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
, where the superscripts
and
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,
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.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1130
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.
