Protonation of nitrous acid and formation of the nitrosating agent NO+ : an Ab initio study

Abstract

The most favourable conformation of nitrous acid HONO have been calculated using STO-3G, 3-21 G, and 4-31 G basis sets. Protonation is predicted to occur on the hydroxylic oxygen to give (4) as the most stable structure. This shows an unusually long N–O(2) bond distance and is best represented as a complex between NO⁺ and H₂O since the geometries are only slightly changed from the isolated molecules. Rotation about the N–O(2) bond requires only a few hundred cal mol^–1 as shown by calculations on the non-planar structures (7) and (8). The site of protonation can be correctly predicted from both the molecular electrostatic potential of HONO and from a calculation of the spatial extent of the lone pairs on the oxygens. The dissociation energy for [H₂ONO]⁺ to give NO⁺ is calculated as 18.2 kcal mol^–1(6.31**//3-21 G), consistent with rate-determining nitrosonium formation under some reaction conditions.