Graduate Thesis Or Dissertation
 

Using Computational Studies to Characterize Aqueous Metal Clusters : From Spectroscopy to Stability

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/n870zw781

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  • In this dissertation, the spectroscopic properties and thermodynamic stability of a class of materials known as metal oxo and hydroxo clusters are studied. In aqueous solutions, many metals can aggregate into discrete clusters of metals bridged by oxo or hydroxo ligands. These clusters are of particular interest to us for their use as precursors for metal oxide thin films. Understanding the stability of these clusters, along with distinguishing ways to uniquely identify particular clusters via spectroscopy, is vital to improving the quality of cluster precursors and the metal oxide thin film products.Vibrational spectroscopy is an appealing technique for the identification of cluster species because of the relative ease to collect experimental IR and Raman spectra. However, uniquely identifying cluster vibrations is difficult due to the similarity of ligands types and vibrational modes between different clusters. Disclosed herein are two studies of vibrational spectroscopy applied to metal hydroxo clusters of Group 13 metals. These studies focus on one cluster structure in particular, the flat tridecamer, M13(OH)24(H2O)2415+, where M= Al3+ or Ga3+. The vibrations of the NO3- counterions used in these studies overlap with the frequencies associated with cluster vibration, which limits the uniqueness of the Raman and IR signals. Nonetheless, full assignment of peaks was made for solid state and solution phase flat tridecamers.NMR spectroscopy is an appealing technique for the identification and characterization of cluster species because of the amount of structural information that can be gleaned from NMR spectra. In one study, we used 1H-NMR to characterize flat tridecamers with a mixture of gallium and indium. Computations struggle to predict NMR spectra because these spectra reflect average structures over time, and static computations rely on single structures and are especially sensitive to small deviations in atom positions. In this study, we computed the 1H-NMR signals for the flat tridecamers Ga13-xInx(OH)24(H2O)2415+, and fully assigned the experimental proton signals to each proton coordination environment that is possible for these clusters. Quantum mechanical computations can also be used to understand the stability of different cluster species relative to each other. In this dissertation, the stability of Al, Ga, and Hf clusters is explored. Hf tetramer clusters, Hf4(OH)8(H2O)168+, were examined with peroxide substitution for a pair of hydroxide or water ligands. These calculations suggest that peroxide prefers to substitute for the terminal water ligands versus the bridging hydroxide ligands. This study also highlights the importance of accurate solvation models for computing the structures and stabilities of clusters in aqueous solutions.For the Group 13 metals aluminum and gallium, the stability of different clusters can be computed based on the identity and binding modes of the different ligands. Using ligand identities to compute hydrolysis energies can reproduce quantum mechanical computations of cluster stability with extremely high accuracy. However, these calculations do not include the cluster counterions, which can affect the stability of high-charge clusters. For a full determination of cluster speciation in solution, counterions are required for accurate speciation across all possible species. The speciation and structural conformations of Al clusters was studied with Cl-, NO3-, HSO4-, and ClO4- counterions.
  • Keywords: Materials Chemistry, Chemistry, Computational Chemistry
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