title: Competing fragmentation processes of β-substituted propanoate ions upon collision-induced dissociation.
abstract: BACKGROUND
  When subjected to collisional activation, gas-phase carboxylate ions typically undergo decarboxylation. However, alternative fragmentation processes dominate when the carboxylate group is located within certain structural motifs. In this work, the fragmentation processes of β-substituted carboxylate ions are characterized to improve correlations between reactivity and structure.
  
  
  METHODS
  Mass spectra were collected using both ion trap and triple quadrupole mass spectrometers operating in the negative ion mode; collision-induced dissociation (CID) of ions was used to study the relationship between product ions and the structures of their precursor ions. Quantum mechanical computations were performed on a full range of reaction geometries at the MP2/6-311++G(2d,p)//B3LYP/6-31++G(2d,p) level of theory.
  
  
  RESULTS
  For a series of β-substituted carboxylate ions, a product ion corresponding to the anion of the β-substituent was obtained upon CID. Detailed computations indicated that decarboxylative elimination and at least one other fragmentation mechanism had feasible energetics for the formation of substituent anions differing in their gas-phase basicities. Predicted energetics for anti- and synperiplanar alignments in the transition structures for decarboxylative elimination correlated with the positions of crossover points in breakdown curves acquired for conformationally constrained ions.
  
  
  CONCLUSIONS
  The feasibility of more than one mechanism was established for the fragmentation of β-substituted propanoates. The contribution of each mechanistic pathway to the formation of the substituent anion was influenced by structural variations and conformational constraints, but mostly depended on the nature of the substituent. Copyright © 2016 John Wiley & Sons, Ltd.
authors.affiliation: Department of Chemistry, Dalhousie University, 6274 Coburg Road, PO Box 15000, Halifax, Nova Scotia, Canada, B3H 4R2.
authors.forename: Luc M
authors.initials: LM
authors.lastname: LeBlanc
authors.affiliation: Department of Chemistry, Dalhousie University, 6274 Coburg Road, PO Box 15000, Halifax, Nova Scotia, Canada, B3H 4R2.
authors.forename: Sean W
authors.initials: SW
authors.lastname: Powers
authors.affiliation: Department of Chemistry, Dalhousie University, 6274 Coburg Road, PO Box 15000, Halifax, Nova Scotia, Canada, B3H 4R2.
authors.forename: J Stuart
authors.identifier: http://orcid.org/0000-0003-4692-2775
authors.initials: JS
authors.lastname: Grossert
authors.affiliation: Department of Chemistry, Dalhousie University, 6274 Coburg Road, PO Box 15000, Halifax, Nova Scotia, Canada, B3H 4R2. robert.white@dal.ca.
authors.forename: Robert L
authors.initials: RL
authors.lastname: White