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Solid-State NMR Spectroscopy, Anisotropic Interactions, and the Elucidation of Molecular Structure

  • Author(s): Kudla, Ryan
  • Advisor(s): Mueller, Leonard J
  • et al.
Creative Commons Attribution 4.0 International Public License
Abstract

The research in this thesis illustrates how chemical shift (CS) and chemical shift anisotropy (CSA) give a detailed view into atomic and molecular structure. Through the collaborative approach of NMR crystallography, utilizing solid-state NMR, x-ray diffraction, and computational methods a deeper understanding of crystal structure and systematic motion is formed. This thesis in constructed in a linear fashion as to shown how more detailed information can be obtained via more complex methods, culminating in the application of all the methods presented within to describe the structure and mechanism of expansion of a the novel anthracene system 9-tertbutylantrhacene ester (9TBAE).

Utilizing the CS, which provides information about the local electronic structure, directed questions such as the structure or arrangement of atoms can be answered. In this thesis problems in which the CS is used to provide detailed information about the location or inclusion of specific atoms in a molecule’s crystal structure will be presented. As crystallographic information for structures with greater complexity is required, the CS alone may be unable to differentiate between multiple similar structures, requiring a method that provides even greater detail of the local electronic environment.

The CSA is capable of providing this greater detail by quantifying a molecule’s discrete orientations in the magnetic field. Obtaining CSA values is non-trivial, requiring the application of pulse sequences with significantly greater complexity than those required to obtain the isotropic CS. Utilizing the approaches of slow spinning, a modified TOSS-t1-DeTOSS sequence, and xCSA, CSA principle components are obtained for increasingly complex systems.

The presentation of these methods culminates in their application on 9TBAE to elucidate not only the molecular structure, but also the mechanism of expansion when 9TBAE nanorods are photoreacted. The data obtained shows the generation of new lattice orientations within the nanorod, and based on these observations, the nanorods expand not due to a change in the volume of the unit cell, but rather due to a rotation of the unit cells. These results demonstrated while most photomechanical materials rely on the generation of a mixed phase bimorph structure, reconfiguration of the product phase can likewise generate a large mechanical response.

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