An Ab initio calculation of the acid-catalysed hydrolysis of N-nitrosoamines. A hypothesis on the rate-determining step
Abstract
The molecular geometries and energies of H2NNO, MeNHNO, and Me2NNO and their N-and O-protonated species have been calculated using ab initio methods. The N-protonated species [NH3NO]+ is found to be the more stable when electron correlation effects are taken into account (by 3–4 kcal mol–1). Substitution on nitrogen by Me groups successively favours O-protonation (by 9–10 kcal mol–1 for each Me group) so that [Me2NNOH]+ is more stable than [Me2NHNO]+. The N–N bond is surprisingly long in the N-protonated species [NH3NO]+(1.971 Å) which resembles an amine-stabilized nitrosonium ion, but progressively shortens on substitution (to 1.587 Ă in Me2NHNO+). No evidence is found for stabilized complexes formed by preassociation between the nitrosamines and H3O+; proton transfer to N or O occurs without a barrier. Dissociation of [R2NHNO]+ to the fragments R2NH and NO+ is in all cases difficult (> 30 kcal mol–1) and increases with Me substitution. Solvation of the protonated substrate at the amino nitrogen raises the dissociation barrier further but a second water molecule solvating the nitrogen of the forming nitroso group facilitates dissociation (by ca. 10 kcal mol–1). The results are interpreted in terms of the rate-determining step for denitrosation being fragmentation of this intermediate.