Unravelling non-adiabatic pathways in the mutual neutralization of hydronium and hydroxide

The mutual-neutralization of hydronium and hydroxide ions is a fundamental chemical reaction. Yet, there is very limited direct experimental evidence about its intrinsically non-adiabatic mechanism. By 3D-imaging of the coincident neutral products of isolated D ₃ O ⁺ and OD ⁻ reactions, we revealed two competing non-adiabatic pathways leading to distinct D ₂ O+OD+D and 2OD+D ₂ product channels and substantial suppression of the proton-transfer mechanism due to a kinetic isotope-effect. Analysis of the 3-body momentum correlations revealed the electronic ground- and excited-state energies of the unstable D ₃ O radicals, formed by e-transfer. Furthermore, we found that in reactions leading to the higher probability D ₂ O+OD+D channel, e-transfer forms the neutral D ₃ O ground state at a short ~4Å distance. In contrast, 2OD+D ₂ products are formed following e-transfer at ~10Å distance and proceed on an excited-state of the neutral D ₃ O. The observed mechanisms are related to the recently reported spontaneous OH radical formation at the surface of pure water microdroplets.