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. Chemistry textbooks describe the products of mutual neutralization in bulk water as two water molecules; however, this reaction has been suggested as a possible mechanism for the recently reported spontaneous formation of OH radicals at the surface of water microdroplets. Here, following three-dimensional-imaging of the coincident neutral products of reactions of isolated D₃O⁺ and OD⁻, we can reveal the non-adiabatic pathways for OD radical formation. Two competing pathways lead to distinct D₂O + OD + D and 2OD + D₂ product channels, while the proton-transfer mechanism is substantially suppressed due to a kinetic isotope effect. Analysis of the three-body momentum correlations revealed that the D₂O + OD + D channel is formed by electron transfer at a short distance of ~4 Å with the formation of the intermediate unstable neutral D₃O ground state, while 2OD + D₂ products are obtained following electron transfer at a distance of ~10 Å via an excited state of the neutral D₃O.