The effect of isotopic substitution on a @O04322@ is referred to as a kinetic @I03327@. For example, in the reaction: the effect of isotopic substitution in reactant A is expressed as the ratio of rate constants \(\frac{k^{\text{l}}}{k^{\text{h}}}\), where the superscripts \(\text{l}\) and \(\text{h}\) represent reactions in which the molecules A contain the light and heavy @I03331@, respectively. Within the framework of @T06470@ in which the reaction is rewritten as: and with neglect of isotopic mass on @T06531@ and the @T06479@, \(\frac{k^{\text{l}}}{k^{\text{h}}}\) can be regarded as if it were the @E02177@ for an @I03328@ reaction between the @T06468@ [TS]^‡ and the @I03338@ reactant A, and calculated from their vibrational frequencies as in the case of a @T06319@. Isotope effects like the above, involving a direct or indirect comparison of the rates of reaction of @I03351@, are called '@I03098@', in contrast to @I03132@, in which a single substrate reacts to produce a non-statistical distribution of @I03352@ product molecules.