Solvation of radical cations in water--reactive or unreactive solvation?

The solvation and reaction of ethylene radical cation in aqueous solution has been studied with Car-Parrinello molecular dynamics simulations. All ab initio simulations were performed using a system of 56 water and one ethylene molecule. Using a favorable symmetrically solvated radical cation as the starting point of the simulation a fast addition of water (within 90 fs) to the radical cation is observed. The primary addition product is rapidly deprotonated (within 100 fs) to yield the ethanol-2-yl radical. A second simulation was initiated through vertical ionization of neutral hydrated ethylene, representing a significantly less favorable situation for the addition process. No addition of water can be observed in this second simulation over a time span of 1.7 ps. Taken together the two simulations are indicative of a rearrangement of the solvent shell which represents the major part of the overall reaction barrier. Under these circumstances, the reaction rate of an otherwise spontaneous reaction is limited by the intrinsic solvent relaxation time. This interpretation of the reactivity of hydrated radical cations reconciles previously conflicting experimental condensed phase and theoretical gas phase studies.