Unimolecular dissociations of ionized azo-tert-butane and acetone azine

Abstract

This M.Sc. thesis presents an experimental and a theoretical study of azo-tert-butane and acetone azine ions which belong to the azo and azine class of compounds, respectively. The experimental study involves an investigation of the fragmentation reactions using tandem mass spectrometry and TPEPICO (threshold photoelectron photoion coincidence) spectroscopy. The theoretical study involves ab initio calculations of neutral and ionized azo-tert-butane, acetone azine and their dissociation products. RRKM theory was used to probe the nature of the unimolecular dissociations by comparing theoretical fits to TPEPICO breakdown curves. Due to an impurity in the sample of acetone azine, only one dissociation channel was studied, namely, methyl radical loss. Contrary to the original hypothesis, that this unimolecular dissociation occurs by a simple bond cleavage reaction, it was found that a rearrangement process takes place. The unimolecular dissociations of ionized azo-tert-butane that were investigated were formation of species corresponding to m/z 57, m/z 85 and m/z 56. Fragment ions m/z 57 and m/z 85 are competing channels, since their formation occurs by the same C-N bond cleavage. However, it was observed that formation of m/z 57 dominates over m/z 85 at higher internal energies. This observation was explained by the difference in the entropy of activation (DeltaS ‡) for the two dissociation channels. Formation of m/z 56 is an isomerization reaction; due to its low significance, the mechanism of this reaction was not studied in depth.