Absorption spectra and kinetics of methyl and ethyl radicals in water

The absorption spectra and some kinetic properties of the methyl and ethyl radicals in water were studied by pulse radiolysis. For methyl radicals the maximum of the absorption spectrum lies at 213 nm (epsilon 1600 M-1 cm-1 ) while the ethyl radical spectrum shows no maximum down to 200 nm. The disappearance of methyl and ethyl radicals follows second-order kinetics. At 250, k/sub (CH3 + CH3)/ = 1.6 + - 0.2 x 109 M-1 sec-1 and k/sub (C2H5 + C2H5)/ = 1.2 + - 0.2 x 109 M-1 sec-1. The effect of temperature on the rate of recombination gives an apparent activation energy of 3.9 + - 0.4 kcal/mol for both methyl and ethyl radicals, and is close to the activation energy for the diffusion of the radicals in water. In the presence of oxygen a new spectrum appears centered at 250 nm with a lifetime in the millisecond range. A study of the effect of pH on this spectrum shows that the new species is not HO2 formed by H atom transfer to the oxygen but RO2 formed by direct addition of the radical to oxygen. For ethyl radicals the rate constant of this addition is k/sub (C2H5 + O2) = 2.9 + - 0.8 x 109 M-1 sec-1. The reaction of the two alkyl radicals with H2O2 was investigated by pulse and steady-state radiolysis. The rate constant between CH3 and C2H5 and H2O2 is comparatively slow k less than or equal to 106 M-1 sec-1 and there is no indication that the reaction proceeds via a chain mechanism. (U.S.)