H/D isotope effects in the formation of hydrogen from the combination of two radicals in aqueous solutions

The H/D isotope effects of the reactions e_aq^-+e _aq^-(a), e_aq^-+H (b) and H+H (c), were determined using high-intensity pulse radiolysis. At room temperature in mixtures of H₂O+D₂O = 1:1, the following H/D isotope effects were observed: α_a = 4.7±0.2, α_b = 1.5-1.9±0.3 (depending on the pH) and αc = 2.6-3.5±0.2 (depending on the pH). The isotope effect of the "molecular" hydrogen formed in the radiolysis of water, α_M = 2.2±0.1, was found to be independent of pH. In view of the isotope effects, reaction (a) was interpreted in terms of the formation of a (e_aq^-) ₂ centre as an intermediate, and reaction (b) as involving the formation of a hydride ion. The results indicate that the "molecular" and the "residual" hydrogen, produced in neutral radiolyzed aqueous solution, are formed via reactions different from those assumed in the diffusion model for the radiolysis of water. Most probably these species are formed from excited water molecules. The incompatability of the results with the diffusion model makes a revision of the latter most desirable.