Kinetic isotope effect

A change in a chemical reaction’s rate caused by replacing an atom with one of its isotopes. Its magnitude can reveal how bonds are formed or broken in the rate-limiting step.

Primary kinetic isotope effect: A kinetic isotope effect arising when the isotopically substituted atom forms or breaks a bond in the rate-limiting step. It is the clearest case: the isotope-sensitive bond changes directly during the rate-determining event.
Isotope: One of two or more atoms of the same element with different numbers of neutrons. An isotope substitution changes nuclear mass while preserving the element’s chemical identity.
Deuterium labeling: The replacement of hydrogen atoms with deuterium to trace or perturb chemical processes. Deuterium substitution is a common way to measure hydrogen-related rate effects.
Equilibrium isotope effect: A change in an equilibrium constant caused by replacing an atom with one of its isotopes. It concerns equilibrium populations, whereas the kinetic effect compares forward reaction rates.
Secondary kinetic isotope effect: A kinetic isotope effect in which the substituted atom’s bond remains intact during the rate-limiting step. It shows that isotope effects can arise from changes near, rather than at, the reacting bond.
Reaction rate: The change in concentration of a reactant or product per unit time. The kinetic isotope effect is defined by comparing rates for isotopically distinct reactants.
Enzyme kinetics: The study of reaction rates catalyzed by enzymes and their dependence on conditions. Isotope effects can reveal which chemical transformations limit an enzyme’s catalytic cycle.
Heavy-atom kinetic isotope effect: A kinetic isotope effect involving substitution of an element heavier than hydrogen, such as carbon or nitrogen. It often provides subtler evidence about bond changes than the commonly measured hydrogen effects.
Transition state theory: A framework that relates reaction rates to the properties of the activated complex and its energy barrier. It explains how isotope-dependent vibrational energies can change the activation free energy.
Rate-determining step: A reaction mechanism step that limits the overall rate under specified conditions. Whether the substituted bond changes in this step strongly shapes the observed effect.

Linked from 19 pages