Date of Award

1-1996

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Supervisor

Professor J.K. Terlouw

Abstract

Low energy rearrangement reactions in selected oxygen-containing ions have been studied by means of mass spectrometry-based techniques (metastable ion (MI), collision-induced dissociation (CID), neutralization-reionization (NR) mass spectrometry as well as multiple collision experiments (MSⁿ)) in conjuntion with ab initio molecular orbital calculations.

The atom connectivity in the product ions has been investigated in detail by tandem mass spectrometry-based experiments on ²H-, ¹³C- and ¹⁸O-labelled isotopologues of some oxygen-containing ions. The thermochemical data (i.e. heat of formation (ΔHf) values) of the oxygen-containing ions have been obtained either experimentally from the appropriate measurement of ionization energy (IE), appearance energy (AE) and/or proton affinity (PA) or computationally.

It has been proposed that the C-H∙∙O bonded counterparts of the O∙∙H∙∙O bonded species may, despite their lower thermodynamic stability, play an even more important role in the decay of oxygen-containing radical cations of the type HOCH(R₁)C(=O)R₂˙⁺. A case in point is ionized acetol (R₁=H,R₂=CH₃), methyl glycolate (R₁=H,R₂=OCH₃), methyl lactate (R₁=CH₃,R₂=OCH₃) and acetoin (R₁=R₂=CH₃). These species all dissociate by loss of R₁CO˙ by double hydrogen transfer (DHT). It is further proposed. from ab initio calculations, that the C-H∙∙O bonded intermediate R₁C(H)=O⋯H-C(=O)R₂˙⁺, I, does not lose R₁CO˙ via a hydrogen atom shift from neutral R₁C(H)=O to ionized H-C(=O)R₂˙⁺.

Instead, charge transfer takes place in I so that the neutral R₁C(H)=O becomes charged and thus it can rotate and donate a proton to H-C(=O)R₂, after which dissociation follows.

Keto-enol tautomerism in the dilute gas-phase has been studied in the unimolecular reactions of ionized ethyl glycolate, HOCH₂CO₂C₂H₅˙⁺, II, and its enol isomer, HOCH=C(OH)OC₂H₅˙⁺, III. It has been shown that the metastable ionized keto isomer II undergoes unidirectional isomerization to the enol ion III, via two consecutive 1,5-hydrogen shifts, from which C₂H₄ is lost to give the ionized trihydroxyethylene, (HO)₂C=CH(OH)˙⁺. NR experiments show that neutral trihydroxyethylene in the gas-phase is a remarkably stable species, which does not tautomerize to the keto isomer glycolic acid, HOCH₂CO₂H.

Some members of the important homologous series of oxonium ions, CnH₂n₊₁O⁺, have been extensively investigated by ²H- and ¹³C-labelling experiments especially at low internal energies. Their intriguing unimolecular chemistry has been interpreted by means of mechanisms in which distonic ions and ion/neutral complexes play crucial roles in the hydrogen transfer and skeletal isomerization steps.

Finally, part of the C₃H₅O₂+ potential energy surface was investigated by ab initio molecular orbital calculations and by mass spectrometry-based experiments to ascertain whether the carbonyl-protonated β-propiolactone ions CH₂CH₂OCOH⁺, IV, can interconvert in the dilute gas-phase with protonated acrylic acid, CH₂=CHC(OH)₂⁺, V, as suggested in a thermolysis study. It is shown that metastable ions IV do not communicate with ions V and the observed equilibrium IV ⇆ V in solution is due to an intermolecular process. Also, ions IV do not undergo cycloreversions to HOCO⁺ + C₂H₄ and to CH₂=COH⁺ + CH₂O, but rather they spontaneously dissociate CH₃CHOH⁺ + CO, CH₃CO⁺ + CH₂O, CH₂=CHCO⁺ + H₂O and C₂H₅⁺ + CO₂. The product ions of these dissociation reactions are characterized by multiple collision experiments and mechanisms for their formation are proposed. Analysis of appropriate isodesmic reactions indicate that the α-COOH group in 1-carboxyethylium ions, CH₃CHCOOH⁺, behaves as a hydrogen atom and therefore this group cannot be said to destabilize the adjacent positive charge.

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