Reaction Dynamics of NO⁺ with Water Clusters

The reaction of NO⁺ with water molecules plays a crucial role in the D-region of the atmosphere because the reaction provides nitrous acid (HONO) and protonated water species (H₃O⁺). In this study, the reaction of NO⁺ with water clusters, NO⁺ + (H₂O) (n = 1-7), was investigated by means of the direct ab initio molecular dynamics method to elucidate the reaction mechanism of NO⁺ in the atmosphere from a theoretical viewpoint. At n = 1 and 2, the reaction of NO⁺ with (H₂O) led to the formation of a complex: NO⁺ + (H₂O) → NO⁺(H₂O) (n = 1 and 2). At n = 3, the formation channel of HONO was open, and HONO was formed according to NO⁺ + (H₂O) → HONO---H⁺(H₂O) (n = 3), through which H₃O⁺ was also formed as H⁺(H₂O)₂. However, the HONO formation efficiency was significantly low for n = 3. In large clusters with n = 4-7, the HONO formation channel became the main channel, and the dissociation of HONO from the HONO--H⁺(H₂O) complex occurred in part: NO⁺ + (H₂O) → HONO---H⁺(H₂O) → HONO + H⁺(H₂O). The energetics and reaction mechanism were discussed on the basis of theoretical results.