The kinetic isotope effect (KIE) refers to the change in reaction rate that occurs when one of the atoms in a molecule is replaced with one of its isotopes. This effect is particularly prominent for elements with isotopes that have a significant difference in mass, such as hydrogen and deuterium (the heavy isotope of hydrogen). In general, reactions involving lighter isotopes tend to proceed faster than those involving heavier isotopes.
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A kinetic isotope effect is a change in a reaction rate constant caused by isotopic substitution. Solvent isotope effects can constrain proton-transfer pathways, but coupling and kinetic masking prevent interpreting every rate ratio as one isolated bond-breaking event.