Note on Secondary Isotope Effects in Reaction Rates

It is shown that secondary kinetic isotope effects, for all isotopes with the possible exception of hydrogen isotopes, are expected to be small unless the force constants involving the isotopically substituted positions are considerably different in the transition state from those in the reactant. Cases involving secondary hydrogen isotope effects are investigated in detail for certain model reactions. It is concluded that hydrogen isotope effects do not constitute an exception to the above rule.