Observation of an Associative State in Aqueous Hydroxide

The dynamics of chemical reactions in solution are of paramount importance in fields ranging from biology to materials science. Because the hydrogen-bond network and proton dynamics govern the behavior of aqueous solutions, they have been the subject of numerous studies over the years. Here, we report the observation of a previously unknown associative state in the hydroxide ion that forms when a proton from a neighboring water molecule approaches the hydroxide ion, utilizing resonant inelastic soft X-ray scattering (RIXS) and quantum dynamical simulations. State-of-the-art theoretical analysis reveals state mixing in the electronically excited states between aqueous hydroxide ions and the solvent. Our results give new insights into chemical bonding and excited-state dynamics in the aqueous environment. This investigation of associative states opens up new pathways for spectroscopic studies of chemical reaction dynamics and lays the foundation for directly accessing dynamic proton exchange in solution.