DIRECT EPR MEASUREMENT OF ARRHENIUS PARAMETERS FOR THE REACTIONS OF H' ATOMS WITH H2O2 AND D' ATOMS WITH D2O2 IN AQUEOUS-SOLUTION

Rate constants for the reaction of hydrogen atoms with hydrogen peroxide in aqueous solution have been determined by use of pulse radiolysis and electron paramagnetic resonance (EPR) free-induction decay attenuation measurements. At 22.7 degrees C a value of (3.48 +/- 0.10) x 10(7) dm(3) mol(-1) s(-1) was measured, and over the temperature range 7.6-84.6 degrees C, an 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 degrees C for hydrogen atom reaction with the hydroperoxy anion, HO2-, has been calculated as (1.24 +/- 0.14) x 10(9) dm(3) 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 degrees C. Measurements were also performed for deuterium atoms, where for D2O2 and DO2- reaction at 25 degrees C, values of (2.30 +/- 0.10) x 10(7) and 2.12 x 10(9) dm(3) 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 atom reaction. From the results of measurements performed in a mixed H2O-D2O solvent, and using semiclassical transition-state theory arguments, it is shown that the reaction barrier is dominated by the hydrogen atom vibrational adiabatic zero-point energy change.