Electronic Photodissociation Spectroscopy of Electrosprayed Anions

<p>Ionic compounds are ubiquitous and play central roles in a variety of chemical, physical, and biological processes. Due to the incredible complexity of many of these processes, it is advantageous to develop a detailed molecular-level description of ions as isolated species. This approach provides the ability to study the intrinsic properties of ions in the absence of perturbing effects from solvent and/or counterions. Additionally, such studies can be used to perform experiments on model systems in which ions serve as \textquotedblleftmolecular laboratories\textquotedblright for developing new insight to fundamental physical and chemical phenomena.</p> <p>This thesis is comprised of work I have carried out on a number of gas-phase anionic systems of both applied and fundamental interest. Early stages of my work focused on the development of instrumentation for performing electronic photodissociation spectroscopy on relatively large and/or fragile anionic species. A complete account of the resulting apparatus is provided, followed by an account of a number of experiments that employed photodissociation spectroscopy to extract intrinsic information on the electronic energy levels, photofragmentation mechanisms, and dissociation thresholds for a variety of ionic species. Experiments on gas-phase mononucleotides provided information necessary for understanding the complex sequence of events involved in the ultra-violet photodissociation of nucleotides, where initial excitation of the nucleobase chromophore is followed by rapid internal conversion, energy redistribution, nuclear rearrangement, and eventually fragmentation via a number of complex mechanisms. Studies on the prototypical gas-phase dianion, IrBr<sub>6</sub><sup>2-</sup>, highlight the importance of repulsive Coulomb barriers in determining the overall stability and spectroscopy of multiply-charged anions, where a variety of potential dissociation mechanisms were identified to be highly dependent upon the nature of photoexcitation. Experiments on gas-phase IrBr<sub>5</sub><sup>-</sup> and IrBr<sub>4</sub><sup>-</sup> help to further characterize the complex potential energy surface of IrBr<sub>6</sub><sup>2-</sup> and provide direct spectroscopic information on under-coordinated transition metal complexes that would be otherwise extremely difficult to obtain due to their transient, reactive nature. Work on two chloroaurate complexes, AuCl<sub>4</sub><sup>-</sup> and AuCl<sub>2</sub>(OH)<sub>2</sub><sup>-</sup>, provides insight into the photoreduction and speciation of gold(III) compounds. Finally, in a slight deviation from the above-themed work, infrared predissociation spectroscopy was used to elucidate the structure of water networks around the nitromethane anion and how solvation affects anion structure.</p>
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University of Colorado Boulder
Advisors - JILA Fellows