Water-catalyzed ultrafast formation of hydronium clusters.

Water catalyzes proton transport through hydrogen-bond networks, yet the minimal structural complexity required to initiate this behavior remains unresolved. Here, we track the birth of the hydrated proton by monitoring the ultrafast formation of size-selected hydronium clusters (H2O) n - 1H+ in real time using femtosecond reaction microscopy. We observe a kinetic collapse in the formation timescale, which drops from ∼259 fs for the water dimer (n = 2) to ∼70 fs for the trimer (n = 3) before converging for larger sizes. This identifies the cyclic water trimer as the minimal catalytic unit. Ab initio molecular dynamics simulations reveal that this acceleration is not driven by the initial proton transfer step, which remains ultrafast, but by the elimination of the reaction barrier governing the separation of the hydronium-solvated products. Our findings demonstrate that the cooperative action of just three water molecules establishes the critical topological threshold for efficient water catalysis, providing a microscopic blueprint for the onset of broader aqueous reactivity.