Definition of isotope effect kinetic

The effect of isotopic substitution on a rate constant is referred to as a kinetic isotope effect.

For example in the reaction

A + B C

the effect of isotopic substitution in reactant A is expressed as the ratio of rate constants kl/kh, 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

A + B [TS] C

and with neglect of isotopic mass on tunnelling and the transmission coefficient, kl/kh 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 (see isotope effect, thermodynamic).

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 (see isotope effect, intramolecular), in which a single substrate reacts to produce a non-statistical distribution of isotopologue product molecules. See WOLFSBERG (1972).