Preferred Label : kinetic isotope effect;
IUPAC definition : 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] sup ‡ /sup 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.;
Origin ID : K03405;
See also
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] sup ‡ /sup 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.