H/D ISOTOPE EFFECTS IN WATER RADIOLYSIS .4. THE MECHANISM OF (H)AQ-REVERSIBLE-ARROW(E-)AQ INTERCONVERSION

We report new measurements of the reaction rates of H with OH- in light water and D with OD- in heavy water. Measurements performed by optical transient absorption and by EPR using the free induction decay attenuation method agree quantitatively. Arrhenius parameters for the H + OH- reaction are A = (1.33 +/- 0.16) x 10(14) M-1 s-1 and E(a) = 38.38 +/- 0.31 kJ/mol. For the D + OD- reaction corresponding parameters are A = (1.19 +/- 0.22) x 10(14) M-1 s-1 and E(a) = 38.16 +/- 0.47 kJ/mol. The reaction rate data for the forward and reverse reactions, taken together with our current understanding of the electron and H atom solvation structure and dynamics, lead us to conclude that the reaction mechanism is best described as a proton transfer from the weak acid (H)aq to the strong base (OH-)aq. The H atom effectively disproportionates, leaving the electron to become hydrated within the H atom solvent cage. By extension, numerous reactions of (e-)aq With protonated aqueous species and water itself can best be understood in terms of a proton transfer to the weak base (e-)aq.