RATE-CONSTANT AND ACTIVATION-ENERGY MEASUREMENT FOR THE REACTION OF ATOMIC-HYDROGEN WITH METHANOL, IODOMETHANE, IODOETHANE, AND 1-IODOPROPANE IN AQUEOUS-SOLUTION

The Arrhenius parameters for the reaction of hydrogen atoms with metha nol in aqueous solution have been determined by use of pulse radiolysi s and electron paramagnetic resonance free induction decay attenuation measurements. At 25.5 degrees C, an absolute scavenging rate constant of (2.84 +/- 0.07) x 10(6) dm(3) mol(-1) s(-1) has been measured, and over the temperature range 10.2-86.4 degrees C, an activation energy of 29.4 +/- 0.8 kJ mol(-1) has been determined. These values have been used to make minor corrections to rate constant and activation energy values for the reaction of aqueous hydrogen atoms with iodomethane, i odoethane, and 1-iodopropane. For these three compounds at 24.3 degree s C, rate constants of (1.17 +/- 0.07) x 10(10), (1.39 +/- 0.05) x 10( 10), and (1.42 +/- 0.06) x 10(10) dm(3) mol(-1) s(-1) and correspondin g activation energies of 10.4 +/- 0.4, 11.8 +/- 0.4, and 11.9 +/- 0.4 kJ mol(-1) have been respectively calculated over the temperature rang e 9-52 degrees C. These fast reaction rate constants and the similarit y of the activation energies suggest that the mechanism of reaction fo r all these iodoalkane scavengers is dominated by halogen abstraction, H-. + RI --> H+ + I- + (.)R. Comparison of the methanol scavenging ra te to gas phase results suggests that tunneling which contributes to t he gas phase reaction may be quenched in aqueous solution.