Even though single
hydrated electrons (e
hyd
–’s)
are stable in liquid water, two hydrated electrons can bimolecularly
react with water to create H2 and hydroxide: e
hyd
– + e
hyd
– + 2H2O → H2 + 2OH–. The rate of this reaction has an unusual temperature
and isotope dependence as well as no dependence on ionic strength,
which suggests that cosolvation of two electrons as a single hydrated
dielectron (e
2,hyd
2–) might be an important intermediate
in the mechanism of this reaction. Here, we present an ab initio density
functional theory study of this reaction to better understand the
potential properties, reactivity, and experimental accessibility of
hydrated dielectrons. Our simulations create hydrated dielectrons
by first simulating single e
hyd
–’s and then injecting
a second electron, providing a well-defined time zero for e
2,hyd
2– formation and offering insight into a potential experimental route
to creating dielectrons and optically inducing the reaction. We find
that e
2,hyd
2– immediately forms in every member
of our ensemble of trajectories, allowing us to study the molecular
mechanism of H2 and OH– formation. The
subsequent reaction involves separate proton transfer steps with a
generally well-defined hydride subintermediate. The time scales for
both proton transfer steps are quite broad, with the first proton
transfer step spanning times over a few ps, while the second proton
transfer step varies over ∼150 fs. We find that the first proton
transfer rate is dictated by whether or not the reacting water is
part of an H-bond chain that allows the newly created OH– to rapidly move by Grotthuss-type proton hopping to minimize electrostatic
repulsion with H–. The second proton transfer step
depends significantly on the degree of solvation of H–, leading to a wide range of reactive geometries where the two waters
involved can lie either across the dielectron cavity or more adjacent
to each other. This also allows the two proton transfer events to
take place either effectively concertedly or sequentially, explaining
differing views that have been presented in the literature.
