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  • March 21, 2019
  • Cheshire, Thomas
    • Affiliation: College of Arts and Sciences, Department of Chemistry
  • ABSTRACT Thomas Paul Cheshire: Higher-Order Effects in Condensed Phase Spectroscopy and Dynamics (Under the direction of Andrew M. Moran) Researchers in the 1970s wondered whether traditional Raman experiments could distinguish homogeneous and inhomogeneous line broadening mechanisms. Since then, a feedback between experiment and theory has spawned and matured the field of multidimensional Raman spectroscopy and laid the groundwork for modeling nonlinear photoinduced reaction pathways. Here two-dimensional resonance Raman (2DRR) spectroscopy is developed to investigate photochemical reaction mechanisms and structural heterogeneity in condensed phase systems. Models are developed to understand 2DRR spectra and extended to incorporate non-radiative transitions. The photodissociation reaction of triiodide serves to uncover the capabilities of 2DRR. A unique pattern of 2DRR resonances is associated with the transition of a nuclear wavepacket from reactant to product. The pattern of resonances is reproduced by modeling the photodissociation as a vibronic coherence transfer. Transient absorption experiments performed on a transition metal complex composed of titanium and catechol, [Ti(cat)3]2-, exhibit signatures of coherent wavepacket motion initiated by back-electron transfer. The model used for triiodide photodissociation applies to this system, and calculations predict that vibrational coherences in the product are independent of whether the reactant undergoes coherent nuclear motion. Vibrational population-to-coherence transitions could accelerate the electron transfer (ET) process, regardless if vibrational dephasing is faster than the reaction rate--a prediction not captured by traditional ET models. 2DRR spectroscopy is further used to investigate oxygen- and water-ligated myoglobin line broadening mechanisms. Vibrational modes proximal to propionic acid side chains of the heme exhibit significant heterogeneity in the 2DRR spectra. A hydrophobic pocket encompasses the heme, but the side chains are exposed to solvent. Molecular dynamics (MD) simulations suggest that fluctuations in the side chain geometries are correlated with the heterogeneity. 2DRR spectra and MD simulations reveal that the side chains function as effective pathways for thermal relaxation. Despite progress, a major challenge still plagues multidimensional Raman spectroscopy. Cascading signals radiated in the same direction as the desired signal can render a signal impossible to analyze. Simulations of 2DRR, femtosecond stimulated Raman spectroscopy, and the accompanying artifacts suggest solute-solute and solute-solvent interactions can significantly affect measured signals if experimental parameters are not carefully selected.
Date of publication
Resource type
  • Moran, Andrew
  • Atkin, Joanna
  • Kanai, Yosuke
  • Berkowitz, Max
  • Cahoon, James
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill
Graduation year
  • 2018

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