S_N2 Reactions of N₂O₅ with Ions in Water: Microscopic Mechanisms, Intermediates, and Products

Reactions of dinitrogen pentoxide (N₂O₅) greatly affect the concentrations of NO₃, ozone, OH radicals, methane, and more. In this work, we employ ab initio molecular dynamics and other tools of computational chemistry to explore reactions of N₂O₅ with anions hydrated by 12 water molecules to shed light on this important class of reactions. The ions investigated are Cl^-, SO₄ ^2-, ClO₄ ^-, and RCOO^- (R = H, CH₃, C₂H₅). The following main results are obtained: (i) all the reactions take place by an S_N2-type mechanism, with a transition state that involves a contact ion pair (NO₂ ⁺NO₃ ^-) that interacts strongly with water molecules. (ii) Reactions of a solvent-separated nitronium ion (NO₂ ⁺) are not observed in any of the cases. (iii) An explanation is provided for the suppression of ClNO₂ formation from N₂O₅ reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SO₄NO₂ ^-) and RCOONO₂, in these reactions is predicted. The results suggest atomistic-level mechanisms for the reactions studied and may be useful for the development of improved modeling of reaction kinetics in aerosol particles.