The nature and absolute hydration free energy of the solvated electron in water

We report the first first-principles solvation-included electronic structure study to energetically compare a variety of candidate structures of the hydrated electron and to determine its absolute hydration free energy ΔG_hyd²⁹⁸(e^-). The calculated results show that both the thermal motion and bulk solvent effects can qualitatively change the relative thermodynamic stability of different structures of the hydrated electron on the basis of a cluster of a given size, and that the most stable structure in solution is not necessarily the most stable one in the gas phase. For a given number of explicitly included solvent water molecules, the most stable structure in solution reveals a unique feature of the chemical nature of the solvated electron in water, i.e., the electron forms two strong electron-hydrogen bonds of the e^-···HO type with the hydrogen-bonded water cluster and two of the hydrogen bonds in the neutral water cluster are broken. On the basis of the most stable structures, the calculated electronic excitation energies are within the observed absorption range of the hydrated electron in water. The absolute hydration free energy of the solvated electron in water has been calculated to be -35.5 kcal/mol by using a reliable computational protocol of first-principles solvation-included electronic structure calculations. This value is in excellent agreement with a recently obtained value of -34.6 kcal/mol. The predicted ΔG_hyd²⁹⁸(e^-) value of -35.5 kcal/mol, when combined with our previously predicted ΔG_hyd²⁹⁸(H⁺) value of -262.4 kcal/mol and ΔG_hyd²⁹⁸(HO^-) value of -104.5 kcal/mol by using the same computational protocol, gives ΔG_hyd²⁹⁸(e^-) + ΔG_hyd²⁹⁸(H⁺) = 297.9 kcal/mol and ΔG_hyd²⁹⁸(e^-) - ΔG_hyd²⁹⁸(HO^-) = 69.0 kcal/mol, in excellent agreement with the corresponding values derived from experimental data.