Kinetic isotope effect
In physical organic chemistry, a kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes. Formally, it is the ratio of rate constants for the reactions involving the light (kL) and the heavy (kH) isotopically substituted reactants (isotopologues): KIE = kL/kH. This change in reaction rate is a quantum effect that occurs mainly because heavier isotopologues have lower vibrational frequencies and, therefore, lower ground-state vibrational energies, than their lighter counterparts.