The Unimolecular Dissociation of (CH₃)₂C^⊕CH₂COOH in the Gas Phase: An Acid-Ester Type Rearrangement Involving the Formation of the Proton-Bound Acetone-Ketene Ion [(CH₃)₂C=O⋯H⋯CH₂=C=O]^⊕

In the gas phase the β-ketocarbenium ion (CH3)2C⊕CH2COOH is proposed to rearrange to the proton-bound acetone-ketene ion [(CH3)2CO⋯H⋯CH2CO]⊕, prior to its dissociation into CH3CO⊕/(CH3)2CO and (CH3)2COH⊕/CH2CO. Collision experiments in a mass spectrometer of reversed geometry on 2H- and 13C-labeled isotopomers indicate that this remarkable rearrangement is characterized by two consecutive [1,5]-proton migrations rather than a direct [1,3]-hydrogen shift.