The loss of NH2O from the N-hydroxyacetamide radical cation CH3C(O)NHOH+: An ion-catalysed rearrangement

A previous study [Ch. Lifshitz, P.J.A. Ruttink, G. Schaftenaar, J.K. Terlouw, Rapid Commun. Mass Spectrom. 1 (1987) 61] shows that metastable N-hydroxyacetamide ions CH3C(O)NHOH+ (HA-1) do not dissociate into CH3CO++NHOH by direct bond cleavage but rather yield CH3CO++NH2O. The tandem mass spectrometry based experiments of the present study on the isotopologue CH3C(O)NDOD+ reveal that the majority of the metastable ions lose the NH2O radical as NHDO rather than ND2O.A mechanistic analysis using the CBS-QB3 model chemistry shows that the molecular ions HA-1 rearrange into hydrogen-bridged radical cations [OCC(H2)H⋯N(H)OH]+ whose acetyl cation component then catalyses the transformation NHOH→NH2O prior to dissociation. The high barrier for the unassisted 1,2-H shift in the free radical, 43kcalmol−1, is reduced to a mere 7kcalmol−1 for the catalysed transformation which can be viewed as a quid-pro-quo reaction involving two proton transfers.