S2 Reactions of NO with Ions in Water: Microscopic Mechanisms, Intermediates, and Products.

Reactions of dinitrogen pentoxide (NO) 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 NO 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, CH). The following main results are obtained: (i) all the reactions take place by an S2-type mechanism, with a transition state that involves a contact ion pair (NONO) 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 NO reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SONO) 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.