Editing Kinetic isotope effect (section)

{{short description|Change in chemical reaction rate due to isotopic substitution}}
{{Use dmy dates|date=November 2022}}
{{image frame|width=500
|content=<math chem>
\begin{matrix}\\
\ce{ {CN^-} + {^{12}CH3-Br} ->[k_{12}] {^{12}CH3-CN} + Br^-}\\
\ce{ {CN^-} + {^{13}CH3-Br} ->[k_{13}] {^{13}CH3-CN} + Br^-}\\{}
\end{matrix}
\qquad
\text{KIE}=\frac{k_{12}}{k_{13}} = 1.082 \pm 0.008
</math>
|caption='''An example of KIE.''' In the reaction of [[methyl bromide]] with [[cyanide]], the KIE of the carbon in the [[methyl group]] was found to be 1.082 ± 0.008.<ref name="Westaway1" /><ref>{{cite journal| vauthors = Lynn KR, Yankwich PE |title=Isotope Fractionation at the Methyl Carbon in the Reactions of Cyanide Ion with Methyl Chloride and Methyl Bromide |journal=Journal of the American Chemical Society|date=5 August 1961|volume=83|issue=15|pages=3220–3223|doi=10.1021/ja01476a012|bibcode=1961JAChS..83.3220L }}</ref>}}
In [[physical organic chemistry]], a '''kinetic isotope effect''' ('''KIE''') is the change in the [[reaction rate]] of a [[chemical reaction]] when one of the [[atom]]s in the [[reactant]]s is replaced by one of its [[isotope]]s.<ref name=Atkins /> Formally, it is the ratio of [[rate constant]]s for the reactions involving the light (''k{{sub|L}}'') and the heavy (''k{{sub|H}}'') isotopically substituted reactants ([[isotopologue]]s): KIE = ''k{{sub|L}}/k{{sub|H}}''.

This change in reaction rate is a quantum effect that occurs mainly because heavier isotopologues have lower [[Molecular vibration|vibrational]] frequencies than their lighter counterparts. In most cases, this implies a greater energy input needed for heavier isotopologues to reach the [[transition state]] (or, in rare cases, [[Bond-dissociation energy|dissociation limit]]), and therefore, a slower reaction rate. The study of KIEs can help elucidate [[reaction mechanism]]s, and is occasionally exploited in drug development to improve unfavorable [[pharmacokinetics]] by protecting metabolically vulnerable C-H bonds.