Catalyzed keto-enol tautomerism of ionized acetone: A Fourier transform ion cyclotron resonance mass spectrometry study of proton transport isomerization

The unimolecular isomerization of CH3COCH3+· 1 into its more stable enol counterpart CH3C(OH)CH2+· 2 is known not to occur, as a significant energy barrier separates these ions. However, it is shown in this work that this isomerization can be catalyzed within a 1 : 1 ion-neutral complex. For instance, a Fourier transform ion cyclotron resonance mass spectrometry study shows that one, and only one, molecule of isobutyronitrile catalyzes the isomerization of 1 into 2. The rather low efficiency of the reaction (12%), as well as the strong isotope effect observed when CD3COCD3+· is used as the reactant ion, suggest that the catalyzed isomerization implicates a substantial intermediate energy barrier. This was confirmed by ab initio calculations that allow us to propose an isomerization mechanism in agreement with this experiment. The efficiency of different catalysts was studied. To be efficient, the catalyst must be basic enough to abstract a proton from the methyl group of ionized acetone but not too basic to give back this proton to oxygen. In other words, the proton affinity (PA) of an efficient catalyst must lie, in a first approximation, between the PA of the radical CH3COCH2· at the carbon site (PAC) and its PA at the oxygen site (PAO), which have been determined to be, respectively, 185.5 and 195.0 kcal mol−1. Most of the neutral compounds studied follow this PA rule. The inefficiency of alcohols in the catalytic process, although their PAs lie in the right area, is discussed. Keywords: Catalyzed keto-enol tautomerism; gas-phase proton transport; isomerization kinetics; proton affinity rule; FT-ICR mass spectrometry