thermodynamic isotope effect

https://doi.org/10.1351/goldbook.T06319
The effect of isotopic substitution on an
equilibrium constant
is referred to as a thermodynamic (or equilibrium)
isotope effect
. For example, the effect of isotopic substitution in reactant A that participates in the equilibrium: A+BC is the ratio KlKh of the
equilibrium constant
for the reaction in which A contains the light isotope to that in which it contains the heavy isotope. The ratio can be expressed as the
equilibrium constant
for the isotopic exchange reaction: Al+ChAh+Cl in which reactants such as B that are not
isotopically substituted
do not appear. The potential energy surfaces of isotopic molecules are identical to a high degree of approximation, so thermodynamic isotope effects can only arise from the effect of isotopic mass on the nuclear motions of the reactants and products, and can be expressed quantitatively in terms of
partition function
ratios for nuclear motion: KlKh=(Qnucl/Qnuch)C(Qnucl/Qnuch)A Although the nuclear
partition function
is a product of the translational, rotational and vibrational
partition
functions, the
isotope effect
is determined almost entirely by the last named, specifically by vibrational modes involving motion of isotopically different atoms. In the case of light atoms (i.e.
protium
vs.
deuterium
or
tritium
) at moderate temperatures, the
isotope effect
is dominated by zero-point energy differences.
See also:
isotopic fractionation factor
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1131 [Terms] [Paper]