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.