Date of Award
Doctor of Philosophy (PhD)
Professor Johan K. Terlouw
The unimolecular chemistry of several organic radical cations has been studied with ab initio molecular orbital calculations and tandem mass spectrometric experiments. The computational chemistry involves. Hartree-Fock (HF), density functional (DFT), Moeller-Plesset perturbation, (MP) and coupled cluster (CC) theories. The tandem mass spectrometry experiments involves metastable ion (MI), collision induced dissociative ionization (CID), and neutralization-reionization mass spectrometry (NRMS) in conjunction with isotopic labeling using ²H, ¹³C and ¹⁸O isotopes. The chemistry has been interpreted by consideration of several fascinating intermediates in the gas-phase which include hydrogen-bridged radical cations, distonic ions and ion-dipole complexes. Two new hydrogen-bridged radical cations have been identified by experiments and were characterized by calculations. The unimolecular chemistry of ionized 1,2-propanediol was re-examined with these methods. It was found that a considerable simplification of a previous mechanistic proposal could be brought about by invoking two different mechanistic concepts, a 1,2-proton shift catalyzed by a dipole and charge (electron) transfer taking place in ion-dipole complexes. This new mechanistic proposal was compared to a more conventional alternative which involves hydrogen atom shifts in distonic ions for both 1,2-propanediol and its lower involves hydrogen atom shifts in distonic ions for both 1,2-propanediol and its lower homologue, 1,2-ethanediol. It was found that a surprisingly large barrier exists for a 1,4-H atom shift in these stable distonic ions. Other low energy dissociation processes in ionized 1,2-propanediol (involving the loss of CH₃･, H₂O, H₂O + CH₃･, H₂O + CH₄) were interpreted by Bohme's 'methyl cation shuttle' mechanism taking place in ion-dipole complexes. Tandem mass spectrometry experiments confirmed previous indirect evidence that commercially available oxalacetic acid, HOOCCH₂C(=O)COOH (OAA) samples do not have the structure of the acid but rather that of its (Z)-enol, hydroxyfumaric acid, HOOCC(H)=C(OH)COOH. It was further established that: (i) the samples contained a minor impurity assigned as a dehydration product of 4-hydroxy-4-methyl-2-ketoglutaric acid; (ii) careful evaporation of OAA yielded mass spectra characteristics of the (Z)-enol form; partial ketonization of the neutral (Z)-enol takes place and these species are either ionized intact or decarboxylane giving a mixture of α-hydroxygen-bridged radical cation, CH₂=O•••H•••O=C-OH･⁺. The long-lived, low internal energy (metastable) ions enolize prior to the loss of C=O yielding an ion-dipole complex of ionized hydroxyketene and water, HOC(H)=C=O･⁺/H₂O. The high energy hydrogen-bridged radical cations also show an intriguing rearrangement ion, CH₂OH₂･⁺, resulting from a decarboxylation process. This reaction likely occurs via communication with ionized glycolic acid and the hydrogen-bridged radical cation CH₂OH₂･⁺•••O=C=O. The methyl ester of β-hydroxypyruvic acid, HOCH₂C(=O)COOCH₃, behaves analogously: upon decarbonylation the high and low internal energy ions are CH₂=O•••H•••O=C-OCH₃･⁺ and HOC(H)=C=O･⁺/CH₃OH respectively. The hydrogen-bridged radical cation formed in the high internal energy process shows an intriguing rearrangement as well. This ion rearranges extensively before dissociation into protonated methylformate, CH₃OC(H)OH⁺ and the formyl radical, HC=O･. This rearrangement occurs via sequential transfers of a proton, electron and another proton within ion-dipole complexes in the gas-phase, akin to that proposed for the diols mentioned above. In the EI mass spectrum of HPA an ion at m/z 59 is observed which was confirmed to have the expected structure HOCH₂-C⁺O. We found the calculated dissociation energy of ionized HPA into HOCH₂-C⁺=O and HOCO･ to be rather different from that derived from an earlier experimental and theoretical study. This observation led us to re-examine the enthalpies of formation for the m/z 59 C₂H₃O₂⁺ system of ions, which includes the methoxycarbonyl ion CH₃-O-C⁺=O, with high level G2 ab initio calculations. A variety of calculated reactions and reconsideration of previous and new experimental measurements resulted in an upwards revisal, by 9 kcal/mol, of the ∆Hf assigned to these ions.
Fell, Lorne Montgomery, "Integrating Computational Chemistry and Mass Spectrometry: A Study of Isomerization Reactions of Oxygen-Containing Ions in the Gas Phase" (1999). Open Access Dissertations and Theses. Paper 1854.
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