Self-catalysis in the gas-phase: enolization of the acetone radical cation

Because of a prohibitively large barrier, the solitary acetone radical cation, CH3C(O)CH3+ (1+) does not rearrange, neither spontaneously nor by activation, to its more stable enol isomer, CH2C(OH)CH3+ (1a+). However, this isomerization occurs smoothly by an ion–molecule interaction with neutral acetone itself. The dimer radical cation, [1+⋯1], generated under conditions of chemical ionization dissociates to m/z 58 and collision-induced dissociation (CID) experiments show that these ions have the enol structure 1a+. Labeling experiments indicate that the reaction can be viewed as a simple 1,3-hydrogen shift within the acetone radical cation of the complex. Ab initio calculations at the CBS-Q/DZP level of theory indicate that this isomerization is best described as a proton transport catalysis rather than as a spectator model. Our calculations show that the incipient radical formed during the proton abstraction is not CH3C(O)CH2, but rather the less stable configuration CH3C(O)CH2 stabilized by CH3C(OH)CH3+. This behaviour can be rationalized by arguments based on ion-dipole interactions. The incipient radical CH3C(O)CH2 is transformed to its more stable configuration CH3C(O)CH2 via surface crossing. However, this process does not occur via the usual “minimum to minimum crossing” but rather by the novel process of “transition state to minimum crossing”. The abstracted proton is then donated back to the oxygen atom of CH3C(O)CH2 to yield the hydrogen-bridged radical cation [1a+⋯1]. The observed tautomerization of the acetone radical cation by acetone itself can be viewed as “self-catalysis”.