Ring-hydrogen participation in the keto-enol isomerization of the acetophenone radical cation

Molecular ions obtained from acetophenone have been observed to undergo pro ton transfer reactions in competition with unimolecular blackbody dissociat ion in a Fourier transform ion cyclotron resonance spectrometer provided wi th an in situ high temperature blackbody source. The ionizing energy depend ence of these two processes and generation of the enol molecular ion by fra gmentation of butyrophenone reveal that the keto ion undergoes blackbody di ssociation exclusively while the enol ion promotes fast proton transfer rea ctions and undergoes very slow blackbody induced dissociation. Experiments with labeled acetophenone either on the methyl group or on the ring reveal that the enol ions can transfer both H+ and D+ suggesting that the mechanis m responsible for the tautomerization process of these radical cations may involve scrambling of the methyl and ring hydrogens, or more than one mecha nism. Theoretical calculations at the B3LYP level predict that the most fav orable pathway for unimolecular isomerization of the keto ion involves init ial migration of an ortho hydrogen to the carbonyl. The subsequent rearrang ement to the enol form is calculated to require enough internal energy that would allow hydrogen walk around the benzene ring in agreement with the ex perimental results. The possibility that isomerization may also occur by a direct 1,3-hydrogen migration is also explored in terms of possible excited electronic states of the ion.