Preferred Label : thermodynamic isotope effect;
IUPAC definition : 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: \[{A} {B}\rightleftharpoons
{C}\] is the ratio Kl/Kh 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: \[{A} {{l}}
{C} {{h}}\rightleftharpoons {A} {{h}} {C} {{l}}\] 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: \[\frac{K {{l}}}{K {{h}}} \frac{(Q_{{nuc}} {{l}}/Q_{{nuc}}
{{h}})_{{C}}};
Origin ID : T06319;
See also
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: \[{A} {B}\rightleftharpoons
{C}\] is the ratio Kl/Kh 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: \[{A} {{l}}
{C} {{h}}\rightleftharpoons {A} {{h}} {C} {{l}}\] 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: \[\frac{K {{l}}}{K {{h}}} \frac{(Q_{{nuc}} {{l}}/Q_{{nuc}}
{{h}})_{{C}}}
