Ab Initio Insights into the Formation of Hydrated Dielectrons: Structure and Recombination Dynamics

Despite extensive research on the behavior and properties of the hydrated electron (e_hyd^-), relatively little is known about the recombination of two hydrated electrons to form a hydrated dielectron. Hydrated dielectrons are hypothesized to be important in the radiation chemistry of water, particularly as an intermediate in the reaction that produces molecular hydrogen: e_hyd^- + e_hyd^- + 2H₂O → 2OH^- + H₂. In this work, we investigate the solvation structures and recombination dynamics of both singlet and triplet hydrated electron pairs in bulk water using DFT-based ab initio molecular dynamics simulations. Our findings reveal that the recombination reaction leading to dielectron formation occurs exclusively from the singlet state, with the dielectron occupying a slightly larger cavity than the single electron. The potential of mean force for open-shell singlet electron pairs has a local minimum at a distance corresponding to a metastable state in which two electron cavities are separated by a single bridging water molecule. From this state, recombination to form dielectrons occurs by two distinct mechanisms: diffusive recombination and direct tunneling. These pathways have an approximately 1:1 branching ratio, a balance set by the symmetry of the local solvation environment. Diffusion occurs when both electrons experience similar, coupled solvation environments, while tunneling is triggered when asymmetric solvation destabilizes one electron, which then tunnels into the more stable cavity of the electron pair in a manner mediated by the bridging water molecule.