The isotope effect is typically quantified using the ratio of reaction rates for the isotopically substituted and non-substituted molecules. This ratio is known as the kinetic isotope effect (KIE) and is represented by the formula:
KIE = k_H / k_D
Where k_H is the reaction rate constant for the non-substituted molecule (usually containing hydrogen) and k_D is the rate constant for the substituted molecule (usually containing deuterium). A significant KIE indicates that the isotope substitution has a substantial impact on the reaction rate.
