<p> Small nitrogen or silicon containing molecules and their ionized counterparts have attracted a great deal of attention from both experimentalists and theoreticians. This is because such species may play an important role in interstellar chemistry. Small nitrogen containing heterocyclic molecules and ions are of biological importance and are of increasing interest to chemists and biochemists. Therefore, studying the chemistry of such species in the gas phase as solitary ions and neutrals is of considerable importance.</p> <p> The ions studied in this thesis were generated in the rarefied gas phase of the mass spectrometer by (dissociative) electron ionization of selected precursor molecules. The characterization of their structure and reactivity was realized using a variety of tandem mass spectrometry based techniques. These include metastable ion spectra to probe the dissociation chemistry of the low energy ions and (multiple) collision experiments to establish the structure (atom connectivity) of the stable ions. The technique of neutralization-reionization mass spectrometry (NRMS) was used to probe the structure and stability of the neutral counterparts of the ions. The results of theoretical calculations involving the CBS-QB3 model chemistry formed an essential component in the interpretation of the experimental findings.</p> <p> The above approach was used to study various isomers of the CH2N2•+ family of ions, in particular the elusive carbodiimide ion HN=C=NH•+. Unlike other isomers including ionized cyanamide H2N-C≡N•+ and nitrilimine HC=N=NH•+, the carbodiimide ion could only be identified by a strategy that involved the use of the technique of collision-induced dissociative ionization (CIDI) mass spectrometry. Guided by the results of the theoretical calculations, the carbodiimide ion, the global minimum on the CH2N2•+ potential energy surface, could also be generated by the molecule-assisted isomerization of its 1,3-H shift isomer H2N-C≡N•+. This occurs through an ion-molecule interaction with a single molecule of H2O under conditions of chemical ionization : a process termed proton-transport catalysis.</p> <p> The amino-substituted carbodiimide ion, H2N-N=C=NH•+, and nine more isomers have all been calculated to be minima on the potential energy surface of the CH3N3•+ family of ions. Four of these, viz. the aminocarbodiimide ion, the aminonitrilimine ion, H2N-C=N=NH•+, the cyclic C-aminoisodiazirine ion, and the ionized biradical, H2N-N=C(H)=N•+, were generated and characterized by tandem mass spectrometry as stable species in the gas phase. Neutralization-reionization experiments show, in agreement with the CBS-QB3 computational results, that aminocarbodiimide and aminonitrilimine are stable species in the gas phase.</p> <p> Next, the α-distonic ions of a number of heterocyclic nitrogen-containing compounds including pyrazole, pyridazine and aminopyrazine, were characterized and differentiated by tandem mass spectrometry. The stabilities of the neutral counterparts of the ylid ions and their conventional isomers were probed by neutralization-reionization experiments. From these experiments, it follows that the ylid ions have stable neutral counterparts, as predicted by the theoretical calculations.</p> <p> The final component of this work deals with the chemistry of a silicon containing species of potential interest in interstellar chemistry. The reactions of protonated silicic acid, Si(OH)3+, and the mechanisms of these reactions were studied using tandem mass spectrometric experiments and CBS-QB3 model chemistry. The low energy (metastable) ions of Si(OH)3+ dissociate by loss of H2O and SiO2 to form O=SiOH+ and H3O+ ions. Neutralization-reionization experiments show that Si(OH)3• is a stable species in the rarefied gas phase.</p> / Thesis / Master of Science (MSc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19269 |
| Date | 11 1900 |
| Creators | Dimopoulos, Georgina |
| Contributors | Terlouw, Johan K., Chemistry |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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