Spontaneous and catalyzed isomerizations of the acetamide radical cation

Abstract

Spontaneous and catalyzed isomerizations of the acetamide radical cationThe use of mass spectrometry for the study of peptides (sequencing, cationization, H-D exchanges, etc.) is a rapidly growing field. 1 Therefore, the chemistry of ionized amides, studied by mass spectrometry, is of considerable interest. This letter reports some new findings concerning the unimolecular and bimolecular chemistry of ionized acetamide, mainly focused on spontaneous and catalyzed 1,3-H migrations, resulting in tautomerization of the acetamide radical cation.Among the four possible tautomers 1-4 of ionized acetamide (Scheme 1), only 1 and 2 have been experimentally described, 2 although no definitive proof was given of their structures. Structure 1 was assigned on the basis of direct electron ionization of acetamide, and structure 2 was logically given to the m/z 59 ion from ionized hexanamide, as the most stable fragment resulting from the McLafferty rearrangement.G2(MP2) level).

Barrier Height Titration by Tunable Photoionization Combined with Chemical Monitoring: Unimolecular Keto/Enol Tautomerization of the Acetamide Cation Radical

“…The slightly, but yet significantly larger threshold of the deuterated ion 1 a +. lends further support to this interpretation because it is fully consistent with the operation of a primary kinetic isotope effect in the keto/enol tautomerization of ionized acetamide 10. Accordingly, these particular thresholds can be regarded as a direct measure of the activation barrier associated with the unimolecular keto/enol tautomerization of ionized acetamide.…”

“…Recently, however, some of us reported evidence for the occurrence of an unimolecular keto/enol tautomerization in the ionized acetamide 1 +. 10. In these experiments, ion cyclotron resonance (ICR) mass spectrometry was used to investigate the bimolecular reactivity of mass‐selected 1 +.…”

“…. While the G2 calculations imply a somewhat larger barrier (1.16 eV),10 the experimental data are preliminary in that the thermal energies of the reactants have been included so far.…”

Structure and Reactivity of theN-Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

“…An alternative explanation is that the radical may rearrange via ion-molecule catalytic processes involving background ESI solvents present in high concentrations in the skimmer-cone ("in source") region. A possible mechanism that might operate involves proton transport catalysis [57] observed for a large number of systems including enol/ keto radical cation tautomers [58] and conventional radical cations/distonic ions [59].…”

Intermolecular proton-transfer in acetic acid clusters induced by vacuum-ultraviolet photoionization

“…As a similar example, the potential barriers in the proton-transfer process from CH to a carbonyl group have been reported to be 1.94 eV for the acetone cations at the G3 level calculation and 1.98 eV for the acetamide cation at the G2 level calculation, respectively. 58,59 For the formation of the proton-transfer from the methyl group in ͑CH 3 COOH͒ 2 + and ͑CH 3 COOH-CH 3 OH͒ + , the barrier heights are expected to be at the same magnitude as that in ͑CH 3 COOH-H 2 O͒ + . Therefore, we conclude that the most stable methyl proton-transferred structures are not formed in the 118 nm photoionization.…”