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 CH3C=O++NHOH by direct bond cleavage but rather yield CH3C=O++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 [O=C-C(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, 43 kcal mol-1, is reduced to a mere 7 kcal mol-1 for the catalysed tranformation which can be viewed as a quid-pro-quo reaction involving two proton transfers.