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 center dot+ (HA-1) do not dissociate into CH3C=O+ + NHOH center dot by direct bond cleavage but rather yield CH3C=O+ + NH2O center dot. The tandem mass spectrometry based experiments of the present study on the isotopologue CH3C(=O)NDOD center dot+ reveal that the majority of the metastable ions lose the NH2O center dot radical as NHDO center dot rather than ND2O center dot. 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(H-2)-H center dot center dot center dot N(H)OH](center dot+) whose acetyl cation component then catalyses the transformation NHOH center dot -> NH2O center dot 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 transformation which can be viewed as a quid-pro-quo reaction involving two proton transfers. (C) 2006 Elsevier B.V. All rights reserved.