J. Phys. Chem. 1992, 96, 5211-5213
5211
Large Isotope Effect Due to Quantum Tunneling in the Conversion Reaction of Electrons to H and D Atoms in Irradiated H20/D20 I c e Hachizo Muto,* Kaoru Matsuura, and Keichi Nunome Government Industrial Research Institute, Nagoya, Hirate-cho, Kita- ku, Nagoya, 462 Japan (Received: February 26, 1992; In Final Form: April 21, 1992)
A H/D isotope effect in water radiolysis was studied at 4.2 K by ESR spectroscopy to elucidate the radiolysis mechanism. Radiation-induced electrons, which are trapped only in the H20/D20mixtures with H contents lower than 3%, were selectively converted to isolated H atoms at 4.2 K and exhibited a very large effect on the conversion rate: (kH/kD)/(H/D)ia> lo3, presumably due to tunneling. The net isotope effect for the atomic hydrogen yield (Y) including the initial production was (YH/YD)/(H/D)ia= 10-102 and 3.5-2.5 for 34.25% and 10-90% H contents, respectively. These findings gave unequivocal evidence for the suggested reaction of electrons with H30+ and implied a possibility of isotope separation.
introduction A study of the radiolysis of water is of fundamental importance to living organisms and nuclear reactor technology. It has been acknowledged that hydrogen atoms (H) are produced in water radiolysis by two distinct mechanisms. One is direct dissociation of excited H 2 0 * molecules, and the other is the ionic reaction of electrons (e-) and hydronium ions (H3O+).I-I3 The extent of their contributions is not known, and direct spectroscopic evidence has not been obtained for the buildup of H atoms following decay of electrons in the latter process. Studies of the H / D isotope effect in water radiolysis are expected to clarify the reaction mechanism and are important for isotope They may also serve for understanding the proton transfer in biological reactions.l4Js It has been reported that the radiolysis of water exhibits an isotopic effect on the hydrogen atom production by a magnitude CY = (H/D),d,a/(H/D)w,t, 2.5 in H 2 0 / D 2 0liquid.+" No effect is reported in the low-temperature solids except for D 2 0containing impurity amounts of H20 (
