Isotope effect

An isotope effect is a change in a physical or chemical property caused by substituting one isotope for another. It arises because isotopes differ in mass and, sometimes, nuclear properties.

Zero-point energy: The lowest energy a quantum system retains even at absolute zero. Different isotope masses shift vibrational zero-point energies, often driving isotope effects.
Primary kinetic isotope effect: A reaction-rate isotope effect involving replacement of an atom whose bond is made or broken in the rate-limiting step. Its magnitude can indicate whether bond cleavage or formation contributes to the rate-limiting transition state.
Heavy-atom isotope effect: An isotope effect produced by substituting an isotope of an element heavier than hydrogen. It is usually smaller than a hydrogen isotope effect because relative mass changes are smaller.
Isotope: One of two or more atoms of the same element with different numbers of neutrons. An isotope effect begins with comparing isotopes of the same element.
Vibrational frequency: The rate at which a molecular bond vibrates around its equilibrium position. Heavier isotopes generally lower bond vibrational frequencies and alter reaction energetics.
Secondary kinetic isotope effect: A reaction-rate isotope effect involving an isotopic substitution near, rather than at, the bond being changed. It can reveal changes in hybridization or geometry that accompany a reaction step.
Tunneling isotope effect: An isotope effect arising when quantum tunneling changes the probability of crossing an energy barrier. Its unusually strong mass dependence can distinguish tunneling from ordinary vibrational effects.
Atomic mass: The mass of an atom, determined mainly by its protons and neutrons. Mass differences between isotopes are the main source of common isotope effects.
Born–Oppenheimer approximation: An approximation that separates electronic motion from nuclear motion in molecular calculations. It explains why isotopic substitution often changes nuclear motion without changing the electronic potential surface.
Isotope fractionation: The separation of isotopes between substances or phases through physical, chemical, or biological processes. Isotope effects generate fractionation that records how materials were formed or transformed.

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