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.