kinetic isotope effect

https://doi.org/10.1351/goldbook.K03405
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 klkh, 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
, klkh 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.
See also:
isotope effect
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1130 [Terms] [Paper]