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|Title:||CHARACTERIZATION OF SMALL MOLECULES USING TANDEM MASS SPECTROMETRY AND COMPUTATIONAL CHEMISTRY IN THE CONTEXT OF ASTROCHEMISTRY|
|Advisor:||Terlollw, Johan K.|
|Abstract:||<p>Ionized and neutral forms of small organic molecules containing nitrogen and/or oxygen have always intrigued experimental and theoretical chemists the world over. The reason is that such molecules could play a vital role in interstellar chemistry, potentially giving rise to larger homologues of groups such as sugars and other organic classes such as amino acids. Therefore, the chemistries of the ionic and neutral counterparts of these species are of considerable interest in the context of astrochemistry and prebiotic synthesis.<br /><br />The ions studied in this thesis were generated in the rarefied gas phase of the mass spectrometer by electron ionization of carefully selected precursor molecules. The reactivity and structural characterization of these species were probed using a variety of tandem mass spectrometry based techniques. These include the acquisition of metastable ion (MI) mass spectra to study the dissociation chemistry of low energy ions and (multiple) collision experiments to establish the atom co nnectivity of such species. The structure and stability of the neutral counterparts of these ions was probed by using the technique of neutralization-reionization mass spectrometry (NRMS). Theoretical findings derived from CBS-QB3 and CBS-APNO model chemistries formed an essential component in the interpretation of experimental observations.<br /><br />The above approach was used to study proton transport catalys is by water in the isomerization of ionized glycolaldehyde, HOCH<sub>2</sub>CHO<sup>•+</sup>, into its enol ion HOCH=CHOH<sup>•+</sup>,A mechanistic anal ys is using the CBS-QB3 model chemistry reveals that an alternative pathway involves ionized glycolaldehyde rearranging into a hydrogen-bridged radical cation consisting of a ketene ion CH<sub>2</sub>=C=O<sup>•+</sup><sup></sup> interacting with two water molecules. The water component of this ion then catalyzes the interconversion of HOCH<sub>2</sub>CHO<sup>•+</sup> into CH<sub>2</sub>=C(OH)<sub>2</sub><sup>•+</sup>, the enol of acetic acid. Tandem mass spectrometry based studies reveal that a mixture of C<sub>2</sub>H<sub>4</sub>O<sub>2</sub><sup>•+</sup> ions are generated out of which CH<sub>2</sub>=C(OH)<sub>2</sub><sup>•+</sup> is the major component.<br /><br />Next, the ionic and neutral C<sub>2</sub>H<sub>2</sub>O<sub>2</sub> potential energy surfaces were studied using CBSQB3/APNO model chemistries. In other words, we probed the stability and reactivity of various ionic and neutral C<sub>2</sub>H<sub>2</sub>O<sub>2</sub> isomers, particul arly the HNC dimer HN=C=C=NH (ethenediimine) and H<sub>2</sub>N-C-C=N (aminocyanocarbene). They were generated from stable precursor molecules i.e. HN=C=C=NH from xanthine and H<sub>2</sub>N-C-C=N from aminomalononitrile. NRMS experiments reveal that the neutral counterparts of these species are stable in the rarefied gas phase. Another precursor molecule, diaminomaleonitrile, is shown to generate HN=C=C=NH<sup>•+</sup> after HCN loss from the hydrogen-bridged radical cation [HNC•••H2N-C-C=N]<sup>•+</sup> via a remarkable quid-pro-quo catalysis.<br /><br />In a similar manner, the stability of the ionic and neutral forms of the HCN dimer, HC=N-N=CH, was probed. The pertinent ion was generated from ionized stetrazine via elimination of N<sub>2</sub>. Us ing CBS-QB3/APNO model chemistries, the mechanism for its generation was probed and confirmed. However, HC=N-N=CH<sup>•+</sup> is found to be generated with sufficient internal energy to facilitate its rearrangement into the more stable HC=N-C(=N)H<sup>•+</sup> ion. This nitrene ion was separately generated from striazine. Collision experiments were used to characterize the ionic species . Despite the stability of the ionic species, NRMS experiments showed their neutral counterparts not to be stable in the gas phase.<br /><br />The final component of this work deals with the generation and characterization of various isomers of ionized N-methylethenediimine CH<sub>3</sub>N=C=C=NH<sup>•+</sup> and N, N-dimethylethenediimine CH<sub>3</sub>N=C=C=NH<sub>3</sub><sup>•+</sup>. Ionized theophylline and paraxanthine were proposed to generate m/z 68 CH<sub>3</sub>N=C=C=NH<sup>•+</sup> ions.<br /><br />Theory and experimental obse rvations reveal that theophylline co-generates CH<sub>3</sub>N=C=C=NH<sup>•+</sup> with its 1,4-H shift isomer, CH2N=C=C(H)=NH<sup>•+</sup>, where the neutral counterpart of the latter is found to be kinetically more stable than that of the former. Paraxanthine is found to generate isomerically pure CH<sub>3</sub>N=C=C=NH<sup>•+</sup> ions. Ionized smethyItetraz ine was proposed to generate another m/z 68 isomer presumed to be CH<sub>3</sub>N=C=C=NH<sup>•+</sup>. Experimental and theoretical results reveal that these ions are generated with sufficient internal energy to transform into various other isomers generating a mixture of m/z 68 ions, out of which, a majority do not have stable neutral counterparts in the gas phase. N,N-dimethylethenediimine CH<sub>3</sub>N=C=C=NCH<sub>3</sub><sup>•+</sup> ions were generated from ionized caffeine. Analogous to the m/z 68 ions from theophylline, these ions were found to be generated in admixture with its 1,4-H shift isomer, CH<sub>3</sub>N=C=C=NCH<sub>3</sub><sup>•+</sup>. Theory shows that these species do not readily interconvert as ions. However, as neutral species, they readily isomerize resulting in a mixture of stable neutral isomers as seen fro m relevant NRMS experiments.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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