Time Resolved Vibrational Spectroscopies as a Tool for Exploring Dynamics of Confined Systems

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Time Resolved Vibrational Spectroscopies as a Tool for Exploring Dynamics of Confined Systems

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This thesis examines a variety of vibrational probe-containing molecules such as triphenyl hydrides, CO2, and metal carbonyls with the goal of better understanding the dynamics for each system. Particular emphasis is placed on understanding how the behavior of a restricted probe, such as one dissolved in a rigid polymer or confined to a nanopore, may differ from the same probe placed in bulk solvent or a more rubbery polymer. The first study described herein scrutinized the vibrational heavy atom effect and its impact on ultrafast vibrational dynamics. A series of three triphenyl hydride compounds was investigated in a range of solvents by Fourier transform infrared (FTIR), infrared (IR) pump-probe, and two-dimensional infrared (2D-IR) spectroscopies. The mass of the central atom in the three compounds was varied systematically down the group 14 elements of silicon, germanium, and tin while keeping the rest of the molecule unaltered. Interestingly, frequency-frequency correlation functions obtained from 2D-IR spectra indicated that an increasingly large central atom produces small, but measurable changes in the dynamics of the solvation shell surrounding each compound. Next, CO2 (g) was examined via 2D-IR spectroscopy as a precursory study to understanding its behavior inside polymers. Processes which lead to dephasing of the vibrational echo such as collisions were largely circumvented by using CO2 diluted in N2 under ambient pressure and temperature. Off diagonal features in the 2D-IR spectra were observed which correspond to population and coherence exchange between rovibrational transitions. Then, CO2 (g) was dissolved inside polymers such as poly(methyl methacrylate), poly (methyl acrylate), and poly(dimethylsiloxane). These polymers with differing properties were chosen to study the impact of the glass transition on the dynamics of the dissolved CO2 probe. Interactions between the polymeric backbone and probe also impacted the dynamics. The parameters obtained from 2D-IR studies directly correlated with the diffusivity of CO2 through the polymer matrices. Next, I inspected CO2 (g) adsorbed to microporous systems such as MIL-53(Al) and ZIF-8. Preliminary FTIR studies suggest that these samples could possess a wealth of dynamic information despite narrow FTIR peaks, much like CO2 dissolved in polymers. Experimental limitations regarding these novel systems are briefly discussed. Lastly, I compared the dynamics of three ruthenium-bound carbonyl complexes: Ru3CO12 in bulk THF, [HRu3(CO)11]- entrapped in an aluminum sol-gel, and [NEt4][HRu3(CO)11] in bulk THF. Ru3CO12 is catalytically inactive but becomes active upon incorporation into an alumina sol-gel matrix. Pump probe and 2D-IR studies indicated that the changed dynamics are primarily due to an altered solvent shell which most likely exhibits long-range ordering. Though it is uncertain whether the increased catalytic activity of [HRu3(CO)11]- is due to the presence of the hydride or this newly ordered solvent shell, the results nonetheless showcase 2D-IR’s efficacy in sensing dynamics of confined environments.


University of Minnesota Ph.D. dissertation. 2022. Major: Chemistry. Advisor: Aaron Massari. 1 computer file (PDF); 178 pages.

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Pyles, Cynthia. (2022). Time Resolved Vibrational Spectroscopies as a Tool for Exploring Dynamics of Confined Systems. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/226643.

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