kinetic isotope effect

The effect of isotopic substitution on a rate constant is referred to as a kinetic isotope effect. For example, in the reaction:

the effect of isotopic substitution in reactant A is expressed as the ratio of rate constants $k l k h$ , where the superscripts $l$ and $h$ represent reactions in which the molecules A contain the light and heavy isotopes, respectively. Within the framework of transition state theory in which the reaction is rewritten as:

and with neglect of isotopic mass on tunnelling and the transmission coefficient, $k l k h$ can be regarded as if it were the equilibrium constant for an isotope exchange reaction between the transition state[TS]^‡ and the isotopically substituted reactant A, and calculated from their vibrational frequencies as in the case of a thermodynamic isotope effect. Isotope effects like the above, involving a direct or indirect comparison of the rates of reaction of isotopologues, are called 'intermolecular', in contrast to intramolecular isotope effects, in which a single substrate reacts to produce a non-statistical distribution of isotopomeric product molecules.