<p>During the past fifty years, mass spectrometry, often hyphenated with chromatography, has developed into the most widely used technique for the quantitative and qualitative analysis of increasingly complex mixtures of (bio)organic molecules.</p> <p>One important aspect of this development concerns the relationship between the structure (atom connectivity) of a molecule and the mass spectrum obtained by electron ionization (EI). In this context, from 1960 - 1990, a wealth of studies has appeared that uses a variety of novel experimental techniques, often in conjunction with isotope labelling, to probe the structure, stability, reactivity and dissociation characteristics of the radical cations generated by EI of various classes of molecules. One highlight was the discovery of surprisingly stable distonic ions and the role they play in the dissociation chemistry of ionized molecules.</p> <p>However, mechanistic proposals based upon experimental observations can often only be considered as tentative. Synergy between experiment and theory would be ideal to remedy this situation, but it was not until recent spectacular advances in computer technology and software that this approach could be implemented. It has led to the growing realization that many rearrangement reactions of radical cations in the rarefied gas-phase involve catalysis. Proton-transport catalysis (PTC) is a prime example : here, a neutral species induces an ion to isomerize via hydrogen-bridged radical cations (HBRCs) as intermediates. An exemplary case described in this thesis concerns the ion-molecule reaction of the cyanamide ion with a single H₂O molecule : experiment and theory indicate that the H₂O molecule catalyzes the swift transformation of NH₂-CN^·⁺ into the more stable carbodiimide ion HN=C=NH^·⁺.</p> <p>The thesis exploits the synergy of tandem mass spectrometry and computational chemistry to study the role of catalysis in the association and dissociation reactions of several systems of radical cations. During these studies, a new type of a catalyzed reaction was discovered: "ion-catalysis", where an organic cation promotes the otherwise prohibitive rearrangement of a neutral. Ion-catalysis is proposed to explain the unexpected loss of NH₂O^· from low-energy N-hydroxyacetamide ions CH₃C(=O)NHOH^·⁺ : the molecular ion rearranges into the HBRC [O=C-C(H₂)--H--N(H)OH]^·⁺ whose acetyl (cation) component catalyzes the transformation NHOH^· --> NH₂O^·. Another highlight involves a hybrid reaction, in which both the ion and the neutral component of an incipient HBRC catalyze one another to rearrange into more stable isomers.</p> <p>Catalysis may also play an important role in astrochemistry and a question addressed in this context is whether pyrimidine, a key component of DNA, may be generated by ion-molecule reactions. It appears that the acrylonitrile ion (AN) does not react with HCN to produce ionized pyrimidine, instead it isomerizes by PTC. However, the reaction of the ion with its neutral counterpart does not involve catalysis, but rather cyclization into the pyrimidine ion ! A related topic concerns the structures of covalently bound dimers of the ubiquitous interstellar molecules HCN and HNC. Neutralization-Reionization Mass Spectrometry in conjunction with model chemistry calculations leaves little doubt that the elusive dimers HN=C=C=NH and HC=N-C=NH are kinetically stable in the rarefied gas-phase, whereas HC=N-N=CH is not.</p> <p>The structure of ions may also be probed by interactions with selected neutral molecules rather than dissociative collision experiments (MS/MS). An exciting case involves the differentiation of isomeric heterocyclic ions by ion-molecule reactions with dioxygen. Here, too, model chemistry calculations play an essential role in understanding the mechanism and the scope of the reaction.</p> / Doctor of Philosophy (PhD)

<p dir="ltr">Interfaces play an important role in a broad range of physical, chemical, and biochemical processes. For example, nutrient transport from and to cells happens at the cellular membrane interface, the corrosion of metals occurs due to chemical reactions at the solid/air interface, and the development of waterproof clothing relies on the modification of the clothing surface with hydrophobic species. The importance and complexity of interfaces make a detailed understanding of the interfacial physicochemical processes central to both the fundamental science and the development of new technologies. Specifically in the fields of energy storage/production and heterogeneous catalysis, understanding the transformations of the active species on surfaces leads to the development of high-performance, stable interfaces. In the thesis presented herein, ion soft landing was used as a preparative technique to understand the chemical changes that ions undergo on surfaces. Ion soft landing is a mass spectrometry technique in which polyatomic ions are deposited onto surfaces while preserving their chemical structure and charge state. The advantage of using ion soft landing to study interfaces is that it enables the preparation of well-defined ionic interfaces by the deposition of mass-selected ions on a defined surface area with high control over the amount of deposited material. Because ion soft landing uses purified ion beams formed in the gas phase, it also allows to study the chemical properties of the analytes in the absence of counterions or solvent molecules. Collectively, these capabilities make ion soft landing a powerful approach for preparing ionic interfaces and studying their chemical properties. A new direction in ion soft landing research takes advantage of gas phase ion chemistry techniques, such as collision-induced dissociation, to generate well-defined reactive fragment ions as unique building blocks for studying chemistry at interfaces. <b>Chapter 2 </b>of this thesis discusses the development of an ion soft landing instrument that enables high transmission of fragment ions for their deposition onto surfaces. Ion soft landing of reactive fragment ions opens up possibilities for studying their stability and reactivity on surfaces providing a path to the controlled preparation of unique ionic interfaces. <b>Chapters 3 </b>and <b>4 </b>describe an unusual spontaneous ligand loss observed for soft landed [Ni(bpy)₃]²⁺⁰, an ion of interest in the field of catalysis, and its stabilization by codeposition with anions. We compared the reactivity of [Ni(bpy)₃]²⁺on surfaces against that of [Ni(bpy)₂]²⁺and [Ni(bpy)]⁺species (both formed by ligand removal in the gas phase). This comparison indicates that the dissociation of [Ni(bpy)₃]²⁺ occurs both due to its reorganization on a surface and by charge-reduction. Both processes substantially reduce ligand binding energy and facilitate ligand loss from the complex.</p><p dir="ltr"><b>Chapter 5 </b>diverges from ion soft landing and instead presents a gas-phase ion chemistry study on the stability of cucurbituril-viologen host-guest complexes to better understand the intrinsic properties that influence the strength of their interaction. We found that there is a “perfect fit” size of the host that maximizes interactions with the guest thus increasing its stability. In addition, guests of smaller sizes that are better incorporated into the host have a substantial stability compared to those that have functional groups extending outside of the protecting cavity of the host. The results of this work reveal a strategy to stabilize viologens in the gas phase for the preparation of functional interfaces using ion soft landing.</p><p dir="ltr">Finally, <b>Chapter 6 </b>shows the results of a work at the teaching/learning interface, specifically regarding an undergraduate research project developed for the Analytical Chemistry I course (CHM323) at Purdue University. The goal of this project was to further develop students’ scientific skills on planning, problem-solving, and critical thinking to assess the performance of two analytical techniques. Specifically, the project described in <b>Chapter 6 </b>was designed in such a way that students had to do research on appropriate analytical techniques to quantify ascorbic acid in an unknown sample, propose an experimental protocol, perform it in the laboratory, and concisely summarize the results of their work in a lab report.</p><p dir="ltr">In summary, the work presented in this thesis encompasses three areas. First, it shows the advantages of using fragment ions produced in the gas phase to study the complex physicochemical processes occurring at interfaces. Second, it presents a study on the gas-phase stability of viologen-based host-guest complexes with the prospect of making viologens accessible for the preparation of functional interfaces using ion soft landing. Finally, it describes an undergraduate laboratory project aimed at developing the scientific skills of students in an analytical chemistry course.</p>

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