Ring-Hydrogen Participation in the Keto-Enol Isomerization of the Acetophenone Radical Cation

Abstract

Molecular ions obtained from acetophenone have been observed to undergo proton transfer reactions in competition with unimolecular blackbody dissociation in a Fourier transform ion cyclotron resonance spectrometer provided with an in situ high temperature blackbody source. The ionizing energy dependence of these two processes and generation of the enol molecular ion by fragmentation of butyrophenone reveal that the keto ion undergoes blackbody dissociation exclusively while the enol ion promotes fast proton transfer reactions 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 mechanism responsible for the tautomerization process of these radical cations may involve scrambling of the methyl and ring hydrogens, or more than one mechanism. Theoretical calculations at the B3LYP level predict that the most favorable pathway for unimolecular isomerization of the keto ion involves initial migration of an ortho hydrogen to the carbonyl. The subsequent rearrangement to the enol form is calculated to require enough internal energy that would allow hydrogen walk around the benzene ring in agreement with the experimental 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.

Kinetic Control in the Regioselective Alkylation of Pterin Sensitizers: A Synthetic, Photochemical, and Theoretical Study

“…The [1,3]-and [1,4]-methyl shifts require very high activation energies. Thus, it is logical to conclude that the O-and N-alkyl pterins, for example, 3 and 4, are non-interconverting species, unlike walk processes observed for molecular rearrangements (33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43), such as the walk rearrangement of bicyclo[2.1.0]pent-2ene (44). Table 5 lists the computed log P and log D values that were obtained with the ChemAxon algorithm.…”

BIRD (blackbody infrared radiative dissociation): Evolution, principles, and applications

“…One way to increase the effective BIRD temperature within the ICR cell is to incorporate a hot filament within the chamber to irradiate the ions. Pioneering investigation of this method was described by Sena & Riveros (1997) in a careful study of dissociation of the acetophenone molecular ion, followed up by further study of this and related systems (Sena & Riveros, 2000; Giroldo & Riveros, 2002). Using a hot filament to illuminate ions contained in a relatively cool ICR cell differs from a true BIRD experiment in a significant way.…”

Activation of protonated peptides and molecular ions of small molecules using heated filaments in Fourier-transform ion cyclotron resonance mass spectrometry

“…Near blackbody radiation can be obtained using a heated filament, and several implementations have used a heated filament as a source of photons for dissociation [24][25][26][27][28][29]. This technique has the advantage of being a pulsed method, like IRMPD and a broadband photon source similar to BIRD.…”

“…They studied the dissociation of acetophenone molecule ions and found an Arrhenius relationship between the dissociation rate constant and the radiation temperature of the heated filament (log e k diss versus 1/T rad ) which was varied over a range from 910 to 1700 K. From modeling these data, they were able to obtain a value for the threshold dissociation energy which fit within the wide range of values that had been measured previously with other methods [24]. Subsequent work by Sena and Riveros examined the keto-enol isomerization of acetophenone [25] and 2 -methylacetophenone molecular ions [26] using this method. The threshold dissociation energy for loss of methyl from the molecular ion of p-cymene has also been obtained from this filament dissociation method [27].…”