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 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: 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. 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.