Direct EPR measurement of Arrhenius parameters for the reactions of H˙ atoms with H2O2 and D˙ atoms with D2O2 in aqueous solution

Abstract

Rate constants for the reaction of hydrogen atoms with hydrogen proxide in aqueous solution have been determined by use of pulse radiolysis and electron paramagnetic resonance (EPR) free-induction decay attenuation measurements. At 22.7 °C a value of (3.48 ± 0.10)× 10⁷ dm³ mol^–1 s^–1 was measured, and over the temperature range 7.6–84.6 °C, and activation energy of 21.12 ± 0.56 kJ mol^–1 was determined. By performing these measurements in basic solution, the rate constant at 22.3 °C for hydrogen atoms reaction with the hydroperoxy anion, HO₂^–, has been calculated as (1.24 ± 0.14)× 10⁹ dm³ mol^–1 s^–1, with a corresponding activation energy of 17.32 ± 0.61 kJ mol^–1 over the temperature range 7.3–41.8 °C. Measurements were also performed for deuterium atoms, where for D₂O₂ and DO₂^– reaction at 25 °C, values of (2.30 ± 0.10)× 10⁷ and 2.12 × 10⁹ dm³ mol^–1 s^–1 were measured, respectively. Only the D₂O₂ activation energy of 25.58 ± 1.68 kJ mol^–1 was accurately determined, but an estimate for DO₂^– was also higher than for the corresponding hydrogen atoms reaction. From the results of measurements performed in a mixed H₂O–D₂O solvent, and using semi-classical transition-state theory arguments, it is shown that the reaction barrier is dominated by the hydrogen atom vibrational adiabatic zero-point energy change.