S N 2 Reactions of N 2 O 5 with Ions in Water: Microscopic Mechanisms, Intermediates, and Products

Reactions of dinitrogen pentoxide (N O ) greatly affect the concentrations of NO , 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 N O with anions hydrated by 12 water molecules to shed light on this important class of reactions. The ions investigated are Cl , SO , ClO , and RCOO (R = H, CH , C H ). The following main results are obtained: (i) all the reactions take place by an S 2-type mechanism, with a transition state that involves a contact ion pair (NO NO ) that interacts strongly with water molecules. (ii) Reactions of a solvent-separated nitronium ion (NO ) are not observed in any of the cases. (iii) An explanation is provided for the suppression of ClNO formation from N O reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SO NO ) and RCOONO , 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.