The role of water molecules in the dissociation of an electron-molecule contact pair.

The hydrated electron, e, is a potent reducing agent and a prototypical quantum solute. Reactions of e often involve a contact pair comprised of a molecule and electron that are hydrated within a single sphere. However, a molecular-level understanding of the solvent-driven coordinate that links the contact pair to the free dissociated e remains elusive. Here, we study this coordinate by kinetically trapping representative metastable intermediates as gas-phase clusters and probing them using photoelectron spectroscopy. We apply this methodology to uracil-water anion clusters, where key intermediates are identified with supporting quantum chemical calculations. Just a single water molecule drives the parent molecule and non-valence electron apart, thereby inhibiting geminate recombination to form the more stable valence-bound uracil anion. The electron-water binding is akin to bare water cluster anions, highlighting the link to larger clusters and e. Our results provide a molecular-level view of quantum solute hydration and, more broadly, of how water-driven electron-transfer reactions proceed.