Picosecond Pulse Radiolysis Study on the Distance Dependent Reaction of the Solvated Electron with Organic Molecules in Ethylene Glycol

The decay of solvated electron e(s)(-) is observed by nanosecond and picosecond pulsed radiolysis, in diluted and highly concentrated solutions of dichloromethane, CH2Cl2, trichloromethane, CHCl3, tribromomethane, CHBr3, acetone, CH3COCH3, and nitromethane, CH3NO2, prepared in ethylene glycol. First, second-order rate constants for the reactions between e(s)(-) and the organic scavengers have been determined. The ratio between the highest rate constant that was found for CH3NO2 and the lowest one that was found for acetone is 3. This difference in reactivity cannot be explained by the change of viscosity or the size of the molecules. Then, from the analysis of decay kinetics obtained using ultrafast pulse probe method, the distance dependent first-order rate constant of electron transfer for each scavenger has been determined. The amplitude of the transient effect observed on the picosecond time scale differs strongly between these solvated electron scavengers. For an identical scavenger concentration, the transient effect lasts approximate to 650 ps for CH3NO2 compared to similar to 200 ps for acetone. For acetone, the distance dependent first-order rate constant of electron transfer is decreasing very rapidly with increasing distance, whereas for nitromethane and tribromomethane the rate constant is decreasing gradually with the distance and its value remains non-negligible even at similar to 10 angstrom. This rate constant is controlled mostly by the free energy of the reaction. For nitromethane and tribromomethane, the driving force is great, and the reaction can occur even at long distance, whereas for acetone the driving force is small and the reaction occurs almost at the contact distance. For nitromethane and acetone, the one-electron reduction reaction needs less internal reorganization energy than for alkyl halide compounds for which the reaction occurs in concert with bond breaking and geometric adjustment.