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Phenyl group participation in rearrangements during collision‐induced dissociation of deprotonated phenoxyacetic acid

Rationale

The identification of trace constituents in biological and environmental samples is frequently based on the fragmentation patterns resulting from the collision‐induced dissociation (CID) of gas‐phase ions. Credible mechanistic characterization of fragmentation processes, including rearrangements, is required to make reliable assignments for structures of precursor and product ions.

Methods

Mass spectra were collected using both ion trap and triple quadrupole mass spectrometers operating in the negative ion mode. Precursor ion scans and CID of ions generated in‐source were used to establish precursor‐product ion relationships. Density functional theory (DFT) computations were performed at the MP2/6‐311++G(2d,p)//B3LYP/6‐31++G(2d,p) level of theory.

Results

Product ions at m/z 93 and 107 obtained upon CID of phenoxyacetate were attributed to phenoxide and o‐methylphenoxide, respectively. An isotopic labeling experiment and computations showed that the phenoxide ion was formed by intramolecular displacement with formation of an α‐lactone and also by a Smiles rearrangement. Rearrangement of phenoxyacetate via the ion‐neutral complex formed in the α‐lactone displacement pathway gave the isomeric o‐hydroxyphenylacetate ion which yielded o‐methylphenoxide upon decarboxylation. Computations provided feasible energetics for these pathways.

Conclusions

Previously unrecognized and energetically favorable rearrangements during the collision‐induced fragmentation of phenoxyacetate have been characterized using isotopic labeling and DFT computations. Notably, the phenyl substituent plays an indispensable role in each rearrangement process resulting in multiple pathways for the fragmentation of phenoxyacetate. Copyright © 2015 John Wiley & Sons, Ltd.

Authors:   Luc M. LeBlanc, Andrew M. J. Crowell, J. Stuart Grossert, Robert L. White
Journal:   Rapid Communications in Mass Spectrometry
Volume:   29
edition:   23
Year:   2015
Pages:   2293
DOI:   10.1002/rcm.7395
Publication date:   29-Oct-2015
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