Isotope effect

An isotope effect is a change in a physical property, equilibrium, or reaction rate caused by replacing an atom with one of its isotopes. A kinetic isotope effect compares rate constants, often as $k_{\mathrm{H}}/k_{\mathrm{D}}$ for hydrogen and deuterium substitution.

Isotopes have nearly the same electronic potential-energy surface but different masses, so vibrational frequencies and zero-point energies change. If a carbon-hydrogen bond is substantially altered in a rate-controlling step, replacing it by carbon-deuterium can produce a measurable rate difference. Such a measurement constrains a reaction mechanism, but its interpretation can also involve tunneling, equilibrium steps, and competing pathways.^[ref]