= Isotope effect
An <isotope effect> is a change in a physical property, equilibrium, or <reaction rate> caused by replacing an atom with one of its <isotopes>. A kinetic isotope effect compares rate constants, often as $k_{\mathrm H}/k_{\mathrm D}$ for hydrogen and deuterium substitution.
Isotopes have nearly the same electronic potential-energy surface but different masses, so vibrational frequencies and zero-point energies change. If a carbon-hydrogen bond is substantially altered in a rate-controlling step, replacing it by carbon-deuterium can produce a measurable rate difference. Such a measurement constrains a <reaction mechanism>, but its interpretation can also involve tunneling, equilibrium steps, and competing pathways.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[isotope effect, term I03327]{ref}
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