorod axis. This application of NMR-assisted crystallography provides a new tool capable of tying the atomic-level structural rearrangement of the reacting molecular species to the mechanical response of a nanostructured sample.
        Cover page: Bridging photochemistry and photomechanics with NMR crystallography: the molecular basis for the macroscopic expansion of an anthracene ester nanorod
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