An isotopic substitution will have a pronounced effect on the reaction rate when the isotopic substitution (i.e. D for H) is part of a chemical bond that is broken, formed or modified in the rate determining step. The magnitude of the effect is dependent on whether the bond with the isotopic substitution is being broken or formed (primary KIE) or if the hybridization of the atom to which isotope is attached is changing (secondary KIE). |
| In summary, primary kinetic isotope effect is a change in rate due to isotopic substitution at a site of bond breaking or bond making in the rate determining step of a mechanism. |
Kinetic isotope effect can also be observed with other pair of isotopes such as H & T;12C& 13C or 14C; 16O & 18O; 35Cl & 37Cl etc., |
| Kinetic isotope effects are expressed as the ratio of the reaction rates in the presence of the reactants when containing one isotope as compared to the other. |
| It is observed experimentally and its values are intermediate between unity i.e. no isotope effect and maximum as large as ~7 (at 25°C) in case of H & D. |
For other elements, these values are low because relative mass difference is small. |
| Secondary kinetic isotope effect, on the other hand, is the change in rate due to isotopic substitution at site(s) than that of bond breaking or bond making in the rate determining step of a mechanism. |
