Competitive homolytic and heterolytic decomposition pathways of gas-phase negative ions generated from aminobenzoate esters

Hanxue Xia, Yong Zhang, Julius Pavlov, Freneil B. Jariwala, Athula B. Attygalle

Research output: Contribution to journalArticlepeer-review

7 Scopus citations

Abstract

An alkyl-radical loss and an alkene loss are two competitive fragmentation pathways that deprotonated aminobenzoate esters undergo upon activation under mass spectrometric conditions. For the meta and para isomers, the alkyl-radical loss by a homolytic cleavage of the alkyl-oxygen bond of the ester moiety is the predominant fragmentation pathway, while the contribution from the alkene elimination by a heterolytic pathway is less significant. In contrast, owing to a pronounced charge-mediated ortho effect, the alkene loss becomes the predominant pathway for the ortho isomers of ethyl and higher esters. Results from isotope-labeled compounds confirmed that the alkene loss proceeds by a specific γ-hydrogen transfer mechanism that resembles the McLafferty rearrangement for radical cations. Even for the para compounds, if the alkoxide moiety bears structural motifs required for the elimination of a more stable alkene molecule, the heterolytic pathway becomes the predominant pathway. For example, in the spectrum of deprotonated 2-phenylethyl 4-aminobenzoate, m/z 136 peak is the base peak because the alkene eliminated is styrene. Owing to the fact that all deprotonated aminobenzoate esters, irrespective of the size of the alkoxy group, upon activation fragment to form an m/z 135 ion, aminobenzoate esters in mixtures can be quantified by precursor ion discovery mass spectrometric experiments.

Original languageEnglish
Pages (from-to)245-253
Number of pages9
JournalJournal of Mass Spectrometry
Volume51
Issue number3
DOIs
StatePublished - 1 Mar 2016

Keywords

  • McLafferty rearrangement
  • aminobenzoate esters
  • even-electron negative ions
  • even-electron rule
  • procaine, benzocaine, collision-induced dissociation

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