Molecular Dynamics Characterization of Dielectron Hydration in Liquid Water with Unique Double Proton Transfers

Radiation chemistry of water and aqueous solutions has always been an interesting scientific issue owing to involving electronic excitations, ionization of solvated species, and formation of radiolytic species and many elementary reactions, but the underlying mechanisms are still poorly understood. Here, we for the first time molecular dynamics characterize the hydration dynamics of two correlated electrons and their triggered unique phenomena in liquid water associated with radiolysis of water using the combined hybrid functional and nonlocal dispersion functional. Hydration of two electrons may experience two distinctly different mechanisms, one forming a spin-paired closed-shell unicaged dielectron hydrate (e(2aq)(2-)) and the other forming a spin-paired metastable open-shell bicaged hydrated electron pair (e(aq)(-)center dot center dot center dot e(aq)(-)) which exhibits intriguing antiferromagnetic spin coupling dynamics (in a range of -40 cm(-1) to -500 cm(-1)). e(aq)(-)center dot center dot center dot e(aq)(-) can recombine to e(2aq)(2-) through a unique solvent fluctuation-controlled gradual-flowing mechanism, and enlarging fluctuation can promote the conversion. Interestingly, we directly observe that e(2aq)(2-) as the precursor can trigger hydrogen evolution via unique continuous spontaneous double proton transfer to the dielectron with a short-lived H-aq(-) intermediate, but e(aq)(-)center dot center dot center dot e(aq)(-) does not directly. This is the first direct observation for the connection between e(2aq)(2-) and spontaneous hydrogen evolution including participation of H-aq(-) in aqueous solution, bridging relevant experimental phenomena. This work also evidences an unnoticed process, the double proton transfer mediated charge separation, and presents the first detailed analysis regarding the evolution dynamics of e(2aq)(2-) for the understanding of the radiolysis reactions in aqueous solutions.

Automated summary

This study reveals the hydration dynamics of two correlated electrons in liquid water and their unique phenomena associated with water radiolysis for the first time through molecular dynamics. The formation of closed-shell and open-shell dielectron hydrates, as well as the conversion mechanism between them controlled by solvent fluctuations, are elucidated. Additionally, a process of double proton transfer mediated charge separation is observed, and the evolution dynamics of e(2aq)(2-) in triggering hydrogen evolution in aqueous solutions is analyzed in detail.