The kinetic isotope effect (KIE) is the ratio of reaction rates between a molecule containing the lighter isotope (usually protium, ¹H) and one containing a heavier isotope (like deuterium, ²H). For a typical C–H bond breaking at room temperature, replacing H with D can reduce the rate by a factor of 6–7, because the heavier atom lowers the vibrational zero-point energy, raising the activation barrier. Even larger effects (up to ~50) occur when quantum tunneling is significant, as in some enzyme-catalyzed reactions.