Pulse Radiolysis of Liquids at High Pressures. III. Hydrated-Electron Reactions Not Controlled by Diffusion

Abstract

Specific rates of the reactions of $e_{\mathrm{aq}}^{-}$ with hydrogen ion, formamide, acetamide, acetoxime, benzyl alcohol, and 2‐chloroethanol have been measured by pulse radiolysis at 29°C and six pressures from atmospheric to 6.4 kbar. The specific rates increase monotonically with increase in pressure to 6.4 kbar except that for the H₃O⁺ reaction for which there is no further significant increase above ∼3.5 kbar. Results for the H₃O⁺ reaction are consistent with that reaction not being diffusion controlled. The activation volume of each reaction increases with increase in pressure. Such a result is attributed to a decrease in cavity volume of the electron with increase in pressure. Activation volumes, in units of milliliters per mole, at 1 atm and 29°C are −2.7, −10.3, −7.0, −9.5, −9.3, and −8.2 for the reactions with H₃O⁺, formamide, acetamide, acetoxime, benzyl alcohol, and 2‐chloroethanol, respectively. Interpretation of the activation volumes gives a partial molal volume of 7 ml mole⁻¹ and, with an electrostriction volume of 3 ml mole⁻¹, a cavity volume of 10 ml mole⁻¹ for the electron in water at 1 atm and 29°C. A previous estimate of ${\mathrm{V̄(e}}_{\mathrm{aq}}^{-}\text{) = 0\hspace{0.17em}}{\mathrm{ml\; mole}}^{\text{−1}}$ , based on study of competitive reactions of $e_{\mathrm{aq}}^{-}$ by γ radiolysis at high pressures, is reconciled with the value of ${\mathrm{V̄(e}}_{\mathrm{aq}}^{-}\text{) = 7\hspace{0.17em}}{\mathrm{ml\; mole}}^{\text{−1}}$ .