posted on 2024-01-07, 13:03authored byPauf Neupane, David M. Bartels, Ward H. Thompson
Many questions remain about the reactions of the hydrated
electron
despite decades of study. Of particular note is that they do not appear
to follow the Marcus theory of electron transfer reactions, a feature
that is yet to be explained. To investigate these issues, we used ab initio molecular dynamics (AIMD) simulations to investigate
one of the better studied reactions, the hydrated electron reduction
of CO2. The rate constant for the hydrated electron–CO2 reaction complex to react to form CO2– is for the first time
estimated from AIMD simulations. Results at 298 and 373 K show the
rate constant is insensitive to temperature, consistent with the low
measured activation energy for the reaction, and the implications
of this behavior are examined. The sampling provided by the simulations
yields insight into the reaction mechanism. The reaction is found
to involve both solvent reorganization and changes in the carbon dioxide
structure. The latter leads to significant vibrational excitation
of the bending and symmetric stretch vibrations in the CO2– product,
indicating the reaction is vibrationally nonadiabatic. The former
is estimated from the calculation of an approximate collective solvent
coordinate and the free energy in this coordinate is determined. These
results indicate that AIMD simulations can reasonably estimate hydrated
electron reaction activation energies and provide new insight into
the mechanism that can help illuminate the features of this unusual
chemistry.