Solvent catalyzed substitution reactions within halogenated benzene/methanol heterocluster ions: A tandem mass spectrometric investigation
Shores, Kevin S
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When a analyte seeded in a high pressure inert gas is expanded into a vacuum chamber, an intense beam of molecular clusters is formed. The consequential skimming and ionization of the cluster beam allows for the analysis of molecular cluster ions using tandem mass spectrometry. Various process that occur within certain heterocluster ions, including reactivity and fragmentation pathways, may be observed by inducing collisions of the cluster ions with inert gas atoms. In certain situations, a reaction channel is only accessed when a minimum number of solvent molecules are present in a heterocluster ion. These reactions are "solvent catalyzed" in that a critical number of solvent molecules effectively lowers the activation barrier such that the reaction becomes the dominant process occurring within that cluster ion. Three tandem mass spectrometric investigations have been performed. The first study was focused on ortho, meta, and paradifluorobenzene/methanol cluster ions, where a solvent catalyzed methoxy substitution reaction and consequential loss of HF is only observed in clusters containing one aromatic molecule and two or more solvent molecules. The second investigation was focused on the reactivity of ortho, meta, and parafluorochlorobenzene/methanol cluster ions, where the replacement of a fluorine with a chlorine substituant revealed the strong preference of methanol to H-bonded with only the fluorine substituant. Consequently, a reaction is only observed at the C-F carbon. The third and final investigation was performed on fluoro, chloro, and bromobenzene/methanol molecules, where the absence of a fluorine substituant on the aromatic ring permits the formation of H-bonds between the methanol molecules and the chlorine and bromine substituants, which results in a reaction at the substituted carbon. The result of all three investigations suggests that the solvent catalyzed nature of the substitution reactions is highly dependent on the structures of the H-bonded precursor heterocluster ions.