The Intricate Nonadiabatic Dynamics of NO⁺ and NO₃^- Mutual Neutralization

Mutual neutralization reactions play a subtle but crucial role in atmospheric chemistry. The intrinsic gap between typical ionization energies and electron affinities allows cation-anion reactions to produce a broad range of neutral products. Specifically, mutual neutralization of NO⁺ and NO₃^- ions in the presence of water has been proposed to play a key role in the formation of atmospheric nitrous acid (HONO) and, consequently, also OH radical formation. Nevertheless, the mechanisms and products of molecular anion-cation reactions are largely unknown. Here, we present a detailed experimental study of the isolated NO⁺ and NO₃^- reactions, using three-dimensional coincidence imaging of the neutral products of low-energy collisions in merged cation and anion beams. We found that while 15 product channels are energetically accessible, the reaction proceeds exclusively via a single nonadiabatic pathway yielding NO + NO₂ + O. Momentum correlation analysis revealed an intricate nonadiabatic mechanism, initiated by a long-range electron transfer at ∼6 Å distance between the ions, resulting in vibrationally hot NO and an electronically excited NO₃ (²E') intermediate that undergoes subsequent dissociation, attributed to a conical intersection with the lower lying NO₃ (²E″) state. The mechanistic picture of NO⁺ + NO₃^- neutralization and identification of specific intermediates provides a basis for considering the competing processes that in the presence of water can lead to HONO formation.