Large Isotope Effects in Tunneling Chemical Reactions.

Our studies on the isotope effects in tunneling reactions are autoreviewed. Large isotope effects are observed in tunneling reactions, which take place by passing through the potential energy barrier for a reaction. A very large isotope effect more than 3 x 10(4) was obtained in tunneling reactions H(D)+H-2(D-2) in gamma-rays-irradiated solid hydrogen at 4.2 K, though there is no isotope effect in a hot atom reaction. The isotope effect on H-2 or D-2 detachment from a 2,3-dimethylbutane cation, produced by gamma-radiolysis, amounts to a very large value of 1.7 x 10(4) at 77 K that indicates a tunneling detachment of a hydrogen molecule. The tunneling reaction of tritium atoms, produced by a He-3(n, p)T reaction, with H-2 or D-2 in liquid helium at 1.3 K shows a large isotope effect of 150. A large isotope effect (greater than or equal to20-50) was observed in the tunneling reaction of vitamin C and the long-lived protein radicals in biological systems. It is concluded that large isotope effects caused by a tunneling reaction play a role in radiation chemistry, a nuclear fusion technology and radiation biology.