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Electronic Photodissociation Spectroscopy of Electrosprayed Anions

TitleElectronic Photodissociation Spectroscopy of Electrosprayed Anions
Publication TypeThesis
Year of Publication2011
AuthorsMarcum, JC
DegreePh.D
Number of Pages221
Date Published2011-05
UniversityUniversity of Colorado
CityBoulder
Abstract

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 “molecular laboratories” for developing new insight to fundamental physical and chemical phenomena.

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, IrBr62-, 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 IrBr5- and IrBr4- help to further characterize the complex potential energy surface of IrBr62- 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, AuCl4- and AuCl2(OH)2-, 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.