Water molecules are not supposed to react with each other at ordinary conditions. Their covalent bonds are strong, and the conventional dissociation pathway — breaking an O-H bond to produce H and OH radicals — requires energies far above ambient. Water's stability is a foundation of chemistry.
Liu et al. (arXiv:2603.12778) discover a previously unrecognized reaction mechanism in water clusters: hydrogen-atom roaming. A hydrogen atom detaches from its parent water molecule and traverses a flat energy landscape before recombining with a different water molecule, bypassing the conventional dissociation pathway entirely. The atom doesn't fully dissociate — it wanders in a region where it's neither bound to the original molecule nor captured by a new one, then completes the transfer.
The dipole moment of the reactant molecules determines whether roaming occurs. High dipole moments create the electrostatic and exchange-repulsion landscape that flattens the potential surface between donor and acceptor molecules. Once roaming begins, the hydrogen atom's polarizability and spin characteristics govern the energy barriers it encounters. The charge distribution of the roaming atom controls barrier widths through electrostatic, orbital, and dispersion forces.
This establishes hydrogen-atom roaming as a new intrinsic reaction class in water — not a curiosity of exotic conditions but a fundamental pathway available whenever water molecules are close enough and oriented correctly. The mechanism matters for understanding radiation chemistry (where energetic particles produce reactive hydrogen atoms in water), astrochemistry (where water ice on grain surfaces undergoes radiation-driven chemistry), and potentially liquid water dynamics where rare but repeated roaming events could contribute to proton transfer networks.