The acetamide radical cation, CH₃C(=O)NH₂^·+, and its enol, CH₂=C(OH)NH₂^·+, undergo several unimolecular reactions in the µs time-frame of which decarbonylation is predominant. This reaction produces the ylid ion CH₂NH₃^·+, rather than CH₃NH₂^·+ (J. Am. Chem. Soc., 109 (1987) 4819). A previously proposed mechanism via ion-dipole complexes is confirmed by the present CBS-QB3 calculations. These calculations reveal the existence of a second mechanism which proceeds via the enol ion and the distonic ion ^·CH₂C(=O)NH₃⁺. Both mechanisms can account for previously reported isotopic labeling experiments. Tandem mass spectrometry based experiments do not provide evidence that the non-decomposing acetamide ions rearrange to any significant extent to the more stable enol form. However, this transformation occurs smoothly by interaction with a neutral acetamide molecule (“self-catalysis”). By integration of experimental data (MS/MS/MS and labelling experiments) and ab initio calculations (CBS-Q (RHF/DZP)) three mechanisms for this assisted tautomerization have been traced. In the first mechanism the neutral acetamide component of the dimer ion accepts a C–H proton from its ionic partner and then donates the proton back to the oxygen atom. This is an example of “proton-transport catalysis”. In the second mechanism, isomerization takes place within the ionic partner via a conventional 1,3-H shift. The neutral partner serves only to lower the energy of the transition state by ion dipole attractions. This is an example of the “Spectator” mechanism. In the third mechanism, proton transfer from the ionic partner to its neutral counterpart is followed by back-donation of a hydrogen atom. This is an example of the “Quid-pro-Quo” mechanism. The behaviour of the acetamide dimer ion is compared to that of the acetone dimer ion which undergoes only proton-transport catalysis.

Keywords: proton-transport catalysis, tandem mass spectrometry, enolization, dimer radical cation, ab initio calculations, CBS-QB3