The loss of NH 2O [rad] from the N-hydroxyacetamide radical cation CH 3C( [dbnd]O)NHOH [rad]+: An ion-catalysed rearrangement

Abstract

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 CH 3C( O)NHOH + ( HA-1) do not dissociate into CH 3C O + + NHOH by direct bond cleavage but rather yield CH 3C O + + NH 2O . The tandem mass spectrometry based experiments of the present study on the isotopologue CH 3C( O)NDOD + reveal that the majority of the metastable ions lose the NH 2O radical as NHDO rather than ND 2O . 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⋯N(H)OH] + whose acetyl cation component then catalyses the transformation NHOH → NH 2O 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.