Discovering the Electronic Properties of Metal Hydrides, Metal Oxides and Organic Molecules Using Anion Photoelectron Spectroscopy
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Date
2015-10-06
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Johns Hopkins University
Abstract
Negatively charged molecular ions were studied in the gas phase using anion photoelectron spectroscopy. By coupling theory with the experimentally measured electronic structure, the geometries of the neutral and anion complexes could be predicted. The experiments were conducted using a one-of-a-kind time-of-flight mass spectrometer coupled with a pulsed negative ion photoelectron spectrometer. The molecules studied include metal oxides, metal hydrides, aromatic heterocylic organic compounds, and proton-coupled organic acids.
Metal oxides serve as catalysts in reactions from many scientific fields and understanding the catalysis process at the molecular level could help improve reaction efficiencies (Chapter 1). The experimental investigation of the super-alkali anions, Li3O- and Na3O-, revealed both photodetachment and photoionization occur due to the low ionization potential of both neutral molecules. Additionally, HfO- and ZrO- were studied, and although both Hf and Zr have very similar atomic properties, their oxides differ greatly where ZrO- has a much lower electron affinity than HfO-.
In the pursuit of using hydrogen as an environmentally friendly fuel alternative, a practical method for storing hydrogen is necessary and metal hydrides are thought to be the answer (Chapter 2). Studies yielding structural and electronic information about the hydrogen bonding/interacting in the complex, such as in MgH- and AlH4-, are vital to constructing a practical hydrogen storage device.
Chapter 3 presents the negative ion photoelectron spectra of aromatic heterocylic organic compounds, tetraphenylporphine- anions, with the metal center (Fe, Ni, or Mn), and the stable closed-shell quinoline- anion. Both of these systems can be viewed as components of metal-organic-frameworks (MOFs), which are used to sequester greenhouse gases such as CO2.
By measuring the hydrogen bond strength between proton-coupled bicarboxylates and other molecules found in the enzyme-substrate complex in the gas phase, provides an upper limit for the forces available to the enzyme (Chapter 4). The intermolecular hydrogen bond strength for formate-formic acid, acetate-acetic acid, imidazolide-imidazole, and phenolate-phenol were experimentally determined to be quite strong (1.0-1.4 eV).
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Keywords
molecules, photoelectron spectroscopy, electron affinites