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|Title:||A mass spectrometric and computational study of hydrogen transfer reactions in radical cations|
|Authors:||Trikoupis, Anna Moschoula|
|Advisor:||Terlouw, Johan K.|
|Abstract:||<p>Hydrogen transfer reactions represent elementary chemical reactions of interest to both chemists and biochemists. In the context of the experimental work presented in this thesis, hydrogen transfers involved in both the unassisted and assisted isomerizations of several organic radical cations have been studied. The structure characterization and reactivity of the ionic species were realized using a variety of tandem mass spectrometry based techniques. Not only conventional metastable ion (MI), collision-induced dissociation (CID) and neutralization-reionization (NR) were used but also the novel hybrid techniques CID/CID and NR/CID. The use of deuterium labelled isotopomers and quantum chemical calculations formed an essential component in the interpretation of the results. Hydrogen transfer reactions also form the key pathway to the unimolecular dissociations of ionized dimethyl oxalate (DMO) and a series of ionized pentenyl methyl ethers. Prior to the expulsion of CO, ionized DMO undergoes intramolecular hydrogen bridging and subsequent C-C cleavage to generate the hydrogen bridged radical cation CH2 =O***H***O=C-OCH 3*+ . Using the experimental results of vacuum ultraviolet photoionization and tandem mass spectrometry based experiments it is also shown that the measured appearance energy (AE) for the generation of the methoxycarbonyl cation, CH3 O-C=O+ (10.5 eV), is not compatible with a simple bond cleavage involving the cogeneration of the radical CH3 O-C=O* . Collision-induced dissociative ionization experiments show that a consecutive two-step dissociation of low energy DMO ions into CO2 and CH3* , without the intermediacy of CH3 O-C=O* occurs. The dissociation chemistry of the low-energy C5 H9 OCH 3*+ ions generated from thirteen isomeric pentenyl methyl ethers derived from stable alkenols, was examined using metastable ion mass spectrometry characteristics. The experiments indicate that there is an influence of the position and substitution pattern of the double bond on the chemistry of ionic species. In addition to the involvement of hydrogen transfers in dissociation reactions, additional ionic systems were studied. These studies involved the characterization of a number of H-shift isomers of the hydroxpyridines and pyridine N-oxide, the characterization of the isomeric dicoordinated borinium ions CH3 O-B-H+ and CH3 -B-OH+ and the iminothisulfine ion H-N≡C-S-S*+ . The final component of this work deals with the heat of formation of sulfine, CH2 =S=O. The value -30 ± 6 kJ/mol has been assigned to ΔHf (CH2 =S=O), as determined by calculations at the CBS-QB3 level of theory. The derived value lies midway between two previously calculated values -9 ± 14 and -52 ± 10 kJ/mol. (Abstract shortened by UMI.)</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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