Ion—neutral complex intermediate for loss of water from fatty acid carboxylates

Fatty acid carboxylates desorbed into the gas phase by fast atom bombardment (FAB) undergo both metastable‐ion and collisionally activated dissociation (CAD) to eliminate water. Upon CAD, the water elimination competes with charge‐remote fragmentation. Deuterium labeling results are in accord with the elimination occurring via an ion‐neutral complex involving hydroxide ion and neutral ketene. Evidence that the precursor of the ion‐neutral complex is the enol form of the carboxylate was obtained by forming the enolate directly by decarboxylation of appropriate dicarboxylic acids. The [M  H  CO₂]⁻ ions of these species undergo a more facile elimination of water than do carboxylates desorbed directly upon FAB.