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Inverse Kinetic Isotope Effects

In order to understand inverse kinetic isotope effects, or IKIEs, a person must be familiar with the concept of kinetic isotope effects, or KIEs. A kinetic isotope effect is a scientific proportion of two molecules within the same chemical reaction. The kinetic isotope effect measures the reaction rate of the molecular constants in the chemical reaction. In an inverse kinetic isotope effect, the heavier molecule reacts faster than the lighter one in the proportion.

Inverse or Normal
- An inverse kinetic isotope effect is one type of effect in the reductive coupling of isotopes. When scientists and chemists discuss kinetic isotope effects, the effects are either categorized as inverse or normal. A normal effect refers to the kinetic isotope effect in which the reaction frequency differences are smaller, whereas the definition of an inverse kinetic isotope effect is the opposite. The isotopes in the chemical bond can modify and determine what type of kinetic isotopic effect comes as the result, based on how they change the bond structure or any time a substitution is added into the chain reaction.
Mass
- The weights and rates of reactivity are determined in molecular equations based on the mass of the atoms in the isotopes. The mass is a constant that allows chemists to analyze an isotopic bond or compound and determine the inverse kinetic isotope effect based on the heavier molecule. In a normal kinetic isotope effect, the heavier molecule with the most mass would not react the fastest, but with the inverse kinetic isotope effect, the heavier molecule would react the fastest.
Reductive Elimination
- Understanding how inverse kinetic isotope effects are found requires you to study the phenomenon of reductive elimination. Reductive elimination is a type of chemical reaction in which the oxidation state is decreased as part of the last step in the catalytic cycle. It is in this stage of the process that inverse kinetic isotope effects are more frequently identified or observed, because it is during this time that the mass of the atoms and molecular bonds change, thereby causing the isotopes to redistribute the weight and cause the inverse reaction rate.
Isotope Fractionation
- Isotope fractionation refers to the equation in which two isotopes are measured to determine the proportion to find the rate of reaction. This equation is also used to judge whether an effect is normal or inverse. In the equation, the two reaction rate constants are divided by one another. They are assessed to determine the pre-fractionation equilibrium, or balance, and then assessed for post-fractionation to decide how the equilibrium changed, or whether it was a variable that skewed the mass of the two isotopes.

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