Date of Award

Summer 8-27-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Korter, Timothy M.

Subject Categories

Chemistry | Computational Chemistry | Physical Chemistry | Physical Sciences and Mathematics

Abstract

Quantum mechanical models are used to calculate a host of physical phenomena in molecular solids ranging from mechanical elasticity to the energetic stability ordering of polymorphs. However, with the many software packages and methodologies available, it can be difficult to select the most suitable model for the problem at hand without prior knowledge. A promising approach for evaluating the performance of solid-state models is the comparison of the simulations to experimentally measured low-frequency (sub-200 cm-1) vibrational spectra. As this region is dominated by weak intermolecular forces and shallow potential energy surfaces, even slight miscalculations in the solid-state packing arrangements can become readily apparent. In this work, terahertz time-domain spectroscopy and low-frequency Raman spectroscopy are used as benchmark experimental targets to develop computational methodologies for simulating and analyzing the lattice vibrations of molecular crystals such as torsions and translations. The developed computational approaches utilize solid-state density functional theory to account for the periodic nature of a molecular crystal and include careful consideration of the effects that functional choice, basis set composition, and energetic tolerances have on the frequencies and spectral intensities of the sub-200 cm-1 vibrations. These computational methodologies serve as standards for accurately modeling low-frequency vibrations across a range of molecular solids from a small molecule that exhibits unusual thermal behavior to the intricacies of an extensively hydrogen bonded oligopeptide.

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Open Access

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