Phenomenological manifestations of quantum-mechanical tunneling. III. Effect on relative tritium-deuterium kinetic isotope effects

The effect of quantum-mechanical tunneling on relative tritium-deuterium kinetic isotope effects is investigated with the aid of model calculations. It is shown that even when tunneling is important, such relative effects, defined as r=ln(kH/kT) [ln(kH/kD)]−1, have values not very different from their values in the absence of tunneling. Thus, for the types of reactions commonly used to investigate tunneling, the limits 1.33 ≤ r ≤ 1.58 are not likely to be exceeded. There is no direct general correlation between the magnitude of r and the extent of tunneling; the temperature dependence of r is qualitatively the same for a wide range of reaction models and barrier heights.